Give predators a chance to make a useful contribution

1 August 1997

Give predators a chance to make a useful contribution

Encouraging beneficial insects in cereals can help boost yield. But is it practical? Louise Impey reports in the latest of our series bringing HGCA-funded researchers and growers face to face on farm

CEREAL growers could help themselves to an extra 0.3t/ha (2.5t/acre) by exploiting predators contribution to pest control, according to John Holland of the Game Conservancy Trust.

His two-year £35,000 study shows naturally occurring enemies of aphids and orange blossom midge do help with control. But most are susceptible to broad spectrum insecticides, so their effect is often disrupted in the field.

"There has been very little information available to farmers on the value of these beneficial insects, both in terms of yield and quality," says Dr Holland. The latest study, which adds to the LINK integrated farming systems project, set out to examine their impact and to try to quantify it.

This involved constructing polythene enclosures within cereal crops, both in integrated and conventional farming systems, so predator numbers could be reduced or eliminated. The effects of these cuts were then compared with those in adjacent areas.

"We found that reducing the number of predators by 80% gave a 31% increase in the number of aphids a tiller," says Dr Holland. "This is equivalent to an extra 130 aphid days, or 0.3t/ha of wheat."

The good news, he says, is that such predators occur all over the UK.

With orange blossom midge, reducing predators boosted the number of larvae an ear by an average of 1.5. And soil populations rose by as much as 180%.

"This shows how a mistimed or inappropriate insecticide treatment may well reduce the level of predation, allowing midge populations to increase in the soil."

Encouraging useful species such as beetles, spiders and hover flies is easy, maintains Dr Holland. Set-aside helps, providing it is managed correctly, as most beneficials need ground cover to thrive.

"There are lots of things that can be done," he says. "Conservation headlands and beetle banks are just two examples."

Minimum tillage and some weed cover also help. "Theyre not always practical. But low levels of annual meadow grass are good, providing both cover and seed as a food source, without being too competitive."

Avoiding broad spectrum insecticides, such as dimethoate, is also recommended.

Drilling date can be used to avoid aphid damage, though it has conflicting effects. Later sowings are less susceptible to autumn BYDV-carrying aphids, but more at risk from summer aphids.

"Field size is important," stresses Mr Holland. "The predators have to be able to get into the field in time to control aphids early on. Many only move 100-200m, so it may be necessary to create beetle banks every 400m."


&#8226 0.3t/ha extra yield.

&#8226 Aphid and blossom midge control.

&#8226 Many encouragement options.

&#8226 BYDV aphids not covered by study.

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