Citrus gall wasp
The citrus gall wasp (CGW), a native insect of northern Australia, is a major pest to citrus and is widespread throughout eastern Australian citrus growing regions including the Riverina and Sunraysia, as well as South Australia and Western Australia. Citrus gall wasps cause galls to form on citrus trees, which weakens them, making them unproductive. Severe infestations can cause reductions in yield and branch dieback. The native host for CGW is the Australian finger lime tree, however CGW has adapted to infest all citrus.
Description
Adult CGW are small (2.5 mm), shiny and black while larvae are small (2 mm), white and stocky without legs. Adult CGW emerge from within galls in late spring or early summer. Timing of emergence is largely determined by temperature; in warmer springs emergence of CGW is earlier than cooler springs. Adult CGW only live for 3 - 14 days, therefore once out they mate immediately and lay eggs, usually under the green bark on citrus trees. Up to 100 eggs will be laid by each female which hatch as larvae within 2 - 4 weeks. Citrus gall wasp larvae live and feed within the gall for 9 - 10 months. During this time a woody growth, known as a gall, forms around the infested area. Galls become visible by December and continue to grow during autumn and winter. By winter the larvae go through a short pupation period, not long after emerging from within the gall as adults.
Trees that are heavily infested by galls have reduced vigour and therefore yields, and in severe infestations branch dieback can occur. Lemons, grapefruit and some rootstocks (e.g. Trifoliata) appear to be more susceptible to attack by CGW.
Control
An integrated approach to managing CGW should be used, including cultural and if required, chemical control. If possible, prune trees and remove galls at least one month prior to wasp emergence. Ensure prunings are removed and destroyed, as wasps can still emerge even once removed from the tree. Avoid pruning in winter as this can stimulate a flush of new growth which is favoured by CGW. Minimise the use of fertilisers in winter or spring which produce flushes of new growth which are preferred by CGW.
Natural enemies important in the control of CGW include two parasitic wasps, Megastigmus brevivalus and M. trisulcus. They lay their eggs within CGW which hatch and develop within the CGW before eventually killing them. Populations in southern regions are low and therefore shouldn’t be solely relied upon for control.
Monitor trees for signs of infestation; once thresholds are reached and if parasite numbers are low, consider applying insecticides. Confidor® Guard is a systemic insecticide which targets larvae and is one of the first insecticides registered for suppression of CGW in citrus.
References
Department of Primary Industries and Regional Development (DPIRD) (2019), ‘Citrus gall wasp control’, https://www.agric.wa.gov.au/plant-biosecurity/citrus-gall-wasp-control
Department of Primary Industries and Regional Development (DPIRD) (2019), ‘Citrus gall wasp in Western Australia, https://www.agric.wa.gov.au/citrus/citrus-gall-wasp-western-australia
Mo, J, Falivene and Creek, A (2018), ‘Citrus gall wasp in Southern Australia’, NSW Department of Primary Industries (DPI), Primefact, Fourth Edition, https://www.dpi.nsw.gov.au/__data/assets/pdf_file/0004/340609/Citrus-gall-wasp-in-Southern-Australia.pdf
Primary Industries and Regions SA (PIRSA) (2019), ‘Citrus Gall Wasp’, Fact Sheet, https://www.pir.sa.gov.au/__data/assets/pdf_file/0017/341450/Fact_Sheet_-_Citrus_Gall_Wasp_-_Nov_2019.pdf
Smith, D, Beattie G and Broadley, R (Viewed Dec 2019), ‘Citrus gall wasp’, Fact Sheet, http://mvcitrus.org.au/mvcb/wp-content/uploads/2012/09/Citrus-Gall-Wasp-Fact-Sheet.pdf
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