Vegetable weevils

Agronomist, Brendan Reinheimer (Elders), has observed vegetable weevils (Listroderes difficilis) in a number of canola crops around Ararat and Stawell, in the Wimmera district of Victoria. The numbers are variable within paddocks, with only a few crops where significant numbers of plants show clear signs of feeding damage. This includes half eaten leaves and some leaves completely removed from the stems. For the most part, paddocks have suffered minimal damage. Brendan says the weevils appear to have moved in from the edges of paddocks and pasture lines.

Darren Scott (SMS Rural Services) has observed feeding damage in a number of canola crops around Horsham, in the Wimmera district of Victoria. Darren says plants have scalloping along leaf edges and holes in the leaves. This resembles feeding by weevils, however none have been found. A number of the affected paddocks have been sown into stubble and have a recent history of minimum or no tillage. Damage resembling weevil feeding has also been reported by sales manager, Luke Zagame (North West Ag Services), in an emerging canola crop near Charlton, in the Northern Country district of Victoria.

Vegetables weevils are dull brown-grey coloured beetles and have two short white angled ‘v-shaped’ markings near the middle of their back. They are approximately 8 mm long and have a very pronounced snout. Larvae are yellow to green in colour, with a flattened grub like body and a smallish brown head. Adult weevils hibernate over summer under debris or tree bark until autumn when they become active. They may then move into the edges of paddocks soon after crop emergence and are commonly found damaging germinating canola. Cereals and grasses are occasionally attacked.

A border spray at crop emergence can help control vegetable weevils before they move into paddocks. Controlling capeweed and marshmallow weeds early in the season may also assist in reducing crop damage. Rotating crops with non-host plants and implementing a long period of ‘weed-free’ fallow prior to sowing may reduce the risk of invasion by weevils.

Checking seedling crops for the presence of larvae and adults is best achieved at night, when weevils typically feed. Alternatively, use pitfall traps that are small containers dug into the soil with their open-mouthed tops flush with the ground surface. Various containers (e.g. plastic cups, glass jars) can be used. A small amount of liquid in the bottom will help to contain any insects that fall into the trap. Traps should be left for at least 24 hours but preferably longer.