Tuesday, 5 April 2016

Citrus Gall Wasp - Bruchophagus fellis

Citrus Gall Wasp is an Australian native insect pest whose natural host is the Australian finger lime. Unfortunately it is now found in the southern citrus growing regions of Sunraysia, Riverland, Riverina and now, in South Australia. Orange production in Australia is most concentrated in these regions. There is a call for not only orchardists to address the spread of this wasp, but backyard growers of citrus to do their bit too. 

The Citrus Gall Wasp infests young flush growth in spring causing woody galls to form around the developing larvae. One of these galls will house many wasps in separate cells. All citrus varieties can be attacked however, lemon, grapefruit and some rootstocks (especially Trifoliata) are most susceptible.

Adult Citrus Gall Wasps emerge from galls in spring and the timing of emergence is influenced by temperature and closely associated with the appearance of the spring growth flush. The adults don't normally move very far, but can be transported over long distances on prevailing winds. Egg-laying starts immediately after the wasps emerge from galls. They live from between 3-14 days depending on temperature and each female can lay up to 100 eggs. Most eggs are laid in the first two days after wasp emergence and are mostly laid under the bark of young spring shoots. Larvae hatch in 2-4 weeks. Heavily galled trees lose vigour which may result in reduced yield and fruit size.

It is imperative not to encourage excessive spring flush by over-fertilising - particularly heavy applications of nitrogen in early spring. 

Avoid using 'sticky' yellow traps as they also kill the beneficial insects. 

There are two naturally occurring parasitic wasps, Megastigmus brevivalvus and M. trisulcus, who are important natural enemies  -  they are similar in size to the gall wasp but are honey-coloured and can be easily distinguished from the black Citrus Gall Wasp. These parasitic wasps lay their eggs directly into the Citrus Gall Wasp eggs, feeding on and eventually killing their hosts. 

If you notice any gall-bearing branches it is best to cut them back as they appear. Removal of all gall material by late winter before the wasps begin their hatching is recommended.

Prune directly below the infected area. Cut just above the first outward facing shoot. 

If your tree is heavily infected, requiring a really hard pruning, you may not get fruit for 2 years - so it is preferable to get on top of these pests when there are not too many on your trees.

Do not dispose of infected material in your green bin or compost heap. The galls will still hatch and their life cycle will recommence. In a perfect world, burning would be the optimum method of disposal, however this is sometimes not possible, so bagging and putting into your general refuse bin is the way to go, or you could chop up and immerse in water to drown them (approximately 2-3 weeks to be sure).

Horticultural mineral oil deters Citrus Gall Wasp egg-laying. Three sprays of this preparation 10-14 days apart during emergence of the wasp can provide good control. Oil sprays will not eliminate the Citrus Gall Wasp but will (can) reduce galls in the following season. Good spray coverage is obviously critical as any unprotected foliage will be susceptible to egg-laying.

Further reading can be found on this GardenDrum article written by Kaye Roberts-Palmer. It is written from a Melbourne prospective however, it has relevance for the Coffs Coast as we really need to be very diligent gardeners to help stop this wasp from spreading even further.

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