5 April 2002

United front can fight increase in resistance

By Andrew Blake

CONTROLLING aphids is set to get trickier, with strains resistant to key insecticides occurring in oilseed rape in Scotland and brassicas in Norfolk and the midlands.

Experts now say control of peach-potato aphid (M persicae) with carbamate chemistry is no longer totally reliable.

With few alternative pesticides available agronomy company UAP is keen to see arable and vegetable crop producers co-operating to minimise damage.

Resistant Modified AcetylCholinEsterase (MACE) aphids increased last autumn in a number of brassica crops. "We first alerted the industry to these risks in the summer," says the firms vegetable specialist Chris Wallwork.

Since then, he has been working closely with UAP arable colleagues to ensure they are aware of the problem.

Perth-based Neil Ross says resistant peach-potato aphids were present in unprecedented numbers in oilseed rape north of the border last autumn.

In the worst affected crops, 70-80% of the undersides of leaves were covered by the pests, leading to complete crop-kill in some fields in Fife and Angus.

"We are going to have to be increasingly vigilant to keep on top of the problem," he says. "An off-label approval for Nicosoap will help, and Chinook seed treatment could be another option."

In Norfolk, David Holgate found poor control in sprouts treated three times at monthly intervals with full dose Aztec (triazamate).

"We were surprised, as the crop had been sprayed using a drop-leg sprayer and coverage was excellent," he says. "But tests by Rothamsted proved MACE resistance, and if it had not been for frosts in December, the crop could have been wiped out."

Temperatures as low as -10C for several days at the turn of the year "really hammered" MACE aphids, which are thought to be more vulnerable to cold than non-resistant types, says IACR Roth-amsted entomologist Steve Foster.

"We have also had a lot of rain since the frosts which they do not like. So the signs for their survival this season are not particularly good."

But some aphids always find their way into the shelter of greenhouses, he says. "They also get into the warmth of beet clamps, and remember that there were a lot more of those about earlier this year."

A key question for seed potato growers in Scotland is how many of the resistant types have survived within stores. "The overall message is do not panic, but do be aware of the potential problem."

New guidelines (see box), specifically for potato growers, will soon supplement those from the "cross-industry" Insect Resistance Action Group, he says.

UAP technical director Chris Bean sits on IRAG and urges growers and advisers to adopt its anti-resistance guidelines. "We should do all we can to avoid relying upon carbamate insecticides such as pirimicarb and triazamate."

But other types of resistance already exist, for example to pyrethroid insecticides, so options are limited.

The only new aphicides unaffected by established resistance mechanisms are imidacloprid, as in Gaucho sugar beet seed treatment, and through Specific Off Label Approval on lettuce and brassicas, and pymetrozine in Plenum for potatoes and via SOLA on lettuce. But overuse of either could put them at risk too, he says.

Off-label approval for nicotine-based sprays on oilseed rape and vegetable crops is useful, but rarely gives 100% control.

Myzus persicae on the undersides of leaves or under the outer leaves in sprouts are hard to hit, so using appropriate spray volumes, nozzles and adjuvants is particularly important in these crops.

Strategies to minimise resistance should concentrate on keeping crops prone to virus infection away from sources of infestation such as potato and beet clamps. "We should also do what we can to encourage beneficial insects and look after them in the crop," says Mr Bean.

"During the season, growers and their agronomists should inspect crops for both pest and beneficial species and mix and match products to get the best control on carefully identified populations." &#42

IRAG GUIDELINES

&#8226 Minimise insecticide use

&#8226 Do not spray unless aphids are at damaging levels.

&#8226 Take account of numbers of beneficial insects.

&#8226 Glasshouse crops are at particular risk. Minimise weed hosts and crop residues. Use parasitoids or predators.

&#8226 Isolate virus-prone crops, such as seed potatoes and sugar beet, from potential sources of virus or aphids and heavily sprayed horticultural crops.

&#8226 Rationalise insecticide use

&#8226 Know your aphids. Use non-carbamate based insecticides on easier-to-kill aphids.

&#8226 Choose products less likely to affect beneficial parasitoids, ladybirds or lacewings.

&#8226 Use persistent systemic soil insecticides against early aphid attack.

&#8226 Stick rigidly to label recommendations to ensure effective coverage and lower risk of poor control.

&#8226 Ring the changes. Alternate or switch pesticide groups wherever possible.

&#8226 Avoid insecticide mixtures. If tank-mixes are deemed necessary, select compounds from different activity groups.

&#8226 Monitor the efficacy of each treatment. If one spray is not working consider another from a different group.

&#8226 Seek specialist advice

&#8226 Visit www.pesticides.gov.uk/committees/resistance/irag.htm

IRAG RECOMMENDATIONS FOR POTATO GROWERS

&#8226 On ware crops only apply aphicides when aphids reach thresholds and build-up is likely to continue. In many seasons only one spray will be needed, if at all.

&#8226 Pirimicarb (Aphox) or pymetrozine (Plenum) are likely to prove most effective against M persicae. Do not reduce rates to increase the number of applications.

&#8226 These selective insecticides should be alternated at the beginning of the season as they are least likely to harm beneficial insects.

&#8226 Be vigilant for resistance, but be aware that poor control can sometimes be due to poor spray coverage.

&#8226 Monitor chitting sheds for the onset of aphids. Deal with dumps before shoots develop.

&#8226 Do not repeat applications of an insecticide if it appears not to work at full rate and has been applied correctly. Use an alternative.

&#8226 Avoid below label rates.

&#8226 Insecticide mixtures are unlikely to delay development of resistance, but mixes of pyrethroids with pirimicarb may be justified to control the spread of virus or more than one pest on the same crop. Where tank mixes are used for this purpose choose components from different classes and apply at full recommended rates.

&#8226 Advice backed by IRAG, BPC, SAPPIO and DEFRA-PSD.

MACE RESISTANT APHIDS

&#8226 From Midlands to Scotland.

&#8226 Unprecedented numbers last autumn.

&#8226 Frosts helpful but threat remains.

&#8226 Stick to IRAG guidelines.