14 May 1999

No panic after rise in aphids

MORE aphids are being caught in water traps at Brooms Barn than ever at this time of year, but sugar beet growers need not panic, says Alan Dewar.

The warm spell over the Bank Holiday is probably to blame for the record numbers. Peach potato aphid (Myzus persicae), largely responsible for spreading virus yellows, appeared three days earlier than predicted.

But most catches to date have been of the less infective potato aphid (Macrosiphum euphorbia).

Growers should check untreated fields, especially in the eastern region now, he advises. Already one Brooms Barn field at the four-leaf stage has exceeded the spray threshold of one wingless aphid every four plants.

"Do not use pyrethroid sprays. They are contact-acting and wont affect aphids on the undersides of leaves and they do not kill resistant aphids," he advises.

Effective insecticides include pirimicarb and triazamate, or a tank-mix of pirimicarb and a pyrethroid if leaf miners or flea beetles are present, he says. Evidence (pirimicarb + deltamethrin) and Dovetail (pirimicarb + lambda-cyhalothrin) are ready formulated options.

The extent to which the trapped aphids are carrying virus yellows will not be known for a week or so. But Dr Dewar suspects the increasing use of a particular seed treatment could be causing what he terms the "Gaucho effect". This could explain the unexpectedly low levels of virus yellows last season, he believes.

"Gaucho is so good at controlling aphids it might be affecting the epidemiology of the virus. I am beginning to think the effect exists. The question is how do you prove it?" &#42