28 February 1997

RESISTANT APHIDS &MOTHS CAUSE GROWERS A FLUTTER

Last year Silver Y moths and insecticide resistant aphids caused big headaches for many arable growers. Edward Long finds out what the risks are for the coming season

LAST August Lincolnshire potato growers found normal insecticides were useless against the peach potato aphid Myzus persicae. That was beginning of the double resistance problem, with organophosphorus, pyrethroid and carbamate sprays all proving ineffective.

Potatoes, sugar beet and leafy vegetable brassicas could all be wide open to attack in 1997.

For 30 years farmers have had to cope with a single type of resistance. Only in 1990 was a second type found overseas. In 1995 two individuals with the double-barrelled resistance were seen in the UK. But it was not until last year that the first reports of a lack of chemical control in the field emerged. With no control from existing materials farmers around Spalding resorted to nicotine to try to protect valuable crops.

"Aphids from the area had high levels of the old resistance, plus the new type which made them insensitive to carbamate, pyrethroid, and the new and yet-to-be-launched triazamate," says research entomologist Alan Devonshire of Rothamsted Experimental Station.

Although the new resistance mechanism is unlikely to have any effect on the performance of granular insecticides, those are declining in popularity. Last year 50% of the national crop was treated with the seed treatment imidacloprid (Gaucho) – British Sugar expects it to account for 65% of 1997 drillings.

Last seasons beet was unaffected by the late flare of aphids with the double resistance to sprays. But this years crops, particularly in East Anglia, could suffer. "This is a real possibility and is causing a lot of concern," says Suffolk-based entomologist Alan Dewar of Brooms Barn Experimental Station near Bury St Edmunds.

"If aphids with the new resistance survive the winter we could be starting the new season facing a serious situation. Beet growers are fortunate as Gaucho currently controls aphids with both types of resistance. But use of nicotine in other crops last year could hasten the selection of resistance, as Gaucho has a similar mode of action.

"If some aphids with the new resistance were to come through the frosty weather, and existing widely used insecticide sprays were useless, what could be done in 1997 to protect crops at risk, including potatoes, field brassicas and sugar beet drilled without the Gaucho protection?" Dr Dewar asks.

Larvae of the Silver Y moth wrought havoc in many crops, including vining peas, last season.

Work at the PGRO has developed pheromone traps (aove) capable of providing a warning of imminent Silver Y moth attack (below), explains Anthony Biddle.


RESISTANT APHIDS


&#8226 Resist all OP, pyrethroid and carbamate sprays.

&#8226 Granular products work.

&#8226 Gaucho effective now, but resistance possible.

&#8226 Risk depends on over-winter survival.