WHAT FAVOURS resistance to pesticides – high doses or low doses? Experts speaking in Glasgow are undecided.

According to Bayer CropScience herbicide resistance expert Hubert Menne dose reduction has brought increased resistance. He cites three instances where low dose rates increased resistance against weeds – two from laboratory experiments and one field example of “fop” resistance against blackgrass.

“Recommended rates are developed to provide high levels of weed control and minimise weed escapes that lead to resistance. Using full dose rates is a key strategy for minimising herbicide resistance.”

But University of Reading researcher Michael Shaw says that for fungicides and insecticides low doses do not increase selection pressure. “The argument goes that using a high dose will kill everything. But it is probably unrealistic in a field situation to impose a dose so high that you can.”

That is because spray deposits will never be applied evenly throughout a canopy. “You will always have over- and under-dosing, so there will always be places where resistant individuals can survive.”

In most situations those individuals will be selected for, because a greater proportion will survive compared with sensitive types, increasing the relative amount of resistance in the population, he explains.

“Reducing the dose will, in general, decrease selection [because a greater proportion of sensitive individuals survive] or leave it unchanged.”