We are searching data for your request:
Upon completion, a link will appear to access the found materials.
Many insects, like butterflies have specific host plants on which they feed or lay eggs. While researching about some butterflies, I found that they lay their eggs on certain specific plants and their larvae feed on the leaves of those plants. I wonderd how do they recognize those specific plant species:
- Do they use visual cues to find or recognize these plants?
- Do they use chemical cues, like sensing volatile compounds released by the plants? I think this might be possible as I found many species of butterflies prefer plants of the citrus family which contain essential oils. Do they just sniff around following the trails of such chemicals to find the host plant,
- Is it the combination of both?
How do insects recognize their host plants? - Biology
This is my course assignment of Evolutionary Ecology of Stockholm University,Sweden in the year of 2009.
Specialization character of phytophagous insects is one of the key examples of insect-plant relationship. Host specificity of insect is required for their larval development. Several factors like trade-off, characteristics of host plant, insect’s neural system are responsible to an insect for becoming a specialist. Though specialization character of phytophagy increase the vulnerability towards extinction, still insect tense to be specialist for protecting themselves from natural enemies, getting the microclimate facility and boost up the survival rate of their offspring.
Key words: Phytopagous insect, Plant, Specialist,Generalist,Trade-off,Neural system
For feeding, reproduction and survival purpose organisms used various types of resources. These resources combined termed as ecological ranges of organism. Similarly insect use the plant resources for fulfilling their different requirements such as shelter, food, protection from different predators. However use of host plant range is relatively different in the phytophagous insect species. Entomologist classified the plant feeding insect into two categories such as generalist and specialist according to the mode of host plant use by them.
Generalist insect can be defined those insect which use use wide range of plant species as their host, whereas the specialist insect using a specified range of host plants in their life stages. Again phytophagous insects are differentiated into three categories such as monophagous, oligophagous and polyphagous. The insect species which feed on plants under single genus termed as monophagous.The oligaphous type consumed wide range of plants of different genera but in a single plant family. Whereas a polyphagous insect refers that they are feeding wide range of plant under different plant families (Bernays and Chapman, 1994).
Most of the phytophagous insects are specialized for choosing their host plant. Finding the causes for their specialization is a long discussed issue in insect host plant relationships. Here I discussed some of the reasons why they become specialist though specialist has higher risk of extinction than generalist type of insect.
Offspring's quality versus quantity
The one of the major reasons which creates the difference between generalist and specialist type of insect that is trade off between offspring quality versus quantity. The searching and evaluation of suitable host plant is done mostly by female insect species. The specialist type of insect provides much attention on the analysis of host plant which provides best resource and environment for feeding and survival of their offspring. They put much emphasis on the quality of larvae then the total of number of offspring. Whereas the generalist type insects have different strategy, they searching large number of host plants for increasing the number of their progeny (Janz and Nylin, 1997)
Neural system and plant choice
It is a long doubtful issue that poor neural system is the main reason for host plant specialization character of phytophagous insect. More theoretical studies than practical influence this hypothesis that phytophagous insects become specialist for their less developed neural system. Another predictions recently established that accuracy percentage of decision take by insect is the major cause for making them specialist towards specific host plant (Tosh et al., 2009).But some previous study suggests that poor neural system of phytopgaous insect make them specialist (Tosh et al., 2003).Meanwhile another study which contradict with this previous study, showing that information processing system of specialist insect help them to find out the better quality nettle (Janz and Nylin, 1997).This study also justify that neuron system of specialist phytophagous insect are good enough for discriminating among large number of host plants. However their information system process is greatly depends on olfactory neuron and which plays a significant role in specific host plant selection by specialist (Bernays, 2001).
Taking decision for choosing various host plants or a single host plant is another important factor for comparing the difference between generalist and specialist type of insects. The decision making process of insect is totally controlled by their nervous system. One study suggests that the nervous system of specialist phytophagous insects is quite simple which has less capability to detect multiple host plants. Specialist insect choice their host plant by recognizing the secondary metabolites (one specific host-plant compound) present in the host plant (Tosh et al., 2003). However another study showing that poor neural system is not only responsible for making phytophagous insects into specialist, actually a broad range of host plants contains different stimuli which takes a lot of time from a generalist insect for selecting the right one for it's purpose (Bernays and Funk, 1999).To save the time for searching of suitable host also influence the phytophagous insect to become specialist.
Host plant features
The chemical content of plant is greatly influence the host plant specialization character of insect.Sometimes Oligophagous or polyphagous insect's behavior turned into monophagous character due to presence of specific chemical substance which present in the host plant body.Even insects move to plant species which are under different families where they are getting the required chemical substance. For example, larvae of cabbage white butterfly also choose different plants which glcosinolates rather than Brassicase (Schoonhoven et. al, 2006).Even larval stage of some insect also prefer sulpher contained plants than free of sulpher, because sulpher chemical is beneficial for both larval and adult stage performance of insect (Marazzi and Städler, 2004).
Besides chemical content of plant, latex concentration of plant also play a vital role for specific host plant choice by phytophagous insect.The number of caterpillars is greater in latex free plant species than latex contained plant species (Diniz et al., 1999).
The morphological character of plant specially age of leaf also one of the features which helps the phytophagous insect to take decision about host plant. Polyphagous type under phytophagous insect likes matured, older leaves rather than younger leaves. The reason behind this decision is again chemical content. Because in comparison of older leaves, tender leaves contained huge quantity of toxic secondary chemical which are very harmful for the polyphagous insect. So, insect showing their priority for nesting in the older leaves (Cates, 1980).
Besides these leaf feeding insects, a large number of insects are feeding on different parts of the plant like wood, flower, fruit, and seeds even rooting system. They are specialized on the specific plant parts rather than generalist feeding behavior (Schoonhoven et al., 2006).
Few other reasons
Chemical content, leaf, secondary metabolites of plant has significant influence of host plant selection, but few other reasons like avoid natural enemies and competitors, surrounding micro climate of the host plant. Female insect who's one of the key responsibility of laying eggs, so they choice the host plant which provide maximum number of larva survives. For increasing the survivor ship of larvae they change their habitat area of same plant which provides an enemy free space for their offspring (Wiklund and Friberg, 2008).
From wikipedia, micro climate defined as a tiny atmospheric region which is different from it is surrounding big area ("Microclimate", n.d.).The major variable factors of micro climate are temperature, light intensity and relative humidity. However the change of insect activity is greatly depends on these variable factors. Relative humidity disadvantageously influences the oviposition rate of female insect whereas temperature advantageously influences it. The light intensity of a small area partially affects the population of insects, because insect choice plant species for their mating site by analyzing the intensity of light (Raghu et al., 2004).
Lastly, the list of reasons for becoming a specialist insect is still incomplete. Protection from enemy, utilization of the facilities of microclimate around the host plant and the most significantly the survivorship of progeny seems to be the causes discovered in present days. Though specialization characteristics of insects increase the chance of their extinction due to decreasing number of specific host plant but why they remain specialist is still a big question in the field of evolutionary biology.
Bernays, E.A.,&Chapman, R. F. (1994). Hostplant Selection By Phytophagous Insects (Contemporary Topics in Entomology). New York: Springer.
Bernays, E.A., and and Funk, D. J. (1999).Specialists make faster decisions than generalists: experiments with aphids.Proceedings - Royal Society of London. Biological sciences, 266, 151-156.
Bernays, E. A. (2001). Neural limitations in phytophagous insects: implications for diet breadth and evolution of host affiliation. Annu. Rev. Entomol., 46,703–727.
Cates, R.,G. (1980). Feeding patterns of monophagous,oligophagous,and polyphagous insect herbivores: the effect of resource abundance and plant chemistry.Oecologia, 46, 22-31.
Diniz, I. R., Moris, H. C., Botelho, A. M. F.,Venturoli, F.,&Cabral, B.C. (1999). Lepidopterian caterpillar fauna on lactiferous host plants in central Brazilian cerrado. Rev. Bras. Biol.(Online),59(4), 627-635.
Janz, N.,&Nylin, S. (1997). The role of female search behaviour in determining host plant range in plant feeding insects: a test of the information processing hypothesis. Proceedings - Royal Society of London. Biological sciences, 264(1382), 701-707.
Marazzi, C.,&Städler, E. (2004). Influence of plant sulpher nutrition on oviposition ad larval performance of the diamondback moth. Entomologia experimentalis et applicata. 111, 225-232.
Raghu, S., Drew, R. A.,&Clarke, A. R. (2004). Influence of host plant structure and microclimate on the abundance and behavior of a tephritid fly. Journal of Insect Behavior, 17(2), 179-190.
Schoonhoven, L. M., Loon, J. J.,&Dicke, M. (2006). Insect-Plant Biology. New York: Oxford University Press, USA.
Tosh C.R., Powell, G., Hardie, J. (2003). Decision making by generalist and specialist aphids with the same genotype. Journal of Insect Physiology, 49, 659-669.
Tosh, C. R., Krause, J.,&Ruxton, G. D. (2009). Theoretical predictions strongly support decision accuracy as a major driver of ecological specialization. PNAS, 106(14), 5698-5702.
Wiklund, C.,&Friberg, M. (2008). Enemy-free space and habitat-specific host specialization in a butterfly. Oecologia, 157,287–294.
Many herbivorous insects are mono- or oligophagous, having evolved to select a limited range of host plants. They specifically identify host-plant leaves using their keen sense of taste. Plant secondary metabolites and sugars are thought to be key chemical cues that enable insects to identify host plants and evaluate their quality as food. However, the neuronal and behavioral mechanisms of host-plant recognition are poorly understood. Here, we report a two-factor host acceptance system in larvae of the silkworm Bombyx mori, a specialist on several mulberry species. The first step is controlled by a chemosensory organ, the maxillary palp (MP). During palpation at the leaf edge, the MP detects trace amounts of leaf-surface compounds, which enables host-plant recognition without biting. Chemosensory neurons in the MP are tuned with ultrahigh sensitivity (thresholds of attomolar to femtomolar) to chlorogenic acid (CGA), quercetin glycosides, and β-sitosterol (βsito). Only if these 3 compounds are detected does the larva make a test bite, which is evaluated in the second step. Low-sensitivity neurons in another chemosensory organ, the maxillary galea (MG), mainly detect sucrose in the leaf sap exuded by test biting, allowing larvae to accept the leaf and proceed to persistent biting (feeding). The two-factor host acceptance system reported here may commonly underlie stereotyped feeding behavior in many phytophagous insects and determine their feeding habits.
Citation: Tsuneto K, Endo H, Shii F, Sasaki K, Nagata S, Sato R (2020) Diet choice: The two-factor host acceptance system of silkworm larvae. PLoS Biol 18(9): e3000828. https://doi.org/10.1371/journal.pbio.3000828
Academic Editor: Anurag A. Agrawal, Cornell University, UNITED STATES
Received: January 20, 2020 Accepted: August 17, 2020 Published: September 16, 2020
Copyright: © 2020 Tsuneto et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Data Availability: All relevant data are within the paper and its Supporting Information files.
Funding: 1. Japan Society for the Promotion of Science (JSPS) KAKENHI Grant Number 17K19261 to RS (https://kaken.nii.ac.jp/ja/grant/KAKENHI-PROJECT-17K19261/) Grant Number 18J00733 to HE (https://kaken.nii.ac.jp/ja/grant/KAKENHI-PROJECT-18J00733/) 2. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing interests: The authors have declared that no competing interests exist.
Abbreviations: AN, antenna ANOVA, analysis of variance CGA, chlorogenic acid ISQ, isoquercitrin LS, lateral styloconic sensillum MG, maxillary galea MP, maxillary palp MS, medial styloconic sensillum Q3R, quercetin-3-O-rhamnoside βCD, methyl-β-cyclodextrin βsito, β-sitosterol
How do insects recognize their host plants? - Biology
The term coevolution is used to describe cases where two (or more) species reciprocally affect each other's evolution. So for example, an evolutionary change in the morphology of a plant, might affect the morphology of an herbivore that eats the plant, which in turn might affect the evolution of the plant, which might affect the evolution of the herbivore. and so on.
Coevolution is likely to happen when different species have close ecological interactions with one another. These ecological relationships include:
Plants and insects represent a classic case of coevolution one that is often, but not always, mutualistic. Many plants and their pollinators are so reliant on one another and their relationships are so exclusive that biologists have good reason to think that the "match" between the two is the result of a coevolutionary process.
But we can see exclusive "matches" between plants and insects even when pollination is not involved. Some Central American Acacia species have hollow thorns and pores at the bases of their leaves that secrete nectar (see image at right). These hollow thorns are the exclusive nest-site of some species of ant that drink the nectar. But the ants are not just taking advantage of the plant they also defend their acacia plant against herbivores.
This system is probably the product of coevolution: the plants would not have evolved hollow thorns or nectar pores unless their evolution had been affected by the ants, and the ants would not have evolved herbivore defense behaviors unless their evolution had been affected by the plants.
How can the interpretation of host range tests be improved?
Prerelease studies in weed biocontrol programs have often been carried out with limited knowledge of the agent's genetics and behavior. Detailed information on these aspects are of basic interest, because both the design of the host-specificity tests and the interpretation of the results largely depend on, and should be adjusted to, the particular characteristics of the candidate species under investigation. I discuss aspects related to larval development first and those dealing with host-selection behavior afterward.
As mentioned above, no-choice larval development tests often reveal that a few larvae of a biocontrol candidate can develop on plant species that are not utilized under field conditions. Such results may not have to be considered further when the required host specificity for the target area includes these partly suitable test plant species. However, if native members of the same taxa occur in the target area, these data have to be interpreted carefully. Statistical analyses are sometimes applied to determine whether the target weed is more suitable for larval development than the attacked test plant species. Yet, if a few larvae are able to complete their development on a nontarget native plant, and if there is genetic variation in traits affecting larval performance, changes in performance may evolve. For example, in a selection experiment, mean larval survivorship of the agromyzid fly Liriomyza trifolii Burgess on a resistant cultivar of Chrysanthemum morifolium Ramat. increased from 15% to 51% within 10 generations ( Hawthorne 1999).
There are several ways to improve the interpretation of the results of standard larval host range tests. As Futuyma and colleagues (1995) have shown, adaptation may be constrained by the lack of genetic variation. Because many of the traits related to preference and performance exhibit continuous phenotypic variation, quantitative genetics is often the appropriate tool for biologists seeking to understand evolution of these traits ( Falconer and Mackay 1996). Statistical methods such as parent–offspring regressions can be used to estimate whether variation in performance on, and preference for, a critical test plant are indeed genetically based, and whether these traits are correlated with each other. Using a quantitative genetic design, Singer et al. (1988) found within a single population of the butterfly Euphydryas editha (Boisduval) a significant tendency for offspring to perform better on the host species preferred by the female parent. There was also a heritable variation in postalighting oviposition behavior, with some females preferring one and others the alternate host. Genetic preference–performance correlations may facilitate host shifts or host race formation ( Singer et al. 1988).
Quantitative genetic designs may also be used to predict the likely course of host range evolution in new insect–plant associations. For example, by comparing larval performance of the butterfly Colias philodice Latreille on its primary host with that on a recently introduced host, Karowe (1990) found that larval survival and growth are reduced, but female fecundity is enhanced, on the introduced plant species. Using quantitative genetic designs, Karowe showed that there exists significant among-family variation for larval performance, and larval performance is positively correlated across the two plant species. This suggests that selection for increased performance on each species can facilitate evolution of increased performance on the other. He therefore concluded that once the prealighting discrimination is overcome, C. philodice expands its host range to include the introduced plant species.
In the evolution and maintenance of specialization, behavioral factors may be more important than morphology or physiology ( Bernays 1998). Host plant selection by gravid females consists of a sequence of behavioral acts, including dispersal, habitat finding, plant finding, alighting, contact evaluation, and oviposition ( Schoonhoven et al. 1998). In general, oviposition behavior seems to be more plastic than larval host range, with many extrinsic and intrinsic factors influencing the host choices made by an insect species ( Bernays and Chapman 1994). Oviposition “mistakes,” whether maladaptive or a risk-spreading strategy ( Larsson and Ekbom 1995), may occur both in cages as well as in the field. The frequency of oviposition mistakes depends on several physiological and behavioral factors: the number of eggs a female carries, whether the eggs are laid on the plant surface or inside plant tissue, and whether the females are mainly egg or time limited ( Minkenberg et al. 1992). Although a few eggs laid on a lower-ranked host are not of great relevance in preference tests with regard to evolutionary questions ( Nylin and Janz 1993, Thompson 1993), they are of concern in biological control testing, especially when the plant species supports complete larval development. If oviposition on a suboptimal host is accompanied by larval or adult learning, behavioral and physiological adaptation to the new host may evolve quickly ( Jaenike and Papaj 1992).
Host ranking and host specificity of biological control candidates should be investigated using carefully designed experiments. It can be misleading to draw conclusions on behavioral preferences from field data, because spatial and temporal availability of potential host plants as well as intra- and interspecific competition may affect the distribution pattern in the field ( Singer 1986). Despite the inherent disadvantage that insects cannot display long-range oviposition behavior in confinement, choice oviposition tests in cages can yield valuable insight into the phenotypic and genetic variation of host ranking and host specificity ( Wiklund 1975, Thompson 1993). By offering insects a range of different target versus nontarget plant ratios, or by using females of different age, one can study response curves of short-range prealighting and postalighting behavior. An example of such a response curve from theoretical ecology is provided by Courtney et al. (1989), who experimentally assessed the relationship between egg load and host acceptance by females of Drosophila busckii Coq. The level of acceptance for the preferred host rose much quicker than for the less preferred host, but at very high egg load, both hosts were almost equally accepted.
However, for the same reason one cannot extrapolate from the egg distribution in the field to behavioral preference, one should not extrapolate from carefully designed behavioral preference experiments in confinement to open-field conditions. For instance, herbivore species with poor mobility actually may not perceive the top-ranked and lower-ranked plant species at a regular, random interval, because the two plant species may have a clumped or spatially separate distribution. This may result, for example, in increased egg load because of a perceived absence of the primary host plant, and lead to the acceptance of hosts which would not, or would only rarely, be accepted in small-scale choice experiments.
An open-field test design originally proposed by Rizza et al. (1988) and Dunn and Campobasso (1993), and refined by Briese (1999), is well suited to address the relationship between behavioral preference and mobility of the biocontrol candidate, as well as ecological parameters such as host plant density and spatial distribution. In this two-phase test, the first phase is a choice test to assess the behavior of the biocontrol candidate when exposed to a plot of randomly arranged control and test plant species. In the second phase, the target weed is killed and removed from the central plot, and satellite trap plots consisting of either target or test plants are set up at a certain distance. This second phase reveals the agent's behavior with regard to nontarget plants in the absence of the target species, and it therefore simulates the situation in which a successful biocontrol agent locally eradicates its target plant ( Briese 1999). By experimentally establishing different ratios of test plant-to-target weed densities, setting up the satellite plots at different distances from the central plot, or releasing different biocontrol agent densities, the effect of ecological parameters on host utilization can be assessed without confinement.
Open-field studies can also be used to elucidate the role and plasticity of individual steps within the sequence of host-selection behavior. For example, following females in the field and recording which plants they land on, and which of these are actually accepted for oviposition, can help to identify the role of pre- and postalighting cues in host-selection behavior ( Papaj and Rausher 1983). Using a similar approach, Parmesan et al. (1995) demonstrated that experience did not alter the open-field oviposition foraging behavior of the checkerspot butterfly Euphydryas editha. Mobility under natural conditions, which has implications for long-range host-selection behavior and the expression of behavioral preference, was assessed in an elegant study by Dempster et al. (1995) for flowerhead-feeding tephritid flies, which have been repeatedly used in biological control programs. By applying to field patches chemical markers (chloride salts) that were taken up by the host plant and translocated to the flower heads and so to the tephritid flies, Dempster et al. (1995) assessed the mobility of these flies in the field. Such an approach could also be used to evaluate the likelihood of a biocontrol agent to find new weed patches once the agent has destroyed the local patch.
For improved interpretation of screening results that do not correspond with the biocontrol agent's realized host range, it is helpful to analyze the cues involved in, and the specificity of, the different steps in the sequence of oviposition behavior. For instance, acceptability of the North American Potentilla gracilis Dougl. by the clearwing moth Tinthia myrmosaeformis (Herrich-Schaeffer), a candidate for biocontrol of Potentilla recta L., did not differ from that of the target weed when the females were transferred by hand onto the plants. However, in small cages females clearly preferred flowering P. recta shoots over flowering P. gracilis shoots. If females were exposed to nonflowering P. recta shoots and flowering P. gracilis shoots, the preference disappeared again, indicating that open flowers of P. recta were an important cue in the narrow-range prealighting behavior of T. myrmosaeformis (Schaffner, unpublished results). In an open-field experiment exposing P. recta and P. gracilis in a randomized block design, P. gracilis was not accepted at all. This suggests that T. myrmosaeformis females use additional cues in the long-range host-selection behavior, and thereby increase their host specificity in the field. What remains to be shown is how endogenous states of gravid females and the density and spatial distribution of target and nontarget species affect host fidelity in T. myrmosaeformis.
In no-choice tests, the chrysomelid beetle Altica carduorum Guer., a potential biocontrol agent for Cirsium arvense (L.) Scop., was able to develop on all Cirsium species tested. However, Wan and Harris (1996) showed that aggregation, which is part of the host-finding behavior, was induced by feeding or mechanical damage on C. arvense only, and not on other Cirsium species. Also, A. carduorum adults were attracted by feces of the opposite sex or of larvae, but only if the feces resulted from feeding on C. arvense. These behavioral studies explain the differences between the no-choice feeding tests and field observations that indicated that this beetle is monophagous on C. arvense ( Wan and Harris 1996).
In summary, in-depth studies on the behavior and the genetics of a biocontrol candidate can considerably improve the interpretation of results obtained by standard host range testing. Instead of using statistical analyses, efforts should be made to understand why and to what extent some test plant species are suitable for larval development or are acceptable for oviposition. Experimental investigations on the phenotypic and genetic variation in preference and performance can lead to predictions of the likely course of host range evolution once the biocontrol candidate is released into the target area. Also, an evolutionary ecology approach may contribute significantly to nontarget risk assessments by identifying the variables that might modify expression of host fidelity ( Roitberg 2000). Predicting the likelihood of a nontarget species being attacked requires knowledge of both the degree of an agent's phenotypic plasticity and the evolutionary consequences of that plasticity.
Evolutionary arms race between plant-eating insects and host plants illuminated
A newly identified relationship between a fly and a weedy mustard-type plant promises to answer many long-standing questions surrounding the evolutionary arms race between plant-eating insects and their host plants
Scientists trying to get a grip on the arms race between plant-eating insects and the defenses put up by their hosts just got a boost from new research by a University of Arizona entomologist published in the early view edition of Molecular Ecology.
Noah Whiteman, an assistant professor in the UA's department of ecology and evolutionary biology, has found a miniature ecosystem consisting of a plant and a tiny fly that spends its entire life cycle on the plant.
What makes this system special is the fact that both its key players -- the plant and the insect -- are what scientists call genetically tractable model organisms: holy grails of any serious science that aim to unravel biological mechanisms down to the level of genes and proteins and signaling molecules.
Decades of research and knowledge rest upon two of the most famous and widely used workhorses in genetics research: Arabidopsis thaliana, an unassuming, weedy plant in the mustard family, and Drosophila melanogaster, familiar to many as the tiny, red-eyed fruit flies hovering around the produce aisle.
However, until now, scientists wanting to study interactions between plant-eating insects and the plants they befell were out of luck: Fruit flies, as the name implies, feed on rotting fruit and couldn't care less about Arabidopsis plants, and vice versa.
Enter Scaptomyza flava, a fly so closely related to Drosophila melanogaster it shares most of its genes, and with a strong appetite for Arabidopsis. Female Scaptomyza flies prick a hole into the plant tissue and lay their eggs inside. Once the larvae hatch, they spend their childhood as leaf miners: tunneling their way through the leaf, munching on the nutritious plant tissue.
A fruitful walk in the field
Although the science underlying plant-insect interactions is no walk in the park, Whiteman's discovery started out as just that.
While a postdoctoral research fellow at Harvard, Whiteman became frustrated by the lack of a model system to study plant-insect interactions on a genetic and molecular level.
After an extensive literature search, he went out into the field and started looking for mustard plants that had flies living in and on them.
"One of the first places I started to look was Fresh Pond, which provides a lot of the water supply for Cambridge. There was a vacant lot so I pulled up my car and looked for yellow flowers. And sure enough, there was this plant called Barbarea vulgaris growing there, which is introduced from Europe and closely related to watercress."
"I looked for mines in the leaves. I brought some leaves and larvae to the lab, wrapped them in paper towels, put them in cages and let them go."
Eventually, flies started coming out. Some of them had red eyes.
"I keyed them out and they turned out to belong to the genus Scaptomyza, an outdated classification because we now know they belong to the Drosophila genus. 'This is exciting,' I thought. I put them all in a cage with Arabidopsis plants. They started attacking the plants not only the larvae, but the adult females, too. They make holes in the leaves with their ovipositors and drink the juices that come out."
Then, one day, while walking his dog in one of Boston's oldest city parks, Whiteman again came across the plants with the telltale tunnels in their leaves.
"I found the same plant there and the same flies in it. There were actually two Scaptomyza species coming out of this plant. One is a true herbivore and was making the mines, and the other was living in the mines of the miner. I sent the flies to the Smithsonian USDA Insect Identification Lab for confirmation but they sent them back and said, 'There is nobody here who could identify this.'"
"There seems to be this idea that there is this big convention where people decide what becomes a model organism, when in fact it's just individuals who decide what can be collected and what will work. The problem with these flies is you can't freeze the eggs, you can't store them. You need to keep things alive."
Sharing a hotel room with flies
When Whiteman traveled across the country to join the faculty at the UA, the flies traveled with him.
"I kept them in the back of my car, in cages. I was bringing them into hotel rooms."
Next, Whiteman's research group had to show that the flies and Arabidopsis plants could be used as a model system.
"There is a lot of research going on trying to figure out what biochemical pathways plants use to cope with insect attacks," said Whiteman. "Now we can tackle these questions in much more detail."
As one might expect, over the course of evolution, host plants have developed numerous ways to ward off parasites, such as chemicals that are toxic to the insects or throw a wrench into their development in some way.
Whiteman and his coworkers found that Arabidopsis plants ramp up their production of proteins that mess with the insects' digestive tract.
"The idea is that the plant is making it difficult for the insect to digest it," Whiteman explained. "It's very complicated, we don't really know what's going on at a molecular level."
To address this question, the researchers compared how well larvae fared on defenseless, mutant plants unable to make the indigestive proteins compared to their kin raised on wild-type plants.
"From those analyses, we do know that it has an effect on the larvae they don't do as well," Whiteman said. "The plants put up their defense in response to the insects being present. If you're a fly, living on something that is trying to kill you is different from living on, say, a rotting apple," Whiteman said.
"We also know that once a plant turns on one defensive pathway, others shut down. They inhibit each other," he added. "To the ecologist this is confusing, but of course, you can't be good at everything at the same time."
"Until now, we weren't able to tease these mechanisms apart, to answer questions like, why is this insect feeding less? We didn't have any tools to study these interactions in a controlled fashion."
"You could put any plant-eating insect on Arabidopsis to study this interaction from the plant side," he said. "In order to make our new model system compelling from the insect side, we had to use some tools from Drosophila genetics."
Hunting down defense genes
His team tested the Scaptomyza flies for the activity of genes known from Drosophila to be important in dealing with toxic plant compounds, and how they responded to the presence or absence of specific host plant defense molecules.
The researchers found that when they reared larvae in knockout plants unable to produce defense molecules, the flies dialed back their expression of detoxification genes.
"It makes sense for them to not turn on those genes unless the plant's defenses are up," Whiteman said.
Conversely, putting larvae on plants with intact defenses turned on their detoxification genes.
To put the new model system on a broad basis, Whiteman and his colleagues determined the genetic code of all the active genes in larvae reared on plants with different abilities to fend off insects.
"Now we have a couple of billion of base pairs of data, and our goal is to identify genes that are selectively induced or repressed in larvae depending on what kind of plant they are reared on," he said. "We found 400-500 significantly induced ones."
As often in ground-breaking research, the initial discovery stirred up a myriad of questions.
"Now we have to figure out, are those genes functionally important?" Whiteman said. "We can show that there is resistance in Arabidopsis and we can identify which pathways are involved in that resistance. The next question is: How is the leaf miner responding to the presence or absence of these toxic molecules? Does it care? Clearly, it does. There is a cost for detoxifying, but what is it?"
Ecological questions are waiting to be answered as well.
"We know that the leaf mining habit has evolved probably 25 times in insects," Whiteman said, "mostly in beetles, butterflies, moths, some wasps and saw flies. It's not present in the other insect orders, but why? How has selection shaped the ability of these insects to colonize an organism with a potent defense response? We want to develop the Scaptomyza lineage as a system for answering questions like these. This is our lab rat system."
Materials provided by University of Arizona. Original written by Daniel Stolte. Note: Content may be edited for style and length.
How acacia ants use vibrations to detect intruders and protect their host plantsAcacia ants on their host tree (Crematogaster mimosae, Acacia zanzibarica). Image credits: Felix A. Hager and Kathrin Krausa.
You could hardly find a more impressive alliance than the one between acacia trees and acacia ants — they even share the same name because their bond is so strong. The trees provide food (in the form of nectar) and accommodation, and the ants protect the trees against would-be nibblers like elephants or giraffes. The trees even provide outposts in the form of hollow spikes, that the ants use to take refuge while defending the trees.
Now, a new study has revealed how ants are capable of telling when such a mammal approaches, and how they’re even capable of differentiating it from wind, sounding the alarm and starting to actively patrol the plant when nibblers are nearby.
This comic depicts how acacia ants are tipped off to the presence of herbivores by vibrations that run throughout acacia trees when an animal (elephant) gets too close or begins to chew. As a result, the insects begin patrolling the acacia’s branches more actively. The ants don’t react when the trees’ movements are caused only by the wind. Image credits: Felix A. Hager and Kathrin Krausa.
Researchers observed first-hand that whenever they got too close to acacias, ants would come running to their defense. But they also realized that they weren’t sure how ants know when to start the defense. The traditional theory says that odor is the main tell, but that just didn’t seem to tell the whole story: they had a hunch that vibrations also play a part.
“We often inadvertently touched the acacia branches and backed off because of the very fast and disruptive attacks of ants that swarmed on us,” says Kathrin Krausa, co-author of the new study. “It struck us that it was assumed that odors associated with plant damage alert the ants. As biotremologists studying vibrations, we felt that this is only half of the story.”
To test their theory, they measured vibrations in acacia induced by two different factors: wind and a browsing goat. They then replicated these vibrations using special devices, observing how the ants behave in the absence of any other cues other then vibrations. The ants didn’t care much about the wind, but whenever the goat-like vibrations were produced, they started actively patrolling the acacia tree and getting ready for defense.
Caught in the act: the research team in the Kenyan savanna: Felix A. Hager, Peter Mwasi Lombo, Kathrin Krausa, and a helpful goat. Image credits: Felix A. Hager and Kathrin Krausa.
“If an ant detects vibrations due to an elephant nibbling at its tree, it needs to find the attacker as soon as possible and decide in which direction to go,” Krausa says. “We were impressed by the ants. Spread all over the tree, they made the right decision and walked toward the vibration source to fight back against the attacker almost every time.”
This isn’t the first study to indicate the intricate ways through which the acacia tree and ants achieve an impressive level of interdependence. Previous studies have found that the ants protect the trees from pathogens and also destroy any threatening plant growing nearby. However, the tree is actually the driving force of this relationship — while we tend to think of plants as inflexible and static, the acacia tree paints a completely different picture. It evolved to produce a sweet nectar that’s addictive to the ants and also changed to enable the ants to better protect it. The passive plant, it turns out, deals an extremely addictive substance to the ants, which then go to great lengths to protect it.
However, it’s the first time this type of vibration study has been carried out.
“We’ve just started to understand this mode of communication,” Krausa concludes. “There is a lot of work waiting for us!”
This Inaugural Perspective was inspired by a new collaborative research project in the laboratories of the participating authors in the area of plant–insect–prokaryotic pathogen interactions. We thank our laboratory members Li Zhang, Richard Hilleary, Bradley Paasch, Adam Seroka, and Hai-Jian Huang for their critical comments during the preparation of this review. Y.J. and S.Y.H. would like to thank organizers and participants of a satellite meeting titled “Vector-borne plant disease agents” held during the International Congress on Molecular Plant–Microbe Interactions, Glasgow, UK, July 12–19, 2019, for many useful discussions. We apologize to any colleagues whose work was not cited due to space limitation. The writing of this Perspective is possible due to funding from China Scholarship Council Fellowship (to Y.J.), Michigan State University Plant Resilience Institute (to S.Y.H.), Yunnan Applied Basic Research Projects (Grants 2018FA012 and 2016FB026 to Y.J.), and Natural Science Foundation of China (Grants 31670273 and 31870259 to Y.J.)
Why do insects like to eat some plants more than others?
In a study appearing in the forthcoming issue of The American Naturalist, Tom E. X. Miller, Andrew J. Tyre, and Svata M. Louda (all of the University of Nebraska, Lincoln) examined herbivore dynamics, specifically why plants aren't all eaten at the same rate. Plant-insect ecologists typically attribute the differences to variation in the nutritional quality or defective chemistry of plant tissues. However, the researchers found that cactus-feeding insects chose host plants based on how the plants allocated resources between growth and reproduction.
"The crux of our findings is actually quite intuitive", says Miller. "These insects prefer to feed on flowers, so it's not terribly surprising that they are abundant on cacti that invest most of their resources in flowers."
"What was surprising," Miller adds, "was how one single trait predicted the variation"
The results also have implications for understanding the evolution of plant allocation strategies. Current thinking on the subject says that these strategies are a trade-off between current reproduction and future survival. The finding that plant reproductive allocation can attract enemies means that sex may be even more costly than previously though. This research also has implications for weed control and protection of rare plant species.
Founded in 1867, The American Naturalist is one of the world's most renowned, peer-reviewed publications in ecology, evolution, and population and integrative biology research. AN emphasizes sophisticated methodologies and innovative theoretical syntheses--all in an effort to advance the knowledge of organic evolution and other broad biological principles.
Tom E.X. Miller, Andrew J. Tyre, and Svata M. Louda, "Plant reproductive allocation predicts herbivore dynamics across spatial and temporal scales." The American Naturalist: November 2006.
Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.
A parasitic relationship is one in which one organism, the parasite, lives off of another organism, the host, harming it and possibly causing death. The parasite lives on or in the body of the host.
A few examples of parasites are tapeworms, fleas, and barnacles. Tapeworms are segmented flatworms that attach themselves to the insides of the intestines of animals such as cows, pigs, and humans. They get food by eating the host's partly digested food, depriving the host of nutrients. Fleas harm their hosts, such as dogs, by biting their skin, sucking their blood, and causing them to itch. The fleas, in turn, get food and a warm home. Barnacles, which live on the bodies of whales, do not seriously harm their hosts, but they do itch and are annoying.
Usually, although parasites harm their hosts, it is in the parasite's best interest not to kill the host, because it relies on the host's body and body functions, such as digestion or blood circulation, to live.
Some parasitic animals attack plants. Aphids are insects that eat the sap from the plants on which they live. Parasitic plants and fungi can attack animals. A fungus causes lumpy jaw, a disease that injures the jaws of cattle and hogs. There are also parasitic plants and fungi that attack other plants and fungi. A parasitic fungus causes wheat rust and the downy mildew fungus attacks fruit and vegetables. Some scientists say that one-celled bacteria and viruses that live in animals and harm them, such as those that cause the common cold, are parasites as well. However, they are still considered different from other parasites. Many types of parasites carry and transmit disease. Lyme disease is trasmitted by deer ticks.
A parasite and its host evolve together. The parasite adapts to its environment by living in and using the host in ways that harm it. Hosts also develop ways of getting rid of or protecting themselves from parasites. For example, they can scratch away ticks. Some hosts also build a symbiotic relationship with another organism that helps to get rid of the parasite. Ladybugs live on plants, eating the aphids and benefiting by getting food, while the plant benefits by being rid of the aphids.