Wasp larvae that eat aphids alive may save apple crops

Few insects strike greater fear into the hearts of orchard-owners than rosy apple aphids. These tiny bugs feed on the leaves of apple trees, draining them of nutrients. Their saliva, meanwhile, contains a toxin which causes those leaves to curl up and harden, providing an excellent place for them to shelter, including from insecticidal sprays. And the honeydew they excrete once they have extracted what nutrients they need from the sap they have ingested encourages the growth of a sooty mould that can further harm a tree. A bad rosy-apple-aphid infestation can reduce an orchard’s yield by 80%.

In the battle against these aphids some propose harnessing the services of tiny wasps called Ephedrus cerasicola and Aphidius matricariae. Like many of their kin, these insects are parasitoids, a trophic status halfway between being parasites and predators. Instead of killing and then eating their prey, parasitoid wasps eat and thus kill them—or, rather, their larvae do.

Females inject their eggs into their victims—in this case rosy apple aphids—once they have mated. The larvae that hatch from them go on to devour their hosts’ organs before pupating and emerging as adults. If the wasps’ services could be harnessed, it might thus be possible to reduce, or even dispense with, chemical insecticides. That would save money. And it would also make life easier for orchard-owners who seek the premium-price-generating imprimatur “organic” for their produce, a label which is incompatible with such chemicals.

Mobilising parasitoid wasps for pest control has a long history, beginning in the 1920s with their employment in greenhouses against whitefly. Unfortunately, attempts to use E. cerasicola and A. matricariae to control rosy apple aphids have so far failed. But Louise Ferrais and Thierry Hance of the Catholic…

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