Potato growers needing to guard against wireworm damage, a growing problem, should get best protection from overall granule treatment, Scottish Agronomy trials suggest.
Arable wireworm, which can render crops unacceptable for packing, has become increasingly troublesome in the absence of the persistent organochlorine pesticide aldrin, says the firm’s senior agronomist Eric Anderson. “Aldrin was withdrawn in 1989, but its effects were seen until about 2000.”
Dual-purpose cereal seed dressings are rarely used, and set-aside’s introduction, more winter cropping and increased use of irrigation have all raised the risk of damage from the pests, he says.
Some varieties, notably Marfona, Maris Peer and Rooster, are more prone to attack, though the first two tend to be lifted before the wireworms become active in late summer. “Maris Piper and King Edward are the least susceptible,” says Mr Anderson.
Deciding whether to treat against wireworms is largely down to experience and field history, he says. “Bait trapping isn’t reliable because of the very low thresholds that can cause damage.”
Pheromone traps need to be in place between May and August in the year before planting, which can be tricky with so much of the crop grown on rented land, he adds.
Two years of SA trials on sites with high wireworm pressure have confirmed the value of overall treatment with Mocap (ethroprophos) incorporated at 60kg/ha compared with an in-furrow application of Nemathorin (fosthiazate) at 150g/100m and to 15kg/ha broadcast overall.
The damage scores were: Untreated – 52%, Nemathorin – 13%, Mocap – 2%. “The approved 15kg/ha rate for Nemathorin just isn’t enough,” says Mr Anderson. “For good wireworm control it needs to be nearer the 30kg/ha used for PCN.” That makes it nearly 50% more expensive than the Mocap treatment, he notes.
“When you start to find wireworm damage in a crop it needs to be burnt off and lifted as soon as possible, particularly with severe infestations.”
Nematicides reduce pressure on crops, but do not eliminate it, he warns. “Mocap is much more effective at restricting the onset and escalation of damage while skins are setting, so the chance of keeping the crop within market tolerance is increased. Granules applied in-furrow potentially leave daughter tubers exposed,” he adds.