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Resistance is futile

December 2009

Resistance is an ongoing problem in pest management. As soon as new chemicals are invented resistance can suddenly appear. The phenomenon occurs because many organisms share an evolutionary life history with the agent that is trying to kill them.

Pyrethroid insecticides for example are derived from Chrysanthemum flowers, which are a type of daisy. Resistance to some pyrethroids was seen only years after they were first introduced because the pest insects already had, stored in their genomes, some elements of the metabolism needed to thwart the new attack, such as enzymes to dismantle the toxic chemicals. It was only a matter of time before mutation and recombination would bring about resistant individuals.

Although there are strategies that can delay the onset of resistance, such as reduced pesticide use and rotating the chemicals used, there is also another way that the metabolic resistance mechanism could be circumvented entirely: time-delayed insecticide ‘crystals’.

Insects mutate and develop enzymes that block or degrade insecticides. But you can also target these enzymes by using enzyme inhibitors so that the original pesticide will work. The new technique therefore is a two-pronged attack: an inhibitor to stop the pests’ natural defences, known as a ‘synergist’, and a pesticide to finish the job.

Scanning electron micrograph (SEM) of crystalline cyclodextrin containing insecticide and the synergist piperonyl butoxide. Image: Rothamsted Research

Timing is critical. It takes 1-12 hours (depending on the insect species) for the synergist of choice, piperonyl butoxide (PBO, originally extracted from the sassafras tree which can now be made synthetically), to cross the insect cuticle and get to work disabling the pests’ defences. Therefore, to get the best results from this approach its optimal if the pesticide starts to work several hours after it is sprayed – just when the insect is at its weakest – and the microencapsulation technology does just that.

It has been tested in some of the world’s pesticide resistance hot-spots with striking success. In trials in Spain, 100% of whitefly larvae were killed by a chemical that they were previously resistant to.

“One major advantage is that this technique works on any pest that is resistant to insecticides,” says Graham Moores from Rothamsted Research (RRes, an institute of BBSRC). “Because the enzyme systems which block insecticides are the same in all insect pests.”

And because exposure to the synergist puts the insect into a ‘hypersensitive’ state, Moores says that less insecticide is used, which has concomitant advantages to the environment.

It’s a strategy that could revolutionise approaches to managing insect resistance and has been described as “… one of the biggest breakthroughs in pesticide application technology that has come along in a long time,” by Dr Robin Gunning from New South Wales Department of Primary Industries (NSWDPI) in Australia who along with Dr Graham Moores developed the new encapsulation technology.

The technology is now moving forward in agriculture following a deal between researchers at RRes, NSWDPI and the Italian chemical company Endura.