Biological Control of Black Wattle

midge-image

The Flowering Gall Midge

(Dasineura rubiformis)and introduced natural enemy of Black Wattle (Acacia mearnsii)

DESCRIPTION

Adult midges are tiny (2mm) and brown/black in colour. They are mostly evident in spring when they can be seen flying around (in the vicinity of the host plants) or ovipositing on the flowers.

LIFE CYCLE

Females oviposit (lay their eggs) into the open flowers during spring. Once hatched, the larvae feed on the ovaries resulting in the formation of gall tissue. Each larva develops within a single chamber in the gall and goes through three larval stages to maturity. A single gall may contain up to five larval chambers. Since most of the flowers in the flower head are utilised during oviposition, characteristic clusters (of up to 35 individual galls) result, each of which resembles the young fruit of a blackberry.

By the end of May/early June, the galls (and larvae within them) are fully developed, and the larvae exit the gall through a small opening and drop to the ground. Here they pupate in silken cocoons in the leaf litter or at shallow depths. The emergence of the adult midges occurs several months later and coincides with the flowering season of black wattle.

IMPACT ON BLACK WATTLE

Oviposition by the midges and subsequent gall development prevents the formation of pods, and at sites where the midges are well established and abundant, pod production has virtually ceased.

Galling by the midge has no impact on vegetative growth of black wattle, so the general appearance of the trees is not affected. However, as with other similar biological control measures, any reduction in seed production, should ultimately reduce the aggressiveness and rate of spread of the plants.

 

 

 

 


Collecting and Re-distributiongalldamage

All release data must be sent to Fiona Impson ARC – Plant Protection research Institute, Private Bag X5017, Stellenbosch, 7599 (impsonf@arc.agric.za)

Mature galls are best collected from trees where the midge is well established during June. Galls can be cut from the branches into containers (cardboard boxes or bags) and then deposited in protected areas against the trunk of black wattle trees at release site. Each batch of galls released should amount to approximately a carrier bag full. To ensure successful establishment, galls should be released within 72 hours of collection. At this stage the galls are fully mature and the larvae within them are ready to emerge and drop to the soil where they will pupate in cocoons. Adults will then emerge from the cocoons during spring.

When choosing a release site ensure the following:

  • The trees have reached reproductive age, i.e they are able to flower
  • Be free from any clearing operations for between 5-10 years to allow the midges to establish
  • Be protected from fire and disturbance as far as possible
  • The release trees are not alongside dirt roads

The following data must be collected during the release:

  • Condition of the galls upon release
  • GPS co-ordinates
  • Contact details of land owner
  • Brief description of how to get to the site

 

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