sustainability through science & innovation

Small lucerne weevil

Small lucerne weevils (Atrichonotus taeniatulus) have been identified for agronomist, Roger Garnsey (Roger Garnsey Agronomy), from an established lucerne paddock near Murrumbateman, in the Southern Tablelands district of New South Wales. Roger reports that the lucerne stand appears unthrifty, with some plants completely killed. Upon digging in the affected areas, Roger has found a number of weevil larvae around the roots of lucerne plants, which show clear signs of feeding damage including stripping along the outside layer of the roots. A low number of adult weevils has also been observed within the crown of plants, and there is a small amount of chewing damage to leaves (characterised by scalloping along the leaf edges) in some parts of the paddock.

The small lucerne weevil is known to attack lucerne, pasture legumes, canola and some weeds. In New South Wales they are regarded as a minor, irregular pest. Large numbers of adults can cause serious damage to subterranean clover pastures by chewing off cotyledons soon after germination. However, they are most damaging to lucerne, where adults feeding on leaves can defoliate plants. Serious damage also occurs when larvae burrow into or chew furrows in the taproot. Eventually plants die, resulting in lucerne stands with dead patches that increase in size each year as the infestation spreads. Wilting and plant death is most noticeable in summer when larvae are fully grown.

Adult small lucerne weevils are grey in colour with some brownish mottling, and are up to 10 mm long. During the day they typically hide in the soil around the base of plants, though some may be found resting together in groups of 3-4 on a single leaf. The larvae are creamy white, legless grubs, up to 8 mm long, with small, pointed, brown jaws.

Infestations of small lucerne weevils spread slowly because they do not fly; they have to walk or be carried to spread through a paddock. Young larvae become active in late winter and can feed on plant roots until mid-January. Adults emerge from the soil from mid-February to March. Eggs are laid at the base of plants, and after hatching in winter, larvae burrow into the soil and begin feeding on roots.  

Insecticide control options are largely ineffective at this time of year. The best time to control small lucerne weevils is in late summer-early autumn when the next generation of adults are emerging. Timing is critical to prevent egg laying. Monitoring crops throughout February and March is the only way to find the first signs of weevil emergence. Rotations to break the weevil lifecycle are effective if all lucerne and tap-rooted plants are killed. Break-crops such as cereals should be considered. A break of at least 12 months is needed. Care must be taken to avoid moving adult weevils in hay, farm machinery and vehicles from infested areas to clean lucerne paddocks. It only takes a single female to start a new infestation.

PestFacts is supported by