Encouraging parasitic wasps to attack aphids could be the key to new methods of control. Sarah Henly looks at the practicalities.
CATS love it and so do aphids, say researchers. Catmint, a summer flowering perennial, common in herbaceous borders, is so called because cats seem to love rolling in it. Whether or not its smell attracts them isnt known, but catmint contains a chemical produced as a sex pheromone by sexual phase female aphids.
These aphids release sex pheromones in the autumn, which attracts winged males, and mating produces overwintering eggs. The resulting aphids threaten many cereal and pea crops.
But why grow catmint when the sex pheromones occur naturally, and aphids are to be discouraged rather than encouraged?
Some years ago, scientists at IACR-Rothamsted and Imperial College noticed that several aphid parasites can locate their aphid host using sex pheromones. Catmint was identified as producing one of the compounds found in the pheromone.
Extracts from catmint were used to produce synthetic pheromones in the laboratory, and the end product is being used to manipulate the behaviour of parasitic wasps, explains Dr Wilf Powell, entomologist at IACR-Rothamsted.
He is co-ordinator of a three-year LINK project being funded by MAFF, the HGCA, the Processors and Growers Research Organisation and the Horticultural Development Council. The work attracted much interest because the sex pheromones are common to many aphid species, and could potentially offer control opportunities in numerous crops.
Parasitic wasp species such as Praon volucre and Aphidius rhopalosiphi are usually present wherever cereal aphids occur. However to control aphid numbers, they must be in the right place in sufficient numbers at the same time as the pest appears.
By enticing the predator using synthetic pheromone lures, Dr Powell and his team have been able to increase numbers of parasitised aphids on potted trap plants. Parasitisation involves the female wasp injecting an egg inside her host, which develops into a larva, killing the aphid as it grows. It spins a cocoon inside, and eventually emerges from the mummified aphid as an adult.
This sight is familiar to glasshouse growers. Mass rearing and release works well in some protected crops, where predatory insects are introduced before pest numbers accumulate.
Such a strategy could work in the field, but only if the chemical pheromone worked as an arrestant as well as an attractant, says Dr Powell. "We are not sure from how far away the synthetic pheromone will draw wasps in, but it appears to intercept them and keep them where we want them long enough to attack nearby aphids."
The thrust is to encourage natural populations of wasps. Further work is needed to discover the best catch crop, both to conserve naturally occurring individuals and to retain any pulled in by pheromone lures as overwintering reservoirs.
It could be that field margins with a diversity of semi-natural plants are sufficient, or buffer strips in set-aside next to the crop make good habitats.
"We need an overwintering reservoir of these predators because both the crop and the aphids disappear in the late summer as harvest ensues. They must be around in sufficient quantities at the time when both winter and spring crops come under threat. It may be necessary to develop different marginal strips for different crops and aphid species," he suggests.
More funding is needed for this and other follow-up trials to determine, for example, the best method of applying the pheromone, and the feasibility of growing catmint as a commercial crop. In the plot trials, capsules containing synthetic pheromones derived from catmint were positioned at intervals along the existing field margins. This may be a cheaper option than a spray if the concept were to take off, believes Dr Powell.
He has no reason to doubt it could work, based on current results. Although sexual reproduction and thus natural pheromone release only occurs in some aphid populations once a year, most aphids being non-sexual females, parasitic wasps are attracted by the pheromone lure at any time of the year. Levels of parasitisation of cereal aphids and pea aphids are encouraging in trials.
There may be opportunities to combine this approach with one of protecting the crop using pest repellents or antifeedants, suggests Dr Powell.
"A simple push-pull model is demonstrated in pea and bean weevil control, where a synthetic pheromone pulls the pest from the crop while an antifeedant made from neem oil reduces feeding damage by any not pulled away.
"The pheromone in this case is for monitoring purposes, while in our work it has a more critical role in control."