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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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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