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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Self-damaging wasps.. not stupid, just dirty!

 One of the first twits that I received this morning was from  Discover Magazine and was related to a post entitled “Parasitic wasps vaccinate aphids by spreading anti-wasp bacteria”. Of course the combination of vaccines and aphid is unusual considering that the insect immune system is devoid of memory and vaccination is actually a sort of immune priming rather than a true vaccination.

The interest in this paper is also hampered considering that aphids have a peculiar immune system since, as recently reviewed by Poirié and Coustau (2011), the immune deficiency (IMD) signalling pathway was apparently non functional in aphids and no genes coding for peptidoglycan recognition proteins (PGRPs) and several well-conserved antimicrobial peptides, such as defensins and cecropins, have been predicted in the pea aphid Acyrthosiphon pisum.

Actually the authors of the paper entitled “Parasitoids as vectors of facultative bacterial endosymbionts in aphids” do not refer to vaccination, even if they described a very intriguing event due to parasitoids! An example is Aphidius colemani showed in the Jarmo Holopainen photo from PBase.

Gehrer and Vorburger demonstrated ”that parasitoids can transfer endosymbionts of Aphis fabae aphids between clones by sequentially stabbing infected and uninfected aphids—a previously undescribed route of horizontal transmission. The wasp’s ovipositor appears to act as a ‘dirty needle’ that can inoculate previously uninfected aphids. If the recipient aphid resists the parasitoid and survives the attack, this can result in a new, heritable infection. Considering the fact that many aphid parasitoids use multiple hosts, it is likely that they can transfer endosymbionts not just within but also between aphid species.”

But that’s not all… the two bacterial symbionts that have been transferred by wasps are Hamiltonella defensa and Regiella insecticola and both have a protective function in aphids. Indeed, the first is known to increase aphid resistance to parasitoids whereas the latter  protect aphids against fungal pathogens. The wasp-mediated transfer of H. defensa is very intriguing from an evolutionary point of view since wasps are spreading a symbiont (H. defensa), which provides aphids with protection against parasitoids (including wasps), so that wasp-mediated transfer is detrimental to their own fitness. This result could seem strange or self-damaging, but actually, as perfectly exemplified by Gehrer and Vorburger, wasps are simply using ‘dirty needles’ so that they are not self-damaging but just paying a price for their dirty work!

Gehrer, L., & Vorburger, C. (2012). Parasitoids as vectors of facultative bacterial endosymbionts in aphids. Biology Letters DOI: 10.1098/rsbl.2012.0144

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