10 May 2002

More vigilance as warm spring gives bugs a head start

Aphid control is the topic

of our latest baseline

advice article, helping you

decide how best to manage

this insect pest threat

CEREAL growers havent experienced a bad aphid year since 1995. But the early spring and prolonged dry period in April will have given the pest a head start this year, warns ADAS entomologist Jon Oakley.

"With the conditions weve had, growers should expect an early flush." Predator ladybirds and parasitoids were also present, he reports, but the other natural enemies, hoverflies, had not arrived from the Mediterranean.

Spraying can pay in some years. "Aphids can cause 0.5t/ha yield loss. But in most areas, theres been no need to spray for the past six years."

Mr Oakley suggests checking when crops reach boot stage, or looking just before T2 spraying. "Use the chance to add an aphicide to an existing spray to save on any additional application costs if needed."

The threshold is half or more tillers with aphids. "Thats regardless of the number of aphids on each tiller. Treatment is based on the assumption that pest numbers will continue to rise."

Where an early spray is warranted, avoid a pyrethroid. "It will also kill the natural predators," warns Mr Oakley. "The best option is pirimicarb." Rates can be reduced to one third without loss of efficacy, he confirms.

Sugar beet

While beet plants remain small, aphids find it difficult to locate them, reports Dr Alan Dewar of IACR-Brooms Barn.

"They are flying, as temperatures have been above 15C," he points out. "But theres no point spraying plants at the cotyledon stage. Wait until they are targetable and aphid thresholds have been reached."

Gaucho-treated crops wont need any action yet, he advises. "The same is true for Temik- treated crops, although those sown at the beginning of March with a low rate of Temik should be checked. The persistence is unlikely to have run out, because it was dry after drilling. But its worth taking a look."

Untreated crops should be sprayed when one green wingless aphid per four plants is present. "Look for wingless aphids as they were born on the plants so have colonised. Winged aphids can and do move on."

Dr Dewar adds that spray choice is between pirimicarb or triazamate. "Of the two, triazamate is more persistent, but it also costs more."

Where leaf miners need controlling, which is likely in parts of Norfolk, different chemistry should be employed. "Neither pirimicarb nor triazamate will control leaf miners. So use a pirimicarb/pyrethroid mix, such as Evidence or Dovetail, and time it for when the eggs hatch."

Gaucho treated crops have up to ten weeks protection, which might be longer following the dry conditions in April. "Moisture and plant growth are needed for plant uptake and there was a shortage of both earlier on."

Potatoes

Monitoring for winged aphids in the potato crop should start from May onwards, advises BPC agronomist Rob Clayton.

"Look on the undersides of leaves and in the middle and lower parts of the potato plant," he says. "As with cereals, recent years havent been bad for aphids."

The main threat is resistance to insecticides, he warns. "Monitoring last year showed three different types of resistance in some peach-potato aphids. When found together, control with insecticides is very difficult."

Resistant peach-potato aphids were found at high levels in central and eastern Scotland last summer, he adds. "Levels in England were much lower."

Pirimicarb or pymetrozine are most effective against the peach-potato aphid and are least likely to harm beneficials. Mixtures of pyrethroids with pirimicarb may be better where other pests have to be controlled.

Dr Claytons advice is to spray where damaging levels of aphids are likely to be reached.

"The type of aphid is important. If you find one peach-potato aphid per plant early in the season, the potential for virus spread is enormous." &#42

Aphid alert – favourable conditions in early spring mean vigilance is vital.

1 Weather Mild winter and early spring have allowed an early flush of aphids.

2 Spraying Not needed in cereals since 1995, but can give 0.5t/ha yield response.

3 Natural control Predators arrive soon after aphids. Ladybirds and parasitoids already in fields, hoverflies still to arrive.

4 Threshold When half or more of tillers have aphids.

5 Cereal products Avoid pyrethroids with early spray, or natural predators will be killed. Use pirimicarb instead.

6 Rates Can be cut to one third.

7 Rainfall Heavy rain gives effective control.

8 Sugar beet Gaucho and Temik treated crops are protected. Spray untreated crops when one green wingless aphid per four plants.

9 Leaf miners Use pirimicarb/pyrethroid mix where leaf miners need controlling. Otherwise triazamate is best.

10 Potatoes Monitor crops from May onwards. Treatment decision depends on aphid identification.