Pea weevil

Scientific name

Bruchus pisorum

Description

In spite of its name, the pea weevil is not a true weevil but belongs to another group of plant-feeding beetles. The adult is about 5 mm long and has a brown body flecked with white, black and grey patches. The wing covers are shorter than the abdomen. The egg is orange, cigar-shaped and 1.5 mm long. The larva is legless and white with a small brown head.

Similar species

The pea weevil is unlikely to be confused with any other pest of peas.

Distribution

A native of west Asia, it is found in most of the world's field pea-growing areas, including Australia. It is not found in Tasmania or some regions of New South Wales.

Crops attacked

Field peas.

Life cycle

Life cycle on field peas: The pea weevil has one generation each year. Adults become active in early spring when temperatures reach around 20°C and move into pea crops from their hibernation sites (under tree bark, posts, along fence lines, around sheds, bins, etc.). Dispersing adults characteristically invade from the edge of the field, and often do not move more than 50 m into the crop. Females must feed on pea pollen to mature their eggs, which usually takes several weeks. When mature, females lay eggs on the outside of green pea pods.

The orange eggs are easily seen by the naked eye and are glued to the pod tissue. When hatched about two weeks after laying, the larva channels through the tissue directly below the egg and does not come into contact with the exposed surface of the pod. More than one egg laid on the surface of a pod usually results in an equivalent number of damaged seeds. The minute larva enters a seed and feeds for 5-7 weeks, in which time it grows to occupy much of the seed. When it is nearly fully grown, the larva chews a circular exit hole, 3 mm in diameter, leaving only a translucent skin above the hole. By this time, the seed has dried and may be harvested during larval development.

The larva pupates in the seed (pupation usually occurs after harvest). The pupa develops into the adult, usually during the time when the grain is put into storage. Adults emerge over several months starting in mid-December. Jolting of the seed during handling may induce a number of adults to emerge. Adult beetles fly to hibernation sites, which may include sheds, silos and grain bins, under the bark of trees, particularly pines, and in cracks and crevices in posts. They remain in hibernation during summer, autumn and winter. They resume activity when spring temperatures reach about 20°C and fly to the edge of the nearest pea crop.

Damage

A circular hole of 3 mm diameter in pea seeds. Heavily infested crop may suffer significant weight loss from feeding by pea weevil.

Risk period: From when pods first appear in spring to drying of the pods in early summer.

Monitoring and action level

Use a sweep net to detect when and how many adult beetles occur in the crop. Start in early spring during flowering, but before first pods have formed. Sweep every 5-7 days around the crop edge, about 2 m into the crop. Take 10 sweeps at a time and count the number of pea weevils caught. Sweep at five to 10 sites along the crop edge, particularly where trees and other hibernation sites are located.

Action level: Spray if an average of one or more adults are collected per 10 sweeps.

Control methods

Chemical control: A spray to kill beetles should be applied before they lay eggs on pods. The spray should be applied if the action level is exceeded and, if so, when the first pods appear in the crop. A spray applied to the border (40-50 m) of the crop, or as indicated by monitoring, is usually sufficient to prevent damage. For current chemical control options see Pest Genie or APVMA.

Cultural control: Early harvest of the pea crop when the insect is still in the larval stage removes it from the field before adults emerge. If the crop is left in the field until mid-December, adults start to emerge and will infest the following season´s crop. Sheep in the pea field following harvest will eat any fallen seed and further reduce the carry-over of pea weevil.

Natural enemies: No effective natural enemies have been recorded in Australia.

Post-harvest fumigation: Infested seed may lose up to 70% of weight after harvest as larvae continue to feed. To prevent this - to meet quality standards for sale and to reduce carry-over into the following season - fumigation of the seed prior to storage may be necessary. However, planting clean seed and pre-harvest control in the field are more cost-effective than postharvest control.

Maintenance of area-freedom (Tasmania and parts of New South Wales): Ensure all purchased pea seed is phosphine-fumigated prior to delivery. The fumigation must occur under gas-proof sheeting for 10 days. If possible, avoid purchasing seed from a pea weevil-infested district.

Further information

  • Crop Insects: The Ute Guide Northern Grain Belt
  • Pests of Field Crops and Pastures: Identification and Control. Editor: P.T. Bailey

Last updated 12 July 2010