Orange Wheat Blossom midge too unpredictable for pyrethroid

UK - April 05, 2006

You can’t tell an Orange Wheat Blossom midge to fit in with the best timing for your insecticide, so arm yourself with all the facts and choose your control method carefully. 

“Controlling Orange Wheat Blossom midge is all about compromise and flexibility,” says Dow AgroSciences’ principal biologist Andy Leader.  “Spray timing, spray choice and effect on beneficials must all be taken into account.  But the greatest variable is the midge itself.  Farmers need to make a single, well-targeted application of an insecticide that delivers maximum yield benefit with minimal long-term effect on beneficial arthropods.” 

“Pyrethroids may provide ‘acceptable’ control of adult midges present at application.  However, acceptable control is rarely enough.  The objective of treatment should be ‘maximum’ control of as many adults as possible.  In the past few years we’ve seen protracted emergence due to drier weather, or a few colder nights.  A pyrethroid would have left a significant percentage uncontrolled whereas the residual action of Dursban WG (chlorpyrifos) would have achieved greater levels of control,” says Mr Leader. 

Grower-scale trials over the past two years consistently show Dursban WG provides the best reduction of grains damaged by Orange Wheat Blossom midge, as chlorpyrifos controls both adults and eggs laid on exposed parts of the wheat floret.   

Temperature affects performance of most insecticides including pyrethroids and chlorpyrifos.  Experience has shown that at 20ºC chlorpyrifos gives effective control for four days and up to ten days when temperatures average 15ºC. Whereas a pyrethroid is likely to give two days control at most, even under the most favourable conditions. 

Mr Leader challenges those who believe that chlorpyrifos may cause excessive damage to beneficial insects.  “Growers will only make an application if the crop is going to suffer an economic loss.  So why not use the best product for the job?  All insecticides kill insects.  But the effect of the insecticide is as much down to timing as active ingredient.  Any wrongly-applied insecticide will do more harm than good by killing beneficial insects.”   

Professor Wilf Powell of Rothamsted Research assessed the effects on beneficial insects when pyrethroids, chlorpyrifos and pirimicarb were applied to wheat in June.   

“With an application in early June, money spiders and ladybirds were at high risk from pyrethroids, but only medium risk from chlorpyrifos.  Parasitic wasps, key predators of Orange Wheat Blossom midge are generally at high risk from both,” explains Mr Leader. 

The study showed that the June timing was less severe against species active on the soil surface, or in the lower crop canopy, such as carabid beetles.  Bees are generally at lower risk in a cereal crop especially if headlands are left unsprayed.  The risks can be minimised further by making one application in response to damage thresholds and at the optimal timing. To help decision-making Dow AgroSciences and HGCA have developed assessment guidelines available in HGCA’s leaflet – ‘Orange Wheat Blossom midge – assessment and control’. 

Recovery of non-target invertebrates depends on mobility and the proximity of non-crop reservoirs such as field margins.  These play a crucial role in maintaining populations of beneficials.  Dow has worked with FWAG to develop a voluntary recommendation to leave a 12m unsprayed headland when Dursban WG is applied in summer against OWBM, allowing non-target insects to rapidly recolonise from the unsprayed area.