Seed weevil and pod midge follow pollen beetle into oilseed rape, warns entomologist

High numbers of pollen beetle have been found in a high proportion of the UK oilseed rape crop, prompting growers to spray. The worry is that seed weevil and pod midge will follow, putting yield under further pressure.

It was the high temperatures early on that brought large numbers of pollen beetle out from the hedgerows where they overwinter, explains ADAS entomologist Steve Ellis.

Seed weevils also migrate into crops from overwintering sites, so our advice to growers is to continue checking crops regularly during flowering and 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. But seed weevil 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 brassica 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 so it should be easy to differentiate between these two insects.

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. One weevil per two plants is the threshold to spray in northern Britain and one per plant elsewhere, advises Dr Ellis.

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

Growers planning to apply a product for seed weevil and pod midge need to  consider beneficial species, notes Makhteshim-Agans Stuart Hill.

As with all pyrethroids, Mavrik disrupts the nervous system in insect pests, such as seed weevil and pod midge, causing rapid paralysis and death.

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, as their formulation masks the repellent effect of the active ingredient.

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.

Chang, C.P. and Plapp, F.W., J.r (1983) Fluvalinate toxicity to the honey bee in relation to pyrethroid mode of action. 30th Annual Meeting of the Entomological Society of America, Nov 28 Dec 02, Detroit.