Washing away insecticide resistance

Over the last sixty years, the use of pesticides in agriculture has increased dramatically. World expenditure on agrochemicals is now US$ 25 billion with sales increasing in developing countries and yet the battle against insect damage to crops is far from won. The time and cost to develop new active ingredients is immense and agrochemical companies are struggling to keep one step ahead of the pests.

credit: Graham Moores

Meanwhile insecticide-resistance continues to rise and is a major problem in certain pests. In Australia, for example, Helicoverpa armigera (cotton bollworm) is a major pest of agricultural and horticultural crops. Widespread spraying with an array of chemicals has resulted in resistance to most insecticide groups, especially synthetic pyrethroids. "It's a huge problem, perhaps the largest problem in food production in the world today," says Graham Moores of Rothamsted Research in the UK, "but we have made a major breakthrough." That breakthrough has involved an important change of focus. Instead of developing a new pesticide, Moores and his colleagues have found a way of inhibiting the capacity of the insects to resist the current ones.

"Almost all insects have the ability to develop resistance to the chemicals used against them due to their very quick reproductive rates," explains Moores. Many plants contain chemicals that are toxic so the insects that eat them naturally have a defence - proteins or metabolic enzymes that breakdown the toxins. These proteins can also protect insects against pesticides. "In a large population there may well be one or two individuals that can resist the chemicals and survive. Very soon they will multiply and build up a resistant population."

Open sesame

In targeting the resistance mechanism, the Rothamsted researchers together with Dr Robin Gunning* from Australia, have examined how a natural extract of sesame oil, piperonyl butoxide, inhibits the production of the metabolic enzymes which are key to building insects' defence against insecticides. Examining the effect on cotton bollworm and other insects, it has been found that the sesame extract takes about five hours to become effective. Dousing insects with the extract a few hours before an insecticide is applied allows time for the enzymes to be fully inhibited resulting in greater efficacy of the pesticide. "It's amazingly exciting", enthuses Moores, "Pests that previously survived now die, and less resistant insects can be killed with lower doses or less potent chemicals."

Initial trials in the UK proved that sesame extract works well to combat insecticide resistance in pests of greenhouse crops such as tomatoes and cucumbers. In Australia and South Africa trials were successful in cotton against the B biotype of Bemesia tabaci (tobacco whitefly) - which is virtually uncontrollable with the use of conventional insecticides - as well as against Aphis gossypii (cotton aphid). "I think it's these economically important crops where we will see the first wave of use," predicts Moores although he agrees that the technology may also have applications for use with crops in developing countries. Most importantly, in commercial terms it is more cost-effective to improve the efficacy of existing chemical products than develop new ones.

One step ahead

Finding the key to unlocking insecticide-resistance has taken long enough. But could insects be equally successful in developing resistance to the sesame extract? Graham Moores hopes not. "This is the first time we've targeted the resistance mechanism. I am convinced that targeting the resistance mechanisms against present chemistries in agrochemicals, rather than searching for new modes of action, could be the most important advance in insect control for some time."

The regime of pre-spraying crops with sesame extract has worked well in laboratory and field trials, but commercial production of piperonyl butoxide has been developed to provide a encapsulated 'two hit' product. Developed by an Italian company, Endura SpA, the novel formulation works by providing a burst of the sesame extract soon after application followed several hours later by a second burst of the active pesticide. The delay between the release of each product can be altered to provide maximum efficacy depending on the pest to be targeted and the environmental conditions during time of application.

With future trials for piperonyl butoxide envisaged in other 'resistance hot-spots' around the world, could this new pest control product lead to an increase in demand for sesame? Possibly, especially in Japan and Australia where it is already a major crop. However, as piperonyl butoxide can be produced synthetically it seems more likely that industrial production of the sesame extract would follow hard on the heels of success in farmers' fields.

*Dr Robin Gunning - Department of Primary Industries, New South Wales, Australia

For further information email Graham Moores