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Bean fly

Bean flies damage stems in many legume crops. Their attacks are capable of causing premature plant death, especially in seedlings.

Scientific name

Ophiomyia phaseoli

Description

Adults are small (3mm long) and shiny black with clear wings.

Larvae (maggots) are cream with dark mouthparts and grow to 3mm long.

Pupae are brown and cylindrical with rounded ends.

May be confused with

  • Soybean stemfly (Melanagromyzae sojae), a pest of soybeans.
  • Soybean podfly (a minor pest only attacking pods).

Distribution and habitat

In Australia, bean fly is most common in tropical and subtropical coastal and subcoastal regions. It's generally not a problem in inland cropping areas.

Hosts

Navy bean, adzuki bean, mungbean. Rarely a problem in soybean.

Damage

Larval tunneling damages the plant's vascular tissue, reducing plant vigour. Severe infestations can cause seedling death.

Crops are at greatest risk for 3–4 weeks from emergence, but later crops are sometimes attacked, causing petiole droop.

Life cycle

Female flies lay their eggs in young leaves.

Larvae tunnel their way to the leaf midvein, then to the petiole and stem, and pupate in the lower stem.

Monitoring and thresholds

Monitor seedling crops twice weekly. Look for:

  • distinctive pale oviposition pinprick windows in the leaves
  • larval tunneling at the base of petioles and in the stems
  • damaged stem tissue in the lower stems
  • pupae and adult flies.

Take action if more than 1 larval tunnel per plant.

Control

  • You can readily achieve control in young crops with systemic pesticides, but you may need to repeat sprays (within 7 days) for heavy infestations.
  • Spraying against bean fly in mature crops may not be cost effective.
  • Ensure cropping areas are free of weed hosts, such as phasey bean and volunteer crop legumes.

Further information