Aphid explosion imminent?

18 October 2001

Aphid explosion imminent?

By Tom Allen-Stevens

AGRONOMISTS are warning cereal growers that there could be an explosion in the aphid population, bringing early-drilled crops at risk.

Unprecedented numbers of insecticide-resistant peach potato aphids have already been found on oilseed rape crops in the Fife and Angus areas of Scotland.

Fears are now growing across the UK that grain and bird cherry aphids will infect early-drilled cereal crops with barley yellow dwarf virus (BYDV).

The rape aphids are believed to have spread from potato crops, where high insecticide use has built up resistance in the aphid population.

Ive never seen them in such numbers before, Scottish UAP agronomist Neil Ross told FWi on Thursday (18 October).

In one crop I inspected there was 70-80% coverage on the underside of the rape crop leaves, which were going yellow and dying.

Approved chemistry appeared to be doing little to help, and resistance to both carbamate (MACE) and pyrethroid (esterase) insecticides was soon confirmed.

The Pesticides Safety Directorate has now issued an off-label approval for nicoteen-based sprays on oilseed rape.

But across the rest of the UK, agronomists have warned of increased sightings of aphids in cereal crops.

IACR Rothamsted entomologist Richard Harrington confirmed that numbers of winged aphids last weekend were very high.

So far aphid numbers over all have been lower than in previous years, but multiplication and movement depends on the weather, he said.

The conditions over the next few days are ideal for numbers to increase.

The winged aphids bring BYDV into the field, he explained. Growers then have 10-14 days grace before populations can reach dangerous levels.

Crops drilled in mid September and treated with Secur (imidacloprid) should still be protected from aphids, says Dalgety agronomist Bob Bulmer.

He advises growers treating crops with pyrethroids to use emulsion in water formulations, such as Minuet (zeta-cypermethrin) to minimise risk of crop scorch.

High soil temperatures mean crops have put on lush, soft growth. The solvents in some pyrethroid sprays can be harsh, especially if mixed with herbicides.

BYDV symptoms are round yellow stunted patches in the field. Most common in the south west and coastal areas, really severe cases can cause 50-60% yield loss.


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