Debate over mite resistance rumbles on

3 July 1998

Debate over mite resistance rumbles on

DEBATE over the significance of insecticide resistance in grain store pests, especially mites, is growing.

Pest control consultant Mike Kelly of Berks-based Acheta acknowledges resistance to organophosphorous insecticides may present practical problems in future. But he claims it is not presently evident in farm stores.

Ken Wildey of the CSL maintains real OP control problems are already showing up. Surveys have shown 92% of mite populations are OP resistant. He predicts widespread failures of residual treatments by 2000.

"For mites, OP resistance is not the most likely reason for control failures," says Mr Kelly. The commonest reasons in the field are:

&#8226 Insufficient OP has been used for the tonnage or crop. Label dose is important. Rape requires twice as much a tonne as cereals.

lA surface OP dust treatment drives mites downwards if the grain bulk is damp enough (more than14.5%mc).

lOP dust has been patchily applied.

lLarger, more noticeable predator mites, naturally tolerant to OPs, have developed.

lLow air and grain temperatures. Mites breed down to about 4C (39F), but at this temperature the OP takes many days or weeks to be effective.

lThe insecticide is past its "best before" date. This can affect part-used bags and stocks bought the previous season and stored open or in poor conditions.

lGrain moisture content is very high, breaking down the OP before all mites are killed.

lMite migration from surrounding straw or hay bales, potatoes, damp debris on wall tops and ledges, etc.

Rough grain handling through conveying systems can give good mite control, notes Mr Kelly.

Dr Wildey believes more than 20 years of OP use has encouraged resistance to the point where failure to control mites will rise dramatically in the next few years. "We consider the problems are upon us now." All Mr Kellys points are valid, he acknowledges. "But added together they only increase the likelihood of resistance developing further."

Within one hour at Cereals 98 Dr Wildey says three growers approached him because surface-applied OP insecticide had failed to control their mites. "At a distance one cannot investigate all the reasons for failure. But from the discussions we had I am confident resistance was a key factor.

"We have field data from a number of sites which confirm that mite resistance to OPs is the major factor in treatment survival. We know the actual amount of pesticide present in which the mites were living, and the treatments involved had delivered the kind of dosage rates likely to be achieved by following label instructions."

In no way is CSLs data biased towards studying and reporting on mites from problem farms, he stresses. "The facts are that in totally random surveys when we have visited farm grain stores, central cereal stores and oilseed stores we collected all strains of mites we could find in the structure of any of the buildings.

"The total number of different strains of flour mite collected from 1987 to 1995 was 625. Of these 547 survived to breed up in the laboratory and all were tested for resistance. Overall the results showed 53% of strains were resistant, but the figure for the most recent years, 1992-95 was 91%.

"It is important, too to remember that the discriminating dose we use to confirm resistance is a massive 8ppm, twice the approved application. If the pests survive our tests they will survive field treatment.

"We can no longer rely, as we have in the past, on pesticides as our long stop to disinfest grain." &#42

A mighty headache in the making? Experts cannot agree on whether poor mite control in many UK grain stores stems from OP pesticide resistance or bad store practice (inset).

Practical failings like product application, poor store hygiene and damp grain are mainly to blame – Mike Kelly.

Official surveys of grain stores show up to 92% of mite populations are resistant to OP insecticides – Ken Wildey.

What do you think?

Do you believe insecticides no longer control mites in your grain store? If so CSL invites you to send details of your experience and, hopefully, some pest samples. Contact: Dr Ken Wildey, Central Science Laboratory, Sand Hutton, York YO41 1LZ. Phone 01904-462682. Fax 01904-462252. Email:

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