This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
Aphids are small insects that are a major pest in crop production. Dealing with these aphid pests often involves the use of pesticides, however growing resistance to these pesticides means that many farmers are now looking to use natural predators such as ladybirds or hoverflies to stop aphids destroying crops.
However the problem with natural predators is that they all tend to be mobile insects. There's no point introducing a large number of aphid-killing ladybirds into your field if they just fly off somewhere else. The ladybirds have to remain attracted to your field, your crops, and your aphids.
What is it that attracts ladybirds to aphids in the first place? One factor is the excretory products that the aphids release. Aphids drink sap from plants (the phloem) and the product excreted is honeydew, a complex mixture of sugars, amino acids, lipids and organic acids. Those who work within microbiology will recognise that honeydew sounds like an ideal medium for growing bacteria, and indeed lots of bacteria do feast on the excreted honeydew.
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These bacteria release volatile compounds, which can vaporise easily and travel long distances. And it is these compounds, produced by the bacteria, that attract the aphid predators to their prey.
Testing samples of honeydew that had been filtered to remove all bacteria revealed very few of these volatile compounds, however reintroducing the bacteria and allowing them to grow produced a whole range of potential insect-attractants. Both of these were then tested for attractiveness to insects by (rather hilariously) putting some hoverflies in a wind-tunnel with plant samples sprayed with either sterile or inoculated honeydew right at the top. They then looked at the number of hoverfly eggs that were laid on each of the plants.
It was shown quite nicely that far more hoverflies were willing to brave the force of the wind to get to the bacterial laden honeydew. The plants with bacterial-honeydew also ended up with a lot more hoverfly eggs (63±6 eggs) than the sterile honeydew (around 2/3 eggs). All experiments can be found in Ref. 1.
Quite why this unusual predator-prey relationship should exist is still a bit of a mystery. It might just be that the predators have learned to recognise the bacterial signal, while the bacteria (not being so directly affected by predators, after all there are things other than honeydew to eat) have simply not had as much evolutionary pressure to stop producing the volatile signal. It is suggested in Ref 2 that the bacteria might be produced or encouraged in some way by the plants as an elaborate defence mechanism, although I find that a little bit far-fetched.
What is probably the most practical use for farmers, the paper also isolated specific volatile compounds and tested them individually, with 3-methyl-2-butenal showing the greatest hoverfly-attracting effect. The production of some form of predator-attraction spray made of natural volatile chemicals could help against aphid infestations by concentrating natural predators around particularly valuable or endangered crops.
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Ref 1 = Leroy PD, Sabri A, Heuskin S, Thonart P, Lognay G, Verheggen FJ, Francis F, Brostaux Y, Felton GW, & Haubruge E (2011). Microorganisms from aphid honeydew attract and enhance the efficacy of natural enemies. Nature communications, 2 PMID: 21673669
Ref 2 = Wootton L (2011). Microbial ecology: Bacterial volatiles give the game away. Nature reviews. Microbiology PMID: 21785449