26 February 1999


Years, like 1998, without

big aphid outbreaks must

not foster complacency,

warns a top entomologist

Andrew Blake reports

PROBLEMS caused by aphids becoming resistant to insecticides can arise quickly, says Stephen Foster of IACR-Rothamsted.

"You only have to look at the control failures on potatoes in Lincs in 1996. We must also recognise that new effective products such as imidacloprid are not panaceas. All insecticides are likely to be vulnerable to the evolution of resistance."

Indeed, low resistance to imidacloprid (Gaucho) already exists in the peach potato aphid, Myzus persicae, on the Continent. So far it has not been found in the UK.

"It does not noticeably affect the product applied at recommended field rates yet," says Dr Foster. "But it may be the first evolutionary step towards greater resistance which could eventually cause significant economic losses such as those suffered by growers in Lincs."

They were battling against the combination of two types of resistance, dubbed esterase and MACE (modified acetylcholine esterase), says Dr Foster.

Resistance is conferred through at least three distinct mechanisms (see panel). "Collectively they could render almost all approved UK insecticides useless," he says.

The limited armoury of approved active ingredients restricts opportunities to allocate chemicals between crops to reduce the likelihood of selecting for greater resistance, he notes.

"All the mechanisms are, therefore, under sustained and widespread selection pressure, especially in high aphid years such as 1996."

IACR-Rothamsted staff are working closely with manufacturers and advisers so that anti-resistance research results and advice can be communicated quickly to growers.

A key channel will be the Insecticide Resistance Action Group (IRAG), a group of specialists from public and private sectors. Vigilance and obtaining expert advice are the key practical steps this season, says Dr Foster. "The chances that Myzus persicae will have an early impact depend on the weather. Cold and wet in winter or early spring will cut the overall aphid population."

In the UK the presence of red rather than the more usual green M persicae, can indicate MACE resistance. "But this is not foolproof as non-MACE forms can also be red. So sightings should be backed up by a biochemical test to verify this form of resistance."

If MACE forms are absent, pirimicarb (Aphox) remains an appropriate insecticide choice. "It is relatively effective against M persicae with high esterase and KDR, while being benign to beneficials such as ladybirds and parasitic insects. The key to control is to avoid spray tactics that select rapidly for all mechanisms leaving no effective options by the end of the season." &#42


&#8226 Broad-spectrum metabolic form, known as esterase-based resistance. Mainly affects organophosphorous products but also confers some protection against carbamates and pyrethroids.

&#8226 Target site knock down resistance (KDR) to pyrethroids.

&#8226 Target site MACE resistance, specific to pirimicarb (Aphox) and triazamate (AZTEC). First seen in peach potato aphid in UK crops in 1996 concentrated in Lincs. Since then many samples collected and tested from sites around the country to assess spread (see figs). Appears to have declined in the region, but has become more widespread, especially in glasshouses.