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Scarabs

Scarabs are a large group of beetles with soil-dwelling larvae. Some species are pests of field crops and can cause significant damage in some years. Cockchafers (common pests in southern Australia) and Christmas beetles are also members of the scarab family.

Scientific name

Heteronychus arator (African black beetle)
Heteronyx spp. (Peanut scarab) )
Pseudoheteronyx basicollis(Black sunflower scarab)

Other names

  • Scarab larvae (particularly the peanut scarab) are often referred to as white grubs

Description

Adults

  • Adults are oval-shaped
  • African black beetle is 10-15mm long, and a rich chestnut when newly emerged, that changes to a shiny black.
  • Peanut scarab is 17mm long and uniformly brown.
  • Black sunflower scarab is 13mm long and shiny black.

Larvae

  • Grow to about 25-30mm long.
  • Have creamy-white bodies and light to dark-brown heads.
  • Curl into a C-shape.
  • Have 6 legs near the head end.
  • Some species have a greyish tinge at the rear end.

May be confused with

Scarab species (particularly the larvae) can be difficult to tell apart.

Distribution and habitat

  • African black beetle is widespread in winter rainfall areas, although it does extend into south-eastern Queensland.
  • Peanut scarab is present in peanut growing regions with ferrosol soils, including the South Burnett, Atherton Tableland and the Darling Downs.
  • Black sunflower scarab is present in Queensland.

Hosts

  • African black beetle is a major pest in southern Australian maize, but is rarely a problem in Queensland field crops.
  • Peanut scarab species are a pest of peanuts and occur in pasture.
  • Black sunflower scarabs attack sunflower. Parthenium weed is a favoured host but the larvae can develop on the roots of many grasses and weeds.

Damage

African black beetle

  • Adults feed on the underground stems of young plants.
  • The central shoots wither and the plants become dead-hearted.
  • Older plants usually survive, but remain weak and liable to lodging.

Peanut scarab

  • Young larvae feed on soil humus and peanut roots.
  • Older larvae attack peanut shells and kernels, reducing yield and quality.
  • Adult beetles feed on seedling leaves.
  • Heavily infested crops may suffer over 30% yield loss.
  • Damaged pods are prone to invasion by species of Aspergillus fungi, which can produce the highly carcinogenic aflatoxin.

Black sunflower scarab

  • Larvae feed on taproots causing wilting and death of plants up to 40cm high. Damage is most prevalent where sunflowers follow wheat, sorghum or grass pasture.
  • Adult beetles feed on foliage, often in a line across the field.

Life cycle

Scarab life cycles can take more than 1 year to complete.

Peanut scarab is a 1-year life cycle native species that has adapted to peanuts:

  • Adults emerge from the soil in late spring/early summer after heavy storm rain to feed on vegetation.
  • Eggs are laid under peanut seedlings.

Larvae overwinter in the soil and pupate in the spring. Early planted crops tend to become more infested than later planted crops.

Monitoring and thresholds

Scarabs are difficult to monitor.

  • Dig and sieve the soil for the presence of larvae and adults before planting.
  • Continue monitoring post-plant, particularly in fields with a history of damage.
  • The presence of peanut scarab small larvae and adults underneath seedling crops is an indicator of likely later crop damage.
  • Look for feeding black sunflower scarab beetles just before sunset (4 beetles p/m2 can cause severe losses to young seedlings).

Control is usually not feasible once a crop is infested with larvae. If damage is anticipated, apply soil-incorporated pesticides at planting to control egg-laying adults and newly hatched larvae

Natural enemies

No effective natural enemies have been identified. Wasp parasitism of peanut scarab larvae is usually very low (less than 1%). Death by pathogen infection (e.g. Cordiceps fungus) is also rare.

Control

While a granular pesticide is registered for peanut scarabs in peanuts, insecticides are of limited effectiveness against black sunflower scarab. Non-chemical methods include:

  • Avoid planting into recently cultivated pasture.
  • Consider crop rotation options (e.g. avoid planting sunflowers directly after wheat or sorghum).
  • Control host weeds and volunteer plants.

Further information