High numbers of cabbage seed weevil are being observed in some oilseed rape crops across the country.
While the pest itself tends to have a relatively small impact on yield, holes created by the seed weevil can then be used by the more harmful brassica pollen midge to lay its eggs.
Dr Sacha White, senior research entomologist at ADAS, says: “The larvae of the pod midge are much more damaging. There are a lot more of them being laid in the pod and they consume a lot more. The larvae weaken the pod wall, causing it to shatter.”
Pod midge can be difficult to identify and therefore target with sprays, which is why a seed weevil threshold is observed, Dr White adds.
“When growers are making that decision on whether to spray, it is worth considering that only contact pyrethroid insecticides are approved for seed weevil control, so timing is crucial. As is weighing up the cost of spraying against any damage that could happen by travelling in the crop at this time, because it is not an easy time to spray.
“It is also worth bearing in mind that pod midge is a really erratic pest, so it is not always a problem.”
Pod midge are weak fliers and cannot travel very far, meaning the pest is most likely to arise from last year’s oilseed rape fields, putting crops near to them most at risk, Dr White adds.
“It is probably worth paying particular attention to fields that are adjacent to last year’s OSR, especially where the farmer may have noticed some pod midge damage.
“If they have not got any previous OSR fields nearby then the risk from pod midge is going to be much lower.”
Southern Britain: More than one weevil per plant
Northern Britain: One weevil every other plant