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Lucerne crownborer

Lucerne crownborer can be a serious pest of summer pulses and a minor pest of lucerne. Zygrita occurs frequently and Corrhenes infrequently.

Scientific name

Zygrita diva, Corrhenes stigmatica

Description

  • Adult Zygrita diva are 15mm long and bright orange with black legs and long black antennae. They usually have 2 prominent black spots on their wing covers, but may have no spots or more spots.
  • Corrhenes stigmatica are similarly shaped but pale mottled brown with striped antennae and lack the dark wing cover spots of Z. diva.
  • Larvae are cream with a wide head and are up to 25mm long.

Distribution and habitat

Present in the Northern Grains region but most common in the tropics and sub-tropics.

Hosts

Soybean and lucerne.

Damage

In some crops, over 80% of plants may be infested.

Larval feeding itself has little impact on yield but, prior to pupating, plants are internally ringbarked and the stem cavity plugged above the pupal chamber. This causes plant death above the girdle and plants in thin stands may lodge before harvest.

  • In southern Queensland, this usually occurs after seeds are fully developed with no yield loss.
  • In tropical regions, larval development is more rapid and there can be considerable crop losses.

Crown borers are very damaging to 'edamame' soybeans where green immature pods are harvested by mechanical pod pluckers. The stems of infested plants are weakened and snap off, contaminating the harvested product.

Life cycle

Infestations usually occur from flowering onwards, with eggs being inserted in the stems of young soybeans. The larvae tunnel up and down through the pith in the stem, but usually pupate in the tap root.

Monitoring and thresholds

  • Spring-planted crops are at greatest risk, especially those close to maturing host crops.
  • Visually examine plants and flowers for signs of thrips. Growing points or flowers can be dipped or stored in alcohol to dislodge thrips for later counting.
  • Adults are highly mobile – check for reinfestation post-control.

Further information