An Agworld user has recently reported finding vegetable weevils in canola near Goomalling. The canola ranged from cotyledon to first true leaf stage.
Four species of weevils typically damage canola: vegetable weevil, desiantha (spotted vegetable) weevil, small lucerne weevil and Fuller’s rose weevil.
Adult vegetable weevils are about 10mm long with two short white stripes at an angle on each side of its abdomen. They will attack canola and some broadleaf pasture plants, especially capeweed. They are often camouflaged amongst stubble and trash and tend to hide during the day.
Vegetable weevils can lay eggs on canola. The eggs hatch into the larval stage in early winter. Larvae feed on the plants on which they hatched. Larvae have a black head, are legless, cream coloured and can be confused with caterpillars.
Desiantha weevil adults are mottled grey-black in colour with grey flecks on the abdomen, and have the typical elongated weevil snout. They grow up to 7mm long and are flightless. Adults chew cotyledons, leaves and stems of canola plants, and may eat small plants down to ground level, while the larvae feed below ground on cereals.
The larvae of desiantha weevils are white legless creatures that grow to 6mm long with orange/brown heads. They remain under the soil where they feed on the germinating cereal seed or stem, they can be difficult to find if in low numbers.
Small lucerne weevil adults are about 5mm long, light grey in colour with a white stripe on each side. Adults will attack geminating lucerne, pasture legumes and canola. Small lucerne weevil will hide under stubble and may not come into contact with insecticides. After spraying, check for new damage in crops or for live beetles before seeding.
Adult Fuller’s rose weevils are about 8mm long, have an elongated body and are grey with a yellow stripe running across the side on the first two body segments and a lateral yellow stripe on each side of the abdomen.
Larvae of Fuller’s rose and small lucerne weevil, are difficult to distinguish between as they both have a white head, with black jaws, are legless and cream coloured. They are found underground, feeding on roots of plants. Small lucerne weevil larvae have been known to cause damage to lucerne.
Adult vegetable weevils often harbour in rock heaps and remnant bush areas and move into the adjoining canola crop from these refuges. The other three weevils have been known to cause damage across the paddock.
Weevils can be very hard to find as they hide and play dead with their legs tucked, which helps them blend in with their surroundings. Pitfall traps placed into the ground can be an effective way to find weevils.
If you are unsure of the type of weevil you have found in your canola crop use the PestFacts WA Reporter app to request an identification by our entomologists.
Diagnosing weevil damage
All adult weevils chew off parts of leaves and cotyledons, giving them a serrated appearance, and can eat plants down to ground level at high numbers.
Desiantha weevil larvae feed on the underground parts of cereal seedlings and can cause a reduction in plant growth, wilting and eventual death of plants.
Damage from vegetable weevil is usually restricted to paddock edges or where capeweed was in abundance the previous year.
Managing canola weevils
Weevils can be a frustrating pest as they can survive spray applications of insecticides at registered rates due to their behaviour of hiding during the day. For this reason, it can be more effective to spray in the evening when weevils are active.
Desiantha weevil numbers can be reduced by effective control of grass weeds in the previous season and of the green bridge following summer rainfall.
The only treatment for Desiantha weevil larvae in cereals is to sow with seed treated with chlorpyrifos.
For insecticide recommendations on managing vegetable weevils in canola, refer to DPIRD’s 2023 autumn winter insecticide spray guide. The vegetable weevil requires higher rates of insecticides than most pests of canola.
For more information on weevils visit DPIRD's Diagnosing weevils in canola and Diagnosing Desiantha weevil in cereals pages.
For more information contact Research scientist Svetlana Micic, Albany on +61 (0)8 9892 8591.
Article authors: Cindy Webster (DPIRD Narrogin), Svetlana Micic (DPIRD Albany) and Alan Lord (DPIRD South Perth).