Leaf beetles in field crops
© J Wessels, Queensland Government
© J. Wessels, Queensland Government
Leaf beetles (family Chrysomelidae) are occasional pests that can cause significant damage quickly when occurring in large numbers.
- Redshouldered leaf beetle (Monolepta australis)
- Swarming leaf beetle (Rhyparida spp.)
- Black leaf beetle (Rhyparida nitida)
Usually only adults (6–7mm long) are found in crops.
- Monolepta beetles:
- 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.
- 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.
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.
No known predators are effective against high populations of leaf beetles.
- 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.
- 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.
- Beetle outbreak—Australian Mungbean Association
- Rhyparida information sheet (PDF, 1.1MB)—Sugar Research Australia
- Registered chemicals database—Australian Pesticides and Veterinary Medicines Authority (APVMA)
- Last reviewed: 20 Aug 2019
- Last updated: 20 Nov 2019