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Leaf beetles in field crops

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