Redshouldered leaf beetle (Monolepta australis)

  • Redshouldered leaf beetle adult showing red-purple patches on shoulders
    Redshouldered leaf beetle showing red-purple patches on the shoulders and ends of the wings. Photo: J Wessels
  • Soybean leaf which has been chewed out by redshouldered leaf beetles
    Redshouldered leaf beetle showing red-purple patches on the shoulders and ends of the wings. Photo: J Wessels
Scientific name

Monolepta australis

Description

The beetles are 6 mm long and yellow, with a dark red (purple) band across the shoulders and two purple spots on the ends of the wing covers. The flaccid yellowish eggs are small (less than 1 mm across) and oval. The larvae are white, slightly flattened with hard brown (sclerotised) plates at both ends, and reach 10 mm in length.

Similar

The adults are easily distinguished from other beetles by their distinctive colouration.

Distribution

Throughout northern Australia and particularly in cane-growing coastal regions such as Bundaberg.

Crops attacked

Adults feed on many types of plants. Of the summer legumes, soybeans, navy beans and mungbeans are particularly attractive hosts. Other crops include cotton, sugarcane and pasture grasses. Larvae feed underground on the roots of sugarcane and pasture grasses.

Life cycle

Eggs are laid in the soil surface, mainly in pastures and sugarcane. The larvae feed on grass roots and pupate in the soil. The life cycle takes about two months during summer and there are three to four generations annually. Adults usually emerge from the soil after heavy rains following a dry spell. If larval populations in the soil are high, the emerging beetles will form a swarm and may migrate into nearby crops.

Damage

Adult beetles attack leaves and flowers. High populations (e.g. 50 per square metre) will shred leaves and denude crops of flowers.

Risk period

Infestations are likely after heavy rainfall. Soybeans are at greatest risk during flowering.

Monitoring and action level

Check crops after heavy rainfall. Monolepta are readily assessed visually or with a beat sheet; however, the adults are extremely flighty and numbers are difficult to accurately count on a beat sheet, as the beetles can literally 'explode' off the sheet in seconds. Estimate the number of groups of 5 or 10 beetles on the sheet to get a 'ball park' population figure.

Populations greater than 20 per square metre will most likely cause significant damage in flowering crops.

Control

Chemical control: Monolepta are readily controlled with many (but not all) caterpillar pesticides already registered in soybeans. For current chemical control options see Pest Genie or APVMA.

Cultural control: Plant legume crops away from susceptible larval hosts if possible.

Conservation of natural enemies: There are no known predators that are effective against high populations of redshouldered leaf beetle. As infestations are often patchy, consider spot-spraying or perimeter spraying where numbers are highest, leaving most of the crop unsprayed.

Further information

Last updated 25 October 2010