Aphid? An insect falling like a cat!

Aphids are wingless individuals (just some of them have wings) that live on leaves and face their predators and parasitoids simply by dropping off plants. Indeed, in order to avoid immediate dangers, aphids do not have any aggressive behaviour, but they simply jump from the plants landing on their legs, regardless of their initial orientation on the plant, by rotating their body during the fall, like a defenestrated cat. As Moshe Inbar and colleagues reported in their paper in Current Biology:

This righting ability is intriguing, as wingless aphids have no specialized structures for maneuvering in mid-air. Instead, they assume a stereotypic posture which is aerodynamically stable only when the aphids fall right-side up. Consequently, the body passively rotates to the stable upright orientation, improving the chance of clinging to leaves encountered on the way down and lowering the danger of reaching the ground.

As you can see in the video below, an important role seems to be played by legs, since aphids without legs are even worse at landing than dead ones. According to this paper, alive aphids with their legs amputated by the scientists only landed the right way up 28% of the tim so that the air resistance on the aphids’ appendages is useful to rotate their bodies as they fall through air regardless of their starting orientation. The anatomy of aphid legs is so important that, differently from cats, aphids do not need to actively twist anything, but they simply rely on physics to passively rotate and land in a proper way… so that they are immediately able to grip the surface and resist further bouncing and rolling.

Aphids are not the unique insects tha can control their falls, since some arboreal ants of several different genera are able to control the direction of their descent when falling from a tree via postural changes, specifically involving the orientation of the legs and gaster (as you can read in Yanoviak et al. 2011).

These are not just funny behaviours, but these studies may yield insight into the origins of winged flight in insects since these primitive aerobatics could be the forerunners to true insect flight. Indeed, as reviewed by Yanoviak et al. (2009) these results suggest that a diversity of aerial behaviours preceded the appearance of wings in the history of insects supporting the hypothesis of a terrestrial origin for winged flight in insects.

Ribak, G., Gish, M., Weihs, D., & Inbar, M. (2013). Adaptive aerial righting during the escape dropping of wingless pea aphids Current Biology, 23 (3) DOI: 10.1016/j.cub.2012.12.010
Yanoviak SP, Kaspari M, & Dudley R (2009). Gliding hexapods and the origins of insect aerial behaviour. Biology letters, 5 (4), 510-2 PMID: 19324632
Yanoviak, S., Munk, Y., & Dudley, R. (2011). Evolution and Ecology of Directed Aerial Descent in Arboreal Ants Integrative and Comparative Biology, 51 (6), 944-956 DOI: 10.1093/icb/icr006

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Not too complex to decide… even if for insects!

The grain aphid Sitobion avenae (in the photo from the website Agroterra) and the bird cherry-oat aphid Rhopalosiphum padi are considered two of the most significant aphid pests of cereal crops worldwide and their damages to agricultural and horticultural crops are compounded by their potential to act as vectors for disease, their high reproductive potential, and their ability to impose massive nutrient loss through the rapid extraction and utilisation of phloem. Due to their presence on different possible host plants, how they make their host selection?

Previous researches indicated that S. avenae prefers winter the wheat Triticum aestivum and may distinguish among different cultivars of the same species of wheat. The preference-performance hypothesis predicts a female oviposition preference to correspond with hosts that are best suited for offspring development that means good food and few enemies for offspring… but is it true also for aphids?

According to the data published by  Michael R. Wilson & Simon R. Leather, Sitobion avenae and Rhopalosiphum padi generally have a preference for nutritionally superior hosts. However, the preference of both species changes towards a nutritionally inferior host after infestations of the harlequin ladybird Harmonia axyridis  (Coleoptera: Coccinellidae), that killed aphids showing a trade-off between high-quality nutrition and avoidance of predation.  Such a capability is often seen in vertebrate ecology, but has been far less investigated in invertebrates and it has been frequently considered unrealistic in arthropods. On the contrary, as assessed by Wilson & Leather,  ”this investigation demonstrated the ability in aphids to make preferences based on the nutritional quality of potential hosts. Furthemore, it appeared evident the ability to perceive past activity of natural enemies and to make decisions based on these perceptions to forfeit the intake of nutritionally superior food and minimise the risk of predation.”

Michael R. Wilson & Simon R. Leather (2012). The effect of past natural enemy activity on host-plant preference of two aphid species. Entomologia Experimentalis et Applicata , 1-7 DOI: 10.1111/j.1570-7458.2012.01282.x

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Not so sweet for climbing plants!

Aphids feed on phloem that is rich in sugars but poor in amino acids. As a result, aphids must feed continuously to ingest phloem in large amounts and then excrete excess sugars in the form of honeydew. Natural enemies of aphids are known to use honeydew as part of their diet and as a cue in host/prey location because honeydew typically accumulates in the vicinity of aphid populations.

Ladybird beetles are major predators of aphids since both the adult and larval stages of ladybird beetles eat aphids and some species are commonly used as biological control agents to control aphid populations. Unfortunately, up till now few studies have evaluated the effect of honeydew on the foraging behaviour of ladybird larvae on aphids, but preliminary studies evidenced that Coccinella septempunctata larvae stayed longer in areas containing honeydew of prey that was easy to catch and so more profitable.  If so,does honeydew on the ground act as a prey-associated cue? does honeydew from low-quality aphid species act as a deterrent?

A recent paper published by Purandare and Tenhumberg reported interesting results about laboratory experiments carried out to explore whether honeydew accumulated on the ground is used as a foraging cue. The study also investigated whether, if honeydew is a foraging cue, larvae show differential responses to honeydew of high-quality prey Acyrthosiphon pisum Harris compared with that of low-quality prey Aphis fabae Scopoli (both: Homoptera: Aphididae) using the ladybird Hippodamia convergens (in the photo from the University of Kentucky) as a prey.

H. convergens larvae stayed longer in areas containing honeydew but did not engage in longer bouts of searching. Furthermore, larvae did not distinguish between honeydew from high- and low-quality aphid prey. As a whole, Purandare and Tenhumberg experiments assessed that H. convergens larvae are not more likely to climb a stick or plant in the presence of either A. pisum or A. fabae honeydew and nor do they seem to distinguish between the honeydew of aphids that differ in profitability.

Despite its applicative interest, honeydew on the ground may not be a reliable indicator of aphid density on surrounding plants, and the usefulness of cues to increase foraging efficiency depends on how reliable cues are. If the benefit of using honeydew as a cue is small, it is possible that not all aphidophagous predator species have evolved a response to honeydew. Moroever, it is possible that under field conditions, honeydew evaporates quickly, is washed away by rain, or that the volatile components of honeydew lose kairomonal activity in a short time. For instance, the kairomonal activity of the aphid Brevicoryne brassicae honeydew has been reported to decrease over time and to be lost completely after 72 h.

The effectiveness of spraying sugar solutions on crop fields to attract and retain natural aphid enemies is therefore at present quite controversial and could be effective with few ladybird species only.

Purandare, S., & Tenhumberg, B. (2012). Influence of aphid honeydew on the foraging behaviour of Hippodamia convergens larvae. Ecological Entomology, 37 (3), 184-192.

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Sex is more attractive than fear… for insects too!

In the last months I found in literature several very intriguing papers about aphids and their biological control. A good example is the paper entitled “Effect of synthetic and plant-extracted aphid pheromones on the behaviour of Aphidius colemani” recently published by O. M. C. C. Ameixa and P. Kindlmann in the Journal of Applied Entomology.

According to this paper the aphid parasitoid Aphidius colemani (in the photo from the Viridaxis homepage) is sensitive to a mixture of odours including both synthetic and plant-extracted nepetalactone (a component of aphid sex pheromone) and (E)-b-farnesene (aphid alarm pheromone). The behavioural responses of A. colemani to three semiochemical groups with different concentrations were studied in a square arena by Ameixa and Kindlmann showing that parasitoid females were significantly attracted by the semiochemicals, when their concentrations were high, in which case the females spent more time in squares with semiochemicals. However, the majority of females preferred plant-extracted nepetalactone, when it was in high concentration, but they consistently did not respond to (E)-b-farnesene.

These results support previous data showing that a high concentration of (E)-b-farnesene became repellent to the egg parasitoid Chrysonotomyia ruforum and that parasitoid females were not attracted by different concentrations of (E)-b-farnesene, but when this component was offered against a background of a non-attractive natural blend of pine volatiles, the combination became attractive… suggesting as a whole that to be detected by the parasitoid, (E)-b-farnesene must be in a combination with other plant volatiles.

As a whole these results are extremely important considering that some trials with genetically modified plants producing (E)-b-farnesene are in progress (as reported here) using (E)-b-farnesene alone making these plants probably not really effective to fight aphids.

Ameixa, O., & Kindlmann, P. (2012). Effect of synthetic and plant-extracted aphid pheromones on the behaviour of Aphidius colemani. Journal of Applied Entomology, 136 (4), 292-301 DOI: 10.1111/j.1439-0418.2011.01638.x

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Sensing aphids

In most aphid species, the volatile sesquiterpene (E)-β-farnesene (Eβf) is released as an alarm pheromone in response to predation and is also emitted continuously at low levels. Some aphid predators use Eβf as a foraging cue, suggesting that the benefits to aphids of signaling via Eβf must be weighed against the cost of increasing apparency to natural enemies.

At the same time, aphid alarm pheromone has been shown to mediate mutualistic interactions between ants and aphids, in which ants protect “myrmecophilic” aphids while collecting aphid honeydew (a rich source of carbohydrates). Nault et al. (1976) observed that ants became very aggressive in the presence of alarm pheromone and increased their rate of attack on aphid predators, but did not attack aphids. In contrast, when alarm pheromone was applied to colonies of aphid species that are not ant-tended (non-myrmecophilic), ants became aggressive towards the aphids themselves (Nault et al., 1976). More recently, ants’ ability to perceive Eβf was confirmed by Mondor and Addicott (2007).

It was frequently believed that ant-aphid mutualisms were tightly linked, coevolved responses. However, it has been proposed that these dyads may evolve, especially in facultative as opposed to obligate mutualists, as ‘loose relationships’ between species explaining why ants are capable of tending multiple aphid species, either simultaneously or sequentially, in their native habitats. The Argentine ant, for example, is capable of tending many different aphid species in its native environment  and many of these aphids are likely to use E-b-farnesene as their alarm pheromone. Interestingly invasive ants, such as Argentine ant in Europe and USA, may form mutualistic associations with native aphids due to selection for this trait in the ant’s native environment.

The study of aphid-ant interaction will be therefore very useful non only to better understand the interaction between these taxa, but also to improve our knowledge about ant invasive species.

(Photo of Alison Bockoven from the blog 6legs2many).

References

• Mondor, E., Addicott, J. (2006) Do exaptations facilitate mutualistic associations between invasive and native species? Biological Invasions, 9 (6), 623-628 DOI: 10.1007/s10530-006-9062-0
• Vandermoten S, Mescher MC, Francis F, Haubruge E, Verheggen FJ (2012) Aphid alarm pheromone: an overview of current knowledge on biosynthesis and functions. Insect Biochemistry and Molecular Biology, 42, 155-63 PMID: 22178597.
• Verheggen, F., Haubruge, E., De Moraes, C., Mescher, M. (2009) Social environment influences aphid production of alarm pheromone Behavioral Ecology, 20, 283-288 DOI: 10.1093/beheco/arp009

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If it was easy, it wouldn’t be a biological problem

In a recent post on genetically modified wheat crops producing the aphid alarm pheromone (E)-beta-farnesene, I suggested some doubts related to results published in literature. I concluded my post writing: “This does not imply that this trial is not interesting at all, since there is scattered evidence in the literature suggesting that alarm pheromone emission might serve as an indirect defense by attracting aphid predators and it will be very interesting to see what will happen to aphid predation”.

A further doubt emerges now from a recent paper published by Ameixa &  Kindlmann in the Journal of Applied Entomology where one of the most surprising results is that (E)-beta-farnesene did not elicit any strong response from the Aphidius colemani aphid parasitoids. Furthermore, it seems that in order to influence the behaviour of A. colamani, the (E)-beta-farnesene must be in a combination with other plant volatiles. This result support a previous study by Mumm and Hilker (2005) who has shown that a high concentration of (E)-beta-farnesene became repellent to the egg parasitoid Chrysonotomyia ruforum.

To achieve a complete understanding of the parasitoid behaviour in this system, further observations and field experiments should be made in the future and probably it is too early for suggesting that genetically modified wheat crop will be the final solution for aphid damages. We have a lot of work to do since, as recently Carl Zimmer stated, in biology being the godawful mess that it is, it seems that different factors work together, rather than in isolation.

References
Mumm R, Hilker M (2005) The significance of background odour for an egg parasitoid to detect plants with host eggs. Chem. Senses, 30, 337-343

Ameixa O, Kindlmann P (2011) Effect of synthetic and plant-extracted aphid pheromones on the behaviour of Aphidius colemani Journal of Applied Entomology, in press.

Image: Bio-bee biological systems

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Cape fear is still far.. at least for aphids!

A recent post on Scientific American reported that in UK some field trials are in progress studying the effects of a genetically modified  (GM) wheat that should strike fear into aphids and attracts deadly predators to devour them.

The genetically modified wheat emits a pheromone which aphids generally release when they are under attack to create panic and prompt the insects to flee (alarm pheromone). As frequenly occurs in nature, some predators may use the prey chemical communication to locate preys. Similarly, this wheat also attracts some parasitoid wasps to provide a second line of defence for crops since these wasps lay eggs in the aphids killing them.

As you can see in the following video by Christine Woodcock, aphids become agitated and move away  after perceiving the alarm pheromone:

According to this assumption on GM wheat, aphids should move toward other plants… BUT…  according to a paper published some years ago in Ecology Letters (here a summary), aphids were shown to react mainly to the frequency of pheromone release and not the actual quantity present, possibly to avoid manipulation by plants. Thus, to reduce damage caused by aphids, the major insect pests in Europe, it may prove effective to apply pulses of alarm pheromone to infested fields in place of a continuous production. At the same time, a recent paper published in BMC Ecology by Grit Kunert, Carolina Reinhold and Jonathan Gershenzon  provided no support for the hypothesis that plant emission of the aphid alarm pheromone has a direct defense effect against aphids and they suggested that  if plants continuously produce EBF aphids may become habituated. In particular Kunert et al concluded that “the results of this investigation demonstrate that EBF produced continuously by transgenic A. thaliana does not act as a direct defense against aphids. The same conclusion might well be applicable to the continuous emission of EBF from other plants though more studies are necessary. (…) This may be due to the fact these transgenic plants release EBF constantly as opposed to the pulsed release caused by natural enemy attacks on individual aphids. ”

This does not imply that this trial is not interesting at all, since there is scattered evidence in the literature suggesting that alarm pheromone emission might serve as an indirect defense by attracting aphid predators and it will be very interesting to see what will happen to aphid predation. Aphids are ancient pest crop insects and I fear (or as aphidologist I’m sure) that we are at present not near to their debacle.

Reference

Kunert G, Reinhold C, Gershenzon J (2010). Constitutive emission of the aphid alarm pheromone, (E)-β-farnesene, from plants does not serve as a direct defense against aphids BMC Ecology (10)

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Aphids do not like earthworms… me too!

The melon aphid Aphis gossypii is a severe pest of greenhouse cucumber, frequently developing extremely damaging populations consisting of several hundred individuals per leaf. This aphid has a short generation time and high fecundity and the presence of a moderate density population is sufficient to create leaf damage. Moreover, the honeydew produced by the aphids contaminates leaves and fruits so that plant growth decrease drastically resulting in a reduced yield.

Recently, some researchers showed that vermicomposts, which are produced by an interaction between earthworms and microorganisms, may enhance plant growth and resistance to some pests and diseases. Vermicompost has neutral pH, suitable structure and crumb, contains high population densities of beneficial organisms and has considerable aeration and water-holding capacity…. properties that make it a desirable alternative for peat and other potting media.

A recent paper by Razmjou et al showed that vermicompost is useful against aphids. Indeed, the application of vermicompost revealed a high potential for reducing A. gossypii populations in cucumber cultures. In particular, aphid numbers increase more slowly on cucumber plants growing in vermicompost-amended soil than on cucumbers growing in vermicompost-free soil. The resulting differences in the severity of infestations were substantial. Plants grown in soil containing 30% vermicompost harboured less than half as many aphids as plants grown in soil without vermicompost. Aphid populations were suppressed even more on plants grown in pots with 50% vermicompost, but at this concentration, vermicompost also had a negative effect on plant performance indices such as plant height, plant dry weight and chlorophyll content.

Previous studied showed that vermicomposts may decrease the population density of some pests such as peach potato aphid, Myzus persicae, the two-spotted spider mite Tetranychus urticae and mealy bug Pseudococcus spp. suggesting that facing pest bugs could be possible also avoiding a diffuse use of chemicals.

So… if you want to try, and most of all if you like earthworms or at at least, if you do not dislike them…. you have just to try and you will have a double gain: 1. fight aphid growth; 2. decompose and recycle the organic matter coming from your kitchen.

 

Reference

Razmjou, J., Vorburger, C., Mohammadi, M., & Hassanpour, M. (2012). Influence of vermicompost and cucumber cultivar on population growth of Aphis gossypii Glover Journal of Applied Entomology DOI: 10.1111/j.1439-0418.2012.01710.x

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