Identifying vegetable weevils – V is for ‘vegetable’

Most weevils that attack canola can be difficult to distinguish, although vegetable weevils are a little easier, partly due to the V shape mark on their back.

Adult vegetable weevils (Listroderes difficilis) have been found attacking a 4-5 leaf canola crop near Rupanyup in Victoria’s Wimmera. Cotyledons and true leaves were damaged or chewed off; leaves had the typical scalloping type of damage that is associated with weevils. Damage was patchy, being severe in some areas particularly those on sandy rises in the paddock. While they are night feeders, these weevils were caught out after being found in pitfall traps.

Distinguishing weevils

Weevils are a confusing group and can be difficult to distinguish. Previously, we put together a simple guide to help identify nine weevil groups through their hosts or appearance. Click here.

Vegetables weevils are one of the few in the weevil group where both adults and larvae will attack the aerial parts of the plant. The adult is about 8 mm in length and has the typical ‘snout’ seen on all weevils. They are dull brown-grey coloured beetles but have two short white angled ‘v-shaped’ markings near the middle of their back (just remember: V is for ‘vegetable’). These markings are somewhat characteristic of the adult vegetable weevil.

Larvae are yellow to green in colour, with a flattened grub-like body and a smallish brown head. The larvae can grow to 13 mm long.

Left: Adult vegetable weevil showing the light ‘v-shaped’ markings. Right: Vegetable weevil larva and feeding damage (Image credit: cesar)

Habits of vegetable weevils

Adult weevils aestivate over summer under debris or tree bark until autumn when they become active. Capeweed is a common weed host in autumn/winter. 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.

Managing vegetable weevils

May-July is a critical monitoring period for weevil larvae and adults. Checking seedling crops for the presence of larvae and adults is best achieved at night, when weevils typically feed (keep in mind that the vegetable weevil adult is flightless and may drop to the ground as you move through it). Alternatively, use pitfall traps that are small containers dug into the soil with 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 fallen insects, and a drop of detergent will also reduce the chance of escape.

For more information on vegetable weevils refer to PestNotes.

 

Field observations

George Hepburn – Agronomist, Tylers Hardware & Rural Supplies (Victorian Wimmera)