14 July 1995

Important role for insecticides in overall strategy

Insecticides have an important role to play in

post-harvest storage – but only as part of an overall management system, according to ADAS

THERE is little point in spending the best part of a year growing a crop of cereals or oilseed rape and harvesting it, only to allow storage pests to eat into yield and quality once its under cover.

So says Cambs-based ADAS entomologist Mike Lole. "Safe storage of grains and oilseeds is rightly the concern of farmers, merchants and manufacturers of food and feedingstuffs alike. Yet over the years surveys consistently show 5-10% of farms have infested grain stores."

Pesticides can help prevent losses, he acknowledges. "But there are other equally important facets to be considered, including store design, hygiene and grain condition."

Cleaning in mind

New stores should always be designed with cleaning in mind, he advises. "This means avoiding leaving dead spaces where crop residues can accumulate and act as reservoirs of pests between storage periods."

Avoiding problems in older stores, especially conversions from other buildings, is more difficult. "But even in these cases access to dead spaces is a big benefit.

"Hygiene in grain stores is of paramount importance." Most of the pests are not found elsewhere, so a good clean-up between selling one harvest and bringing in the next can prevent carry over from year to year, explains Mr Lole. Layers of dust and dirt also reduce the effectiveness of insecticide sprays or smokes.

Treat with pesticide

"Once the store is clean – and theres a range of industrial machinery for the purpose – the fabric should be treated with pesticide. The most efficient way to do this is to spray it on.

"Smoke bombs look impressive and have their uses for treating otherwise inaccessible ducts, etc. But most of the active ingredient they dispense ends up on horizontal surfaces – the floor not the walls."

There is a reasonable choice of products. And despite the discovery of resistant strains, notably of saw-toothed beetle, in laboratory conditions, all should provide good control provided growers stick to the label advice, says Mr Lole.

Mike Kelly of the Central Science Laboratory echoes that view. "There is some concern about resistance – the potential is still there. But in the field the chemicals still work in grain stores, and it behoves everyone to use them according to the label."

Although commercial stores frequently apply such chemicals directly to grain, relatively few farms should need to do so, suggests Mr Lole. "It should be necessary only where there is a history of problems – usually where the structure of the building is less than ideal. Its wise to check with processors before going ahead with admixture."

Dusts have the advantage that they can be easily mixed into the top layers of grain to provide a barrier to fresh infestation, he explains. "Liquids are generally nicer to use." However, special metering units are required.

For normal spray treatments the choice depends largely on the store materials. "Choose a wettable powder for applying to very absorbent surfaces to minimise the amount of active ingredient taken up by the structure."

Ideally treatment is best done a couple of months before harvest to allow the insecticide time to work. "But it will, of course, be necessary to wait until the store is empty."

Once harvest begins it is worth remembering that cool, dry grain is least susceptible to pest attack. "For cereals the critical grain moisture content is 14%." At or below this mites do not survive. "For oilseeds the figure is lower – about 7%.

"Temperature, not moisture, is the more limiting factor for insects, and it is important to remove harvest heat as soon as possible by blowing with cool air. The target temperature is 12C (54F).