A pheromone a day keeps the midges away

Chemical ecologists have taken a key step towards understanding the sex life of a notorious insect pest. The discovery spells good news for apple growers.

The apple leaf midge, Dasineura mali, causes millions of pounds of damage each year to apple orchards across the UK, mainland Europe, New Zealand and North America.

Midge larvae cause the edges of apple leaves to curl up into so-called galls, reducing photosynthetic area and stunting tree growth. This is especially damaging in nurseries and young orchards, where thousands of pounds of damage can hit a single farm, though growing evidence suggests yield can also be reduced in established orchards.

David Hall, professor of chemical ecology at the University of Greenwich, UK, and colleagues have identified and synthesised the female apple leaf midge sex pheromone. The group is applying for a patent on the novel chemical structure, which remains unpublished.

The aim of the project was to use pheromone traps for monitoring emergence of the midges to time applications of selective insecticides, says Hall. ’Once the eggs have hatched, and the larvae formed the leaf galls, they are very difficult to kill,’ he said. Timing insecticide treatment would also minimise damage to natural insect predators.

The group found that each female midge produces only 1.5 picograms of the pheromone. The researchers collected the molecule from over 2000 females and analysed it by gas chromatography linked to electroantennographic recording - which monitored the response of a male midge - and then by mass spectrometry. Once the chemical structure had been identified, the pheromone was synthesised, and the synthetic material shown to be identical to its natural counterpart.

Sticky traps baited with a microgram of the pheromone attracted male midges from up to 50m away. One trap caught 1500 males midges in a morning.

’Now that we have the pheromone and it has proved to be so phenomenonally active, we are reconsidering the possibility of also using it for actual control by mass trapping, lure-and-kill and/or mating disruption approaches, as has been done with sex and aggregation pheromones of many moths and beetles,’ Hall told Chemistry World.

The pheromone is ’not spectacularly difficult to synthesise,’ he said. The synthesised product is chiral but, although the insects probably produce only one entantiomer, the racemic mixture works well. ’So that simplifies things,’ said Hall, ’and you are using such tiny amounts.’

But although the amounts are tiny, other researchers might expect them to be orders of magnitude smaller, says US pheromone expert and resident entomologist at Ecobugs.com Jeremy Heath. Hall’s baits were coated with microgram levels of pheromone. But Heath’s work on live midges found that a trap containing only six live females, producing just picogram levels of pheromone between them, was as effective as Hall’s microgram baits.

This suggests either that ’there is something about the way pheromone is released from the female that is inherently different than the way scientists typically deliver pheromone from a lure or odour cartridge,’ said Heath, or that ’the compounds we think are pheromones are really just carriers of the true material.’ Heath looks forward to publication of the UK team’s data.

Bea Perks