Oilseed rape crops are under threat from high seed weevil and brassica pod midge populations, growers are being warned.

This latest pest warning comes after crops suffered serious damage from pollen beetle attacks in early spring, says ADAS entomologist Steve Ellis.

“High temperatures brought large numbers of pollen beetle out from the hedgerows where they overwinter,” Dr Ellis explains. “Seed weevils also migrate into crops from overwintering sites, so our advice to growers is to continue checking crops regularly during flowering, but treat only if thresholds are exceeded.”

The need to spray for pollen beetle ends when the crop flowers as the pest can easily get to pollen without damaging the plant, he says.

But seed weevils lay eggs in up to 50 pods. The larvae consume up to a quarter of the seed and leave the pod vulnerable to attack by pod midge.

The pod midge lay eggs in the holes left by seed weevil in developing pods, but can leave 20-30 larvae per pod, rather than just the one left by weevils.

The seed weevil is black or slate grey, has a matt finish and a snout, while pollen beetle is shiny and black.

“Seed weevils need to feed for a couple of weeks before laying eggs, after which treatment is useless, as larvae cannot be controlled inside the pod,” Dr Ellis says.

“One weevil per two plants is the threshold to spray in northern Britain and one per plant elsewhere.”

Pod midge are up to 1.5mm long, with long legs. Threshold to treat depends on seed weevil damage, Dr Ellis says.

Makhteshim-Agan’s Stuart Hill says Mavrik (tau-fluvalinate) is a popular choice of insecticide for use in flowering rape because it can be tank-mixed with fungicides.

“Most other pyrethroids cannot be tank-mixed, as their formulation masks the repellent effect of the active ingredient,” he explains.

“However, it is still an insecticide, so best agricultural practices should be followed when spraying crops in flower.”

British Beekeepers’ Association guidelines are:

• Aim to spray when bees are not actively foraging, such as in the evening or early morning and when temperatures are below 10C

• Take care to minimise drift to nearby flowering plants or hives in and around the treated field

• Check with beekeepers for locations of local hives

• Repeat this process annually as beekeepers may change locations of hives

• Give beekeepers at least 24 hours’ notice of spraying and provide the name and HSE number of product(s) being used

• If using with a triazole fungicide use only approved tank-mixes.