Vegetable weevil

Craig Hole (Landmark) has reported vegetable weevils (Listroderes difficilis) around Naracoorte; which is west of Edenhope, in the Wimmera district of Victoria. The weevils have been found attacking canola. The crops have been sprayed with alpha-cypermethrin and chlorpyrifos, which Craig says has significantly reduced weevil numbers.

Vegetable weevil larvae can be hard to find but are usually seen on the underside of leaves. They are yellow to green in colour, with a flattened slug like body and a smallish brown head. They grow up to 12 mm in length. Adults are approximately 10 mm long, dull brown-grey in colour with two short white angled ‘v-striped’ markings near the middle of their back. They have a very pronounced snout.

Vegetable weevil eggs are laid in autumn and develop into larvae in winter. Capeweed is one of their favoured host plants. The larvae live for a few months before pupating. Adults emerge from the pupae and feed until summer when they hibernate under debris or tree bark until the following autumn when they become active again.

Adult weevils usually move into the edges of paddocks soon after crop emergence and are commonly found damaging germinating canola. They usually feed at night. Adults and larger larvae chew sections out of the leaves of seedlings during autumn and winter. High numbers can eat plants down to the ground level, especially around the edges of paddocks and where broad-leaved weed populations are high. Small larvae are more frequently seen in winter and feed on the terminal growth.

A border spray at crop emergence will help control vegetable weevil before they move into paddocks. Controlling capeweed and marshmallow weeds early in the season may assist in reducing crop damage. Checking seedling crops for the presence of larvae and adults is best achieved at night. Cereals and grasses are least preferred hosts. Rotating crops with non-host plants, implementing a long period of ‘weed-free’ fallow prior to sowing and planting susceptible crops away from previously affected areas, may reduce the risk of invasion.