Leaf Beetles

Leaf beetles (family Chrysomelidae) are occasional pests that can cause significant damage quickly when occurring in large numbers.

Scientific name

Monolepta australis
Rhyparida spp.
Rhyparida nitida

Other names

  • Redshouldered leaf beetle (Monolepta australis)
  • Swarming leaf beetle (Rhyparida spp.)
  • Black leaf beetle (Rhyparida nitida)

Description

Usually only adults (6–7mm long) are found in crops.

  • Monolepta beetles:
    • yellow-orange
    • dark red band across the shoulders
    • red spot on each wing cover.
  • Rhyparida beetles are brown to black.

May be confused with

  • Monolepta are highly distinctive and unlikely to be confused with other species.
  • Rhyparida may be confused with flea beetles (occasional pests of cotton and horticulture).

Distribution and habitat

Found throughout northern Australia and northern New South Wales, but most commonly in cane-growing coastal regions.

If larval populations in the soil are high, the emerging beetles will swarm and may migrate into nearby crops, particularly after heavy rainfall.

Hosts

  • Larvae feed underground on the roots of grass species.
  • Adults feed on many types of plants, including horticultural tree crops, eucalypts and pasture grasses. Field crop hosts include:
    • Monolepta: soybean, navy bean, mungbean, peanut, cotton, maize and sorghum
    • Rhyparida: sugarcane and maize.

Damage

Monolepta beetles attack leaves and flowers in pulse crops.

  • High populations shred leaves and damage reproductive structures.
  • Damage is often patchy and more common at field edges.

Rhyparida larvae are more likely to cause damage than adults in field crops.

  • In sugarcane, boring into the plant base weakens or kills the stool.
  • In maize, severe infestations have caused 40% crop loss due to death of young plants.

Life cycle

Leaf beetles are most active from late spring and summer. The life cycle takes about 2 months, and several generations can occur per year.

  • Eggs are laid in the soil surface.
  • Larvae feed on roots of grass species and pupate in the soil.
  • Adults often emerge after heavy rains.

Monitoring and thresholds

When monitoring for beetles, pay particular attention to field edges closest to windbreaks containing eucalyptus species, particularly Eucalyptus torelliana (cadaghi gum).

  • Rhyparida may be difficult to detect as they often feed at night and hide under leaves during the day.
  • Monolepta are easy to see, but are extremely flighty and difficult to count.
    • Populations greater than 20 per square metre are likely to cause significant damage in flowering soybean.
    • For pulse crops, use caterpillar defoliation thresholds.

Natural enemies

No known predators are effective against high populations of leaf beetles.

Control

Chemical

  • Monolepta are readily controlled with many of the caterpillar pesticides already registered in soybeans.
  • Infestations are often patchy, so consider spot-spraying or perimeter spraying where numbers are highest, and leave most of the crop unsprayed to conserve natural enemies.

Cultural

  • Avoid planting cereal crops immediately after grass pasture where Rhyparida are present.
  • Plant legume crops away from susceptible larval hosts if possible.
  • Avoid planting susceptible sugarcane varieties.

Further information