Tag Archives: Aphids

The complex life of the pea

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As a behavioral ecologist, I’m spending a surprising amount of time reading and writing about plants these days.  It turns out that plants are amazingly complex and interactive; you just need to know where and how to look.  Today we will discuss the humble pea plant, how it is infected with a virus that is carried by an aphid that sucks its xylem, and how a herbivorous weevil fits into the whole system.   The virus is the pea enation mosaic virus (PEMV), which causes pea leaves to yellow and wither, and also creates enations (scaly tissue) to develop on a leaf’s undersides. The aphid vector (a vector is the organism that carries a disease) is Acrythosiphon pisum, while the herbivorous weevil is Sitona lineatus.

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Pea plant infected with pea enation mosaic virus. Credit Paul Chisholm.

David Crowder has been studying plant/insect interactions for many years, and knew that most researchers who studied interactions between plants and insect vectors focused their attention on the plants, insects and the disease, but did not consider how other species in the community might affect this relationship. Paul Chisholm was a PhD student in Crowder’s lab; and working with two other researchers, they explored whether S. lineatus, an abundant herbivore of peas, influenced viral transmission.  They expected that if the pea was first attacked by the weevil, it might be more susceptible to subsequent viral infection.  Conversely, if the pea was infected by the virus, it might be less able to chemically defend itself against subsequent herbivory by the weevil.

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(Left) Pea aphids. Credit: Shipher Wu under a Creative Commons Attribution 4.0 International Public License. (Right) Very adult pea leaf weevils, Sitona lineatus. Credit: Gail Hampshire under a Creative Commons Attribution 4.0 International Public License

It is easy to visually distinguish between PEMV-infected and uninfected plants, so the researchers could assess whether infected plants tended to suffer more defoliation by weevils than did uninfected plants.  They visited 12 different fields in northern Washington and western Idaho, USA, and measured defoliation by counting the number of feeding notches left by the weevils after feeding on 3 – 10 infected pea plants and an equal number of nearby uninfected plants on each field (more feeding notches = more defoliation).  They discovered that PEMV-infected plants tended to suffer substantially higher herbivory than did uninfected plants

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Herbivory (as measured by number of feeding notches) caused by weevils on paired uninfected (black bars) and PEMV-infected (blue bars) pea plants sampled from 12 different fields. Error bars for all figures represent 1 SE.

Given the correlation between herbivory and infection, Chisholm and his colleagues then explored whether (1) the weevil preferred to feed on infected plants, and/or whether (2) infective aphids preferred to feed on plants that had been damaged by herbivorous weevils.  Both questions were answered with behavioral choice assays done in a greenhouse. First, the researchers created two groups of pea plants.  The first group, sham infected, were fed on by aphids not carrying PEMV for 48 hours, while the second group of plants were fed on by PEMV-infected aphids for the same duration. Aphids were removed and PEMV infection developed within 15 days in the PEMV infected plants.  The researchers then set up one sham infected and one PEMV-infected plant in a test cage, and released two weevils equidistant from the plants, allowing them to feed for six days.  They discovered that aphids fed much more voraciously on the PEMV-infected plants.

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Mean leaf area removed from sham infected and PEMV-infected pea plants.

For the second experiment, the researchers again created two types of plants: undamaged – no herbivory, and damaged – 48 hours of weevil herbivory.  Weevils were then removed, and one leaf from each plant was connected to each end of a tube, while still attached to each plant.

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Experimental setup with tube attached to one leaf of each experimental plant.  Aphids were introduced into the central tube. Credit Paul Chisholm. 

The researchers added either 25 infectious or 25 non-infectious aphids, and allowed them 3 hours to choose a leaf.  PEMV-infected aphids preferred damaged leaves, while uninfected aphids showed no preference.

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PEMV-infected aphid preference for undamaged (no herbivory) or damaged (weevil herbivory) leaves.

Chisholm and his colleagues then turned their attention to whether weevil herbivory made pea plants more susceptible to PEMV infection.  In one experiment they allowed PEMV-infected aphids to feed on plants for 3 days, and then introduced 0, 1 or 3 weevils who fed on the plants for another 6 days.  They used a protein assay to estimate the PEMV-titer (concentration) of each plant and discovered that the plants that were exposed to greatest herbivory had the highest PEMV titer (see graph below).  In a second experiment the researchers allowed weevil herbivory before adding the aphids, and found no effect of prior herbivory on PEMV titer.

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Relative PEMV-titer of infected leaves after they were subjected to herbivory by zero, one or three weevils for six days. different letters above bars indicate significant differences between treatments.

What causes these plant responses to challenges by PEMV and weevils?  The researchers discovered that levels of three important plant hormones increased either in response to PEMV infection, weevil herbivory or both.  At this point it is not clear how these different hormone levels interact to bring about the changes we’ve described.

The researchers conclude that weevil behavior has a profound influence on the interactions between aphids, the viruses they carry and the pea plants they feed on (and infect).  The weevil is not a vector for the virus, yet it affects the virus directly by altering plant behavior and physiology and indirectly by altering the behavior of the vector (the aphids).  PEMV outbreaks are more likely when weevils are abundant, as aphids prefer damaged plants, and feeding by weevils increased the PEMV titer in infected plants.  Crowder argues that interactions in which a non-vector species influences the relationship between a host and its vector (and the pathogen it carries) are probably extraordinarily common in crop systems.  So if we want to understand crop susceptibility to pathogens we need to cast a broad net and consider both the direct and indirect effects of a community of species that can influence how the crop responds to infection.

note: the paper that describes this research is from the journal Ecology. The reference is Chisholm, P. J., Sertsuvalkul, N. , Casteel, C. L. and Crowder, D. W. (2018), Reciprocal plant‐mediated interactions between a virus and a non‐vector herbivore. Ecology, 99: 2139-2144. doi:10.1002/ecy.2449. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2018 by the Ecological Society of America. All rights reserved.

Females are better speakers and better listeners than males – at least in plants

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My age puts me smack dab in the middle of the woo-woo generation, when many people engaged in activities, or shared in belief systems, that were criticized as unscientific, spacey or just plain bizarre.  For example, talking to your plants was purported to make them bigger, greener or more florid.  This hypothesis generated a huge number of science fair projects, but no clear answers (so far as I know – but I admit that I have not done the appropriate research!).  But, it turns out that plants do talk to each other and to some animals.  When attacked by herbivores, many plant species will emit volatile organic compounds (VOCs) into the air that can have two effects.  First, these VOCs can alert nearby plants that herbivores are in the area, and that they should start producing defense compounds in their tissues that will repel these herbivores.  Second, these VOCs can alert predators that herbivores are present, and they should swing by and eat them.

Several studies have shown that female and male plants may differ in several ways that could affect communication.  Females typically invest more in reproduction, grow more slowly and invest more in defense against herbivory. Xoaquin Moreira and his colleagues wondered if sexual dimorphism in defense investment would result in differences between males and female in how they talk to each other. They chose the woody shrub Baccharis salicifolia, in which females grow more slowly but invest more in chemical defense and thus are infested by fewer herbivores than are males.  They focused their study on chemical responses of the plant to the highly-specialized aphid Uroleucon macolai, which only feeds on two Baccharis species.

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Baccharis salicifolia hosting an army of herbivorous aphids. Credit: X. Moreira.

The researchers used greenhouse experiments to explore how Baccharis uses VOCs for communication.  To control aphid movement, each treatment was done in a mesh cage, with one centrally located VOC emitter plant (of either sex), and one female and one male receiver plant equally distant from the central plant. Control emitter plants were untreated, while herbivore-induced emitter plants were given 15 mature aphids, which fed and reproduced on the plants for 15 days.  After 15 days Moreira and his colleagues removed all of the emitter plants and all of the aphids, and then inoculated each receiver plant with two adult aphids.  The researchers measured aphid reproductive rate on the fifth day as their measure of aphid performance, or of plant resistance to aphids.

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Emitter Baccharis salicifolia plant flanked by one male and one female receiver plant. Credit X. Moreira.

Aphids did much more poorly on male and female receiver plants that were associated with male herbivore-induced emitter plants (top graph below).  This implies that these receiver plants became resistant to aphids as a result of their exposure to an airborne substance released by the male emitter plant.  When the researchers used female emitter plants they found something very different.  There was no effect on male receivers, but still a very strong effect on female receivers, which had a much lower aphid reproductive rate than the female plants exposed to untreated female emitter plants (bottom graph below).

MoreiraFig2

Reproductive performance of aphids raised on control receiver plants (emitter plant with no aphids – clear bars) and herbivore-induced emitter plants (gray bars).  Two left bars show performance on male receiver plants, while two right bars show performance on female receiver plants. Top graph shows data for male emitters and bottom graph shows data for female emitters. Error bars = 1 SE. *** indicates P < 0.001.

Showing differences between sexes in communication is important, but the next step is to figure out how this happens.  In previous research, Moreira and his colleagues identified seven different VOCs that Baccharis emitted after aphid herbivory.  So they explored whether there were differences between males and females in how much of each VOC they emitted in response to aphids.  As before, they subjected some plants (of each sex) to herbivory and others were untreated controls. They then bagged each plant, and passed the collected vapors over a charcoal filter trap at a constant rate for an equal period of time.  After extracting the substances from the charcoal, the researchers used a gas chromatograph to identify and quantify the VOCs.

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Setup for collecting VOCs from Baccharis salicifolia. Credit X. Moreira.

The most impressive finding was a fivefold increase in pinocarvone release by female herbivore-induced plants in comparison to controls.  In contrast, in males there was only a minor pinocarvone effect.

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Relative increase in VOC emission following aphid attack in female (clear triangle) vs. male (filled triangle) Baccharis salicifolia. The induction effect is the log response ration (LRR) which is the natural log of (emission by the herbivore induced plants divided by the emission by the control plants).  Error bars are 95% confidence intervals.

Having discovered that females emit much more pinocarvone than males, the next question was whether females are more sensitive to pinocarvone, or in fact to any of the other VOCs.  So Moreira and his colleagues exposed plants to one of three treatments: 100 ul of pure pinocarvone, 100 ul of six VOCs including pinocarvone, and a control (no VOCs).  They discovered that all experimental treatments reduced herbivory in comparison to the controls, but that there was no difference between males and females in how they responded.

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Reproductive performance of aphids raised on female plants (left graph) or male plants (right graph) subjected to pinocarvone or a blend of six VOCs (including pinocarvone) in comparison to reproductive performance on untreated control plants (dashed line on top of each graph).  Shading surrounding dashed line indicates 1 SE.  Error bars are 1 SE.

This lack of different response between male and female plants to pinocarvone was a bit surprising; the researchers speculate that both males and females have pinocarvone receptors, but that female receptors are more sensitive (or numerous). If true, natural emissions of pinocarvone may suffice to induce a response in female but not male plants. But the artificial emitters may have released enough pinocarvone to stimulate male plants to respond as well. Clearly there is much more work to do here.

The researchers also wanted to know whether plants were more sensitive to VOCs produced by genetically identical plants (clones) in comparison to genetically-distant plants.  They discovered no influence of genetic relatedness on plant response to herbivory.  This is important, because from an evolutionary standpoint, there is no obvious reason why a plant would want to warn an unrelated plant that it was about to get eaten. An adaptive explanation is that relatives may tend to live near each other, so an emitter plant still benefits indirectly by promoting the survival of relatives who carry a proportion of genes identical to its own genetic constitution. One possible non-adaptive explanation is that a plant may use VOCs as a way of quickly communicating with itself, informing distant tissues that they need to produce defense compounds.  Nearby plants may simply be eavesdropping on this conversation, and using it to their advantage.

note: the paper that describes this research is from the journal Ecology. The reference is Moreira, X., Nell, C. S., Meza‐Lopez, M. M., Rasmann, S. and Mooney, K. A. (2018), Specificity of plant–plant communication for Baccharis salicifolia sexes but not genotypes. Ecology, 99: 2731-2739. doi:10.1002/ecy.2534. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2018 by the Ecological Society of America. All rights reserved.

 

Biological control: birds vs. (insects vs. insects)

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We all know that birds eat crop-destroying bugs, so we might think that farmers would welcome insectivorous birds to their fields with radiant rakes or happy hoes. But not so fast! Research by Ingo Grass and his colleagues alerts us to the reality that not all insects are created equal. Some insects eat crops, but some insects eat insects that eat crops.

Aphids are one of the worst scourges of the agricultural world. They suck the phloem sap from many plant species; this action can kill the plant directly, and also cause infections by plant pathogens and viruses. Fortunately for farmers, many animals enjoy eating aphids, including birds such as the Eurasian Tree Sparrow, Passer montanus, and insects such as ladybird beetles and hoverfly larvae.

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Hoverfly larva consumes an aphid while a second aphid looks on. Credit: Beatriz Moisset.

Grass and his colleagues knew that sparrows eat both aphids and hoverflies, but they did not know how the effects of bird predation on these insects cascaded down to the oats and wheat crops grown near Gottingen, Germany. Their research tested the hypothesis that sparrows eat so many hoverflies that aphid abundance actually increases (despite also being eaten by sparrows), and oat and wheat abundance decreases (top food web in the diagram below). If so, they reasoned that removing the birds would increase hoverfly abundance, thereby decreasing aphids and increasing grain abundance (bottom food web).

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Agricultural food web with (top) and without (bottom) sparrows. Arrows show consumption, with dashed arrows indicating weak effects, and solid arrows and doubled organisms indicating strong effects.

The researchers set up an experiment with 11 nest boxes strategically placed between an oat field and a wheat field. Each box was equipped with a camera, so the researchers could see what the parents fed to their nestlings. In addition, Grass and his colleagues set up eight 4 X 5 meter plastic mesh exclosures which excluded birds, but allowed insects free access. They periodically surveyed 50 plants in each exclosure and in equal-sized control plots for hoverflies and aphids over the course of the sparrow breeding season. Because these birds can have three broods, this project kept them (the sparrows and the researchers) busy from early May to late July.

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The birds fed very little on the two grain fields during the first brood, but towards the end of their second brood, they turned their attention to feeding on insects from the two grain fields, and later to eating the ripening grain. One important finding is that bird predation severely reduced hoverfly abundance. By early July hoverfly abundance was about 1 per 50 shoots when birds were present, and more than 3 per 50 shoots when birds were excluded (top graph below).

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How did hoverfly consumption translate to aphid abundance? As you can see from the bottom graph, by early July, aphid abundance without birds was considerably lower than aphid abundance in the presence of birds. Taken together, these findings indicate that European Tree Sparrows consume hoverflies, which ultimately leads to an increase in aphid abundance.

Grass and his colleagues conclude that insectivorous birds can interfere with natural pest control of cereal production in central Europe. When birds were experimentally excluded, aphid densities declined 24% in wheat and 26% in oat crops. European Tree Sparrows were doubly bad for the crops, as they also harvested substantial quantities of grain from these fields to feed their third brood. The researchers argue that management of biological control systems for agriculture requires a broad food-web perspective that accounts for trophic cascades, such as the interactions that occur among sparrows, hoverflies, aphids and various types of economically important grain crops.

note: the paper that describes this research is from the journal Ecology. The reference is Grass, Ingo, Katrin Lehmann, Carsten Thies, and Teja Tscharntke. 2017. Insectivorous birds disrupt biological control of cereal aphids. Ecology 98 (6): 1583-1590. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2017 by the Ecological Society of America. All rights reserved.