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Cabbage stem weevil

1. Cabbage stem weevil. Note the distinguishing white rectangular mark just behind the head on the wing case.
2. Cabbage stem weevil adult ©Blackthorn Arable Ltd.

Identification

The adults are grey and up to 3 mm long with a typical weevil long curved proboscis. The larvae are white, legless, up to 5 mm with a brown head and are typically 'C' shaped.

Symptoms

The adults weevil usually lays a single egg in a seed pod and it has been estimated that an adult cabbage seed weevil can lay as many as 50 eggs in a season. After hatching the larvae feed on the developing seeds. The damage done by the developing larvae is relatively minor but the secondary damage caused by pod midges which lay their eggs via the puncture holes is more economically damaging.

Life-cycle

Seed weevils invade oilseed rape crops as temperatures increase during May. After about 3 weeks of feeding the females begin to lay eggs in the pods and this continues until the seeds are formed. When fully fed the larvae leave the pods to pupate in the soil with the adults emerging later in the summer. The cabbage seed weevil has 1 generation per year.

Importance

Cabbage seed weevils are widespread throughout the UK although they are generally considered to be less abundant than in previous years. Adult feeding on the young flowers and pods has little impact on yield, so treatment is not necessary during migration into crops. Larvae feeding in the pods can damage up to a quarter of the developing seeds and where a high percentage of pods have been affected this can equate to an overall yield loss of 5-10 %, however additional yield losses may result from brassica pod midge which can exploit feeding damage and egg laying scars to deposit their eggs.

Threshold

ADAS thresholds are 0.5 weevils/plant in northern Britain and 1 weevil/plant elsewhere. Decis can be applied at any time during the flowering period if cabbage seed weevil numbers are at threshold levels, but best results will be obtained from applications made at the end of flowering on the main raceme (GS 49), usually 75 % petal fall. Later applications may not prove effective as Decis is primarily a contact insecticide.

 

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