Sitona weevils

Agronomist, Mick Duncan (Northern Agriculture), has observed chewing damage to a seedling lucerne crop near Tamworth, in the North West Slopes and Plains district of New South Wales. The majority of damage is confined to areas of the paddock along fence lines. Mick reports the damage consists of scallop-shaped holes along the edges of leaves, as well as entire seedlings chewed off at the ground. Upon closer inspection, Mick has identified sitona weevil (Sitona discoideus) under clods of dirt in the affected area. Agronomist, John Mudford (AGnVET Services), has also reported sitona weevils in an established lucerne paddock near Coonabarabran, in the North West Slopes and Plains district of New South Wales. Quite high numbers of weevils can be found directly on the plants, and feeding damage has been observed on the stems.

Sitona weevils attack lucerne, annual medics and subterranean clover. Adults are 3-5 mm long and greyish-brown in colour with three characteristic pale stripes on the thorax. They have a short, broad snout and are capable flyers. Larvae are white and legless, with an orange-brown head capsule and grow up to 5 mm long.

Larvae live in the soil and mostly feed through winter and spring, attacking root hairs and root nodules, which can slow plant growth and reduce the plants’ ability to fix nitrogen. Adults are active from late spring until autumn. They can fly considerable distances, so infestations can start without a previous history of damage. Heavy infestations can lead to complete defoliation of plants.

It is best to search for adult sitona weevils at night by inspecting plants and looking directly on the soil surface. Pitfall traps are also an effective technique to assess the abundance of weevils.

There are several insecticides registered for control of sitona weevils, however there are no well-established economic thresholds. Adults should only be controlled if there are large numbers causing extensive leaf loss. Larvae are protected in the soil and cannot be effectively controlled with chemical sprays. There are numerous beneficial insects that will attack sitona weevils, including the sitona weevil parasitoid wasp (Microctonus aethiopoides), which causes death of adult weevils and can reduce fecundity of females.