Leaf beetles in field crops

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.

• 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).

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.

• 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.

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.

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.

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.

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

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.