5 June 1998

OP-resistant mites mean

physical control is vital

By Andrew Blake

DONT rely on insecticides to provide control of mites in stored grain.

After two decades of exposure to organophosphorus products the pests have become so resistant that, until alternatives become available, physical control must be the main defence, advises Ken Wildey of the Central Science Laboratory.

He echoes reports of high mite numbers in stores (Arable May 15). "We too have recognised the serious problems associated with mite infestations over the past six months or so."

Currently approved insecticides are not the solution, he maintains. In the absence of other options, besides phosphine fumigation, growers should concentrate more on achieving the store conditions mites dislike – cool and dry.

"Mite resistance to OP insecticides is now at such a high level the majority of UK stored product mite strains are uncontrollable, even at the maximum approved application rates," he says.

CSL has found 71% of mite populations from commercial cereal stores survive 14 days of continuous exposure to twice the maximum permitted dose of Actellic (pirimiphos-methyl). In oilseed rape stores the equivalent figure is 93%, he notes.

"This backs up our field observations of storage mites living and breeding on the surface of grain bulks heavily treated with insecticide. In these cases the mites are effectively OP-proof."

Sorbent dusts, already used to control cockroaches in the food industry, are potentially promising OP alternatives, says Dr Wildey. "They are very effective and long lasting. They absorb the mites waxy coating and dehydrate them. The difficulty is that although they are not pesticides in the traditional sense they would still have to go through the approvals procedure."

Whether manufacturers perceive agricultural storage as a big enough market to justify that commercially remains to be seen, he suggests. A new EU Biocide Directive could ease their passage, but will not be in force until 1999.

A Rentokil spokesman confirms the firm has such a product for use against public health pests and is interested in other applications where it could be effective, including agriculture.

Surveys show too many growers fail to appreciate mite-safe moisture contents for storage, maintains Dr Wildey. "A surprising number tell us they still intend to store oilseed rape at 10% moisture. Even 9% is too dodgy. Two years ago some even planned to use the same levels as they do for cereals – 12-15%."

To be sure of physical mite control rape should be reduced to 8%, maintains Dr Wildey. "For grain the correct figure is no more than 15%. The critical area is the surface layer which tends to reabsorb moisture after drying."

Signs that rape varieties differ in their requirements highlights the need to ensure moisture meters are accurately calibrated, he adds. "Not all varieties are the same, though we dont have enough information yet to be specific."

Keeping store temperatures down with appropriate ventilation also discourages mites, advises Dr Wildey. "The target should be as low as possible. Growers should be able to get it down to 5-10C within three to four months. Dont stop blowing too early. If you have the correct equipment it may cost only cost 10p/t."

TRADE ISSUES

UK grain stores are no more infested with mites than those in any other country, says Alan Almond of British Cereal Exports. "The UK has a good reputation for pest-free grain." To protect that position the HGCA is already using funds from the grower levy to back work on mites, he notes.

MANAGING MITES

&#8226 Dont bank on insecticides.

&#8226 Dry & cool grain correctly.

&#8226 Beware of surface moisture.

&#8226 New products in the offing?

OP MAKERS RESPOND

&#8226 Malcolm Smith for Actellic supplier Zeneca acknowledges some mite populations are resistant to it. "There is no denying resistance is out there." In those cases the only chemical alternative is phosphine fumigation, he says. Switching to another OP insecticide would offer no benefit, he adds. "They are all from the same group."

&#8226 Robin Mumby for Nickerson Seeds, supplier of OP insecticide Satisfar (etrimfos), says the firm has seen no evidence of resistance developing to the product, though he acknowledges the possibility. The active ingredient is different to that in Actellic, he points out. The two cases of complaint following sales of up to 10t of dust before Christmas this season turned out to involve predatory mite species which have never been controlled by either chemical, he says.

TRADE ISSUES

UK grain stores are no more infested with mites than those in any other country, says Alan Almond of British Cereal Exports. "The UK has a good reputation for pest-free grain." To protect that position the HGCA is already using funds from the grower levy to back work on mites, he notes.

Most stored product mites are now resistant to OPinsecticides, warns CSLgrain expert, Ken Wildey. Growers need to make greater use of low temperatures and moisture contents to combat the pests.

MANAGING MITES

&#8226 Dont bank on insecticides.

&#8226 Dry & cool grain correctly.

&#8226 Beware of surface moisture.

&#8226 New products in the offing?

OP MAKERS RESPOND

&#8226 Malcolm Smith for Actellic supplier Zeneca acknowledges some mite populations are resistant to it. "There is no denying resistance is out there." In those cases the only chemical alternative is phosphine fumigation, he says. Switching to another OP insecticide would offer no benefit, he adds. "They are all from the same group."

&#8226 Robin Mumby for Nickerson Seeds, supplier of OP insecticide Satisfar (etrimfos), says the firm has seen no evidence of resistance developing to the product, though he acknowledges the possibility. The active ingredient is different to that in Actellic, he points out. The two cases of complaint following sales of up to 10t of dust before Christmas this season turned out to involve predatory mite species which have never been controlled by either chemical, he says.