Focused approach is best to combat aphids in spuds

Potato growers are being urged to look out for the virus-carrying pests and be prepared to control them as warmer weather stirs crop-damaging aphids into activity.

A surge in numbers of sycamore aphids caught in 16 nationwide suction traps during recent hot spells has heightened awareness, notes Richard Harrington of Rothamsted Research.

“They are quite big and green and get people excited, but they do not damage crops and have no role in spreading viruses.”

Overall aphid activity is about two weeks later than usual, so for crops planted in good time the threat from the key virus carriers – the potato and peach-potato species – should be slightly lower than in recent years, suggests Dr Harrington.

But already both species have been found in south-western water traps in the BPC’s monitoring network* for seed growers, notes CSL entomologist Phil Northing.

The traps, available free to BPC levy payers, can help determine the need to spray against them, but it is only a guide, warn specialists.

Catching high numbers of aphids that pose little or no risk to potatoes can trigger unnecessary spray warnings, points out the SAC’s Andy Evans.

“There’s really no substitute for getting in the crop and regularly inspecting for aphids yourself,” he says.

BPC agronomist Mark Prentice agrees.

“The traps are useful but we’ve never considered them a complete replacement for all other intelligence.”

The results are only part of a decision support system worked out over the years which includes key factors like the proximity of other crops and seed quality.

Mr Northing echoes this, pointing out the BPC network is for seed growers, does not provide warnings, and is only part of a larger risk assessment picture.

Even for seed crops Dr Evans advocates a “wait-and-see” policy based on a robust and fast acting insecticide programme in the early stages of crop growth, when plants are most susceptible to virus transmission.

This could involve a mix of Plenum (pymetrozine) plus Hallmark Zeon (lambda-cyhalothrin) to give a quick knockdown of virus carriers, provide repellent anti-feeding activity, and control of any over-wintered resistant aphids.

But Dr Harrington is cautious.

“Non-persistent viruses like PVY can be transmitted in just a few seconds by a whole raft of aphid species that do not feed on the crop but only probe the epidermis for the chemical cues that tell them they are in the right place.”

Losses from PVY in epidemic years can be up to £18m, notes Mr Prentice.

“Most ware crops will tolerate a limited aphid population,” says Dr Evans.

“But as soon as they start to multiply rapidly, that’s the most effective time to strike.

If you go too early then there’s time for aphids to re-colonise.

A more focused and tailored approach, ideally on a field-by-field basis, will achieve the most economic control strategy.”

Despite the limitations of water traps as the sole decision tool Mr Prentice is keen to see them deployed.

“We’ve seen many users invest in extra traps to improve accuracy and create local networks such as in the Yorkshire wolds.”

With new products on the market this season the Insecticide Resistance Action Group hopes to revise its management guidelines, notes ADAS’s Bill Parker.