New application system
Cool and cover might be mite answer – CSL
MITES are a real threat to the quality of UK stored products. They damage cereal germs and hollow out oilseeds. Large numbers can trigger allergies and eating them may cause clinical symptoms.
"So its crucial to prevent mites from building up in store," says CSL entomologist David Armitage.
Under Intervention no live pests are permitted. But the same rules permit grain intake at 15% moisture. At that level mites can still develop, he says.
"The easy answer would seem to be pesticides. Unfortunately, the organophosphates approved for use on stored products, though generally effective against insects, are not particularly suitable for mites."
The predatory mite, Cheylerus eruditus, is unaffected by OP pesticides and many strains of the main species, the flour mite, Acarus siro, can survive twice the maximum approved dose.
"Physical control is always an option. But unfortunately some species can reproduce at 5C, so low temperature alone is not the solution.
"Their Achilles heel is humidity. They cannot usually reproduce below 65% relative humidity. So knowing the relationship between moisture content and rh in between grains is essential for control."
Continuous, hot-air dryers lower the moisture too quickly to allow mite breeding, but bulk drying systems using ambient air are risky, warns Dr Armitage.
"We know the pests will increase explosively until the drying front passes through. Their decline then depends on final moisture content. But we are unsure which drying strategies minimise mite proliferation."
Even when grain is dry, the UKs mild maritime climate presents another challenge. "Surface grain picks up atmospheric moisture over winter. Typically it rises to about 17% moisture for at least three months, falling to 12-13% if stored throughout the summer."
That winter surface moisture, which is high enough for mites to thrive, equates to 75-80% rh.
In the past, surface treatments of OP dusts and cooling have countered this. But resistance means OP alternatives are needed.
Diatomaceous earths are being explored but none are currently registered for UK use, so another approach may be to prevent normal moisture uptake.
HGCA research is testing semi-permeable or impermeable sheets to cover the grain and stop moisture absorption.
Sealed and airtight
The idea came from work on rapeseed which found that surface moisture contents in a sealed, airtight bin varied between only 7 and 9% (60 and 72% rh) in summer and winter. This compares with 5-12% (40-85% rh) in an open-topped bin, with knock-on effects on mite populations.
Trials with 25kg grain bulks using different materials have cut moisture uptake by 1%, reducing mite numbers 10-fold.
Farm observations confirmed the expected maximum moisture content of 17% in bins over winter. But the surface moisture in an adjacent floor store was much lower.
"Stopping surface moisture uptake may be a key in the war against mites. But the secret could be a strategy combining this with cooling. Further studies on the relationship between bulk moisture content and surface absorption may be rewarding, judging by the differences seen in the bin and floor stores."
Mite development
The Intervention moisture content of 15% is in equilibrium at about 70% relative humidity. At a post-harvest temperature of 25C that still permits some mite development.
Lowering the temperature to 5C, will lower the rh by about 8%. But as grain rewarms in spring, the rh rises again and mite infestations may re-occur. "A safer drying target would be 14-14.5% moisture (in equilibrium with 60-65% rh) at 25C," says Dr Armitage. *
New application system
FAST, safe, and accurate treatment of grain to meet the needs of buyers and assurers alike. Thats the claim for Perrys newly-developed insecticide application systems.
Such even application of insecticide to grain is essential, reckons company director David Perry.
"Low dosing could lead to increases in resistance to insecticide, whilst overdosing is a waste of money and could lead to grain rejection. Worst of all, growers may think they have safely treated, when insect numbers could be building to epidemic populations in untreated pockets," he says.
Liquid and dust applicators start at £750 and £530 plus VAT respectively for manually-set models. However, these require a constant, known flow of grain, which can prove a problem with modern dryers, he notes.
There are no such difficulties with the fully-automated liquid system, which includes a weigher, electronically controlled applicator, and computer recording system. However, at about £8,000, it is only likely to be justified on larger farms or commercial stores, he admits. Passing grain through a holding bin would smooth the flow to a manual model, Mr Perry concedes. *
Perrys new grain insecticide applicator can handle 100t/hr, says company director Nigel Perry.