Sitona Weevil

Sitona discoideus

Family:

Order: Coleoptera

Description:

Adult

Nocturnal and hides in the ground during the day.

Colour - Dull grey to dark grey brown. Three pale stripes behind the head.

Body - Slightly flattened. 3-5 mm long, 2 mm wide.

Wings - 2 Pairs. Front wings modified to form hard protective cases (elytra) for rear wings. When at rest they meet in a straight line down the centre of the back. Rear wings membranous and folded both lengthwise and across so they fit under their protective cases. When in flight the elytra are held at an angle and the rear wings beat rapidly. Rear wings may be reduced or absent in some species. Wing-span up to 200 mm. Both pairs attached to the hind body.

Mouthparts - Chewing. Pointing forward. Short and broad weevil snout.

Antennae - 7-11 segments.

Legs - Rear legs larger than front legs. Feet (Tarsi) have 3-5 segments with 1 or 2 claws on the end segment.

Head - Rigid. Large rounded compound eyes.

Thorax - Front segment associated with head to form distinct fore body. Front segment covered by a hard plate (pronotum). 2 rear segments fused and associated with the abdomen to form hind body.

Abdomen - 10 segments in male, 9 in female. Often only 8 can be counted by eye. Spiracles on segments 1-7 and often on 8 also.

Egg - Usually simple and ovoid.

Habits - Can't fly when cold. Adults feed in the morning and evening and hide in the ground at other times.

Larva -

Colour - white with darker brown head.

Body - No legs, C shaped. 5 mm long, 2 mm wide.

Mouthparts - Chewing. Transverse action.

Antennae - 1 segment.

Legs - Has no legs or 6 legs. 5 segments.

Head - Usually a hard capsule. No ridges on forehead.

Thorax - No functional spiracle on middle segment.

Abdomen - 8-11 segments. No prolegs. Spiracles on segments 1 to 8.

Habits - Live underground feeding on roots. Difficult to find.

Pupa -

Biology:

Life Cycle:

Eggs usually laid near base of plants in autumn. Larva hatch and feed underground on roots of pasture emerging at the break of the season. In spring they pupate to become the adult weevil form. The adults can fly long distances.

Habitats:

Origin and History:

Distribution:

Significance:

The larva and weevils attack medics and lucerne. The adult eats U shaped notches in the leaves and in severe cases completely defoliating the plant. Seedlings may be killed. The larva attacks roots below the ground causing patchy losses of plants. Affected plants become pale or yellow and unthrifty and may wilt and die.

Management and Control:

Insecticides alone are ineffective on the subterranean larva.

Control of adults is only economic in severe infestations.

Controlling adults to prevent eggs being laid and reducing larval damage has not been very effective. The migratory nature of the adults often overshadows the control achieved in the paddock. Sitona infestations appear cyclical and severe outbreaks usually die down after two or three years.

A small wasp that parasitises the eggs has been introduced from Europe to help control sitona weevil.

Related Species:

Garden weevil, Vegetable weevil, Sitona weevil.

Similar Species:

References:

CSIRO. The Insects of Australia. Melbourne University Press. (1991) p682, 970.

WADA. Insects and Allied Pests of Extensive Farming. Department of Agriculture - Western Australia Bulletin No. 4185. p96.

Acknowledgments:

Collated by HerbiGuide. Phone 08 98444064 or www.herbiguide.com.au for more information.