20 February 1998

Aphids and nematodes twin threat to UK yields

LIBERAL use of aphicides in potatoes is encouraging rapid development of resistant aphid strains which are increasingly difficult to control (Arable, Feb 13).

"Aphid resistance is outstripping aphid control technology," Martyn Cox, Hutchinsons technical development manager, told a potato growers meeting in Ipswich last week. "They are able to develop resistance quicker than we can develop new effective aphicides."

Widespread use of pyrethroid chemistry is to blame, Mr Cox believed. "They are cheap, and people are putting them on in multiple doses through the season."

Growers should only spray ware crops when aphids reached threshold levels of five insects a compound leaf, he advised. Aphox (pirimicarb) is the best choice, as it is less harmful to aphid predators.

"A lot of crops last year contained a few aphids. Levels stayed low throughout the season, so there was no need to spray," he said.

Patch treating nematodes using distribution maps produced by GPS technology is a false economy, according to Mr Cox.

"If you detect PCN in one part of the field, you have almost certainly got it across the whole of the field." To detect variation in populations accurately, 16 bags of soil each containing 50 individual samples would need to be taken from each hectare, he reckoned. "That would cost £150/ha just for the sampling."

No firm offering a GPS service comes anywhere near that figure. "They often take just one bag a hectare from a 20sq m area. That is not big compared to a whole hectare."

Taking the same number of samples from a bigger area does not help, he added. "In just 20m you can go from 10 to 110 eggs/g of soil. And even where sampling does not detect PCN, you can still have 10m eggs/ha."

Checking volunteer potatoes for cysts offers a better guide, Mr Cox believed. "You will pick up low levels of PCN sooner than by soil sampling." &#42

Nematodes and aphids deserve respect. Both can hammer yield if managed poorly, says Hutchinsons Martyn Cox.