Insecticides, and potentially herbicides and fungicides, affected by enhanced metabolic resistance could have their efficacy at least partially restored, according to research presented at the International Plant Protection Congress organised by the British Crop Production Council.

By pre-applying a compound, known as a synergist, that inhibited the enzymes insects used to detoxify pesticides, activity of various insecticides against resistant insects had been increased to near levels seen against susceptible insects, Graham Moores of Rothamsted Research said.

Just as excitingly the technique also improved activity of insecticides against susceptible populations, and therefore, potentially could be used to reduce insecticide doses.

But it wasn't quite as simple as spraying a mixture of the insecticide and the synergist, piperonyl butoxide (PBO), he said. "It takes between four and 10 hours for PBO to penetrate the insect's cuticle and begin to inhibit the enzymes detoxifying the pesticide, so spraying a straightforward mixture may not be the best way of overcoming resistance.

"Ideally you need to use the synergist first, but farmers don't want to spray twice."

That drawback was being overcome through using microencapsulation formulations that released the PBO first and the insecticide several hours later, he explained.

The technique works independently of chemical class - improved control has been recorded with cypermethrin and pirimicarb against Myzus persicae (peach-potato aphid), for example, and imidacloprid against Bemisia tabaci (tobacco whitefly), which is a major pest of protected vegetable crops.

The results against Myzus also, perhaps surprisingly, demonstrated that the encapsulated insecticides could sometimes overcome not only metabolic resistance, but also target-site resistance. "We think this is because removal of the background metabolic enzymes leaves the insect in a hypersensitive state prior to exposure with insecticide."

By tweaking the formulation researchers have changed the time span between the PBO and insecticide being released. That's important because insect species have their own optimum pre-treatment time, but it could also allow the possibility of "protecting" beneficial insects present in the crop, Dr Moores explained.

Potential uses in the UK for synergised products include control of resistant Myzus aphids in potatoes and other crops, and pollen beetles in oilseed rape, Dr Moores said. "There is also no reason why the concept shouldn't be extended to fungicides and herbicides, which suffer from metabolic resistance."

One potential disadvantage of the technique was the possibility of encouraging other forms of resistance, such as target-site resistance, to develop more quickly, he admitted. "But we don't know how likely or unlikely that is, although it is subject to ongoing research."

PBO is not yet approved in the UK for agricultural use, and Dr Moores believed it would be around three years before the first commercial products would be launched. Other synergists could also potentially be used, he added.