CITRUS GALL WASP
Citrus gall wasps (Bruchophagus fellis) are small (3mm) shiny black wasps native to northern Australia. There they have natural predators (two parasites) which keep the number of citrus gall wasps under control. As the wasps have gradually moved south (thought to be via the movement of infected citrus trees), they have appeared in many areas without their natural predators, and consequently have exploded in numbers and caused considerable damage.
The citrus gall wasps have a preference for the native limes, as well as lemons, oranges and grapefruits. Mandarins are less affected, and cumquats are unaffected as they are not from the citrus genus. Adult wasps emerge in spring, often timing emergence with the onset of a flush of new growth. After mating, the female immediately lays her eggs into the soft new season flush. Eggs hatch after 2 – 4 weeks and the young larvae burrow into the soft bark. Distinctive woody galls form around the larvae during summer. The new generation emerges the following spring, completing the year-long life cycle. Adult wasps live for only one week after emergence.
Citus Gall Wasp control is difficult in Melbourne as at this stage predator release is not possible (as happens in Queensland). Best practice at the moment includes placing sticky traps into the trees around spring to catch emerging wasps and prevent them mating and laying eggs into the new growth. Adult wasps don’t fly far. They can be moved good distances by wind but will tend to re-infect the tree they emerged from. Hence traps are useful to prevent re-infection. Remove galls before spring, wrap and place in your rubbish bin (NOT your green waste bin). You can also reduce the amount of soft spring flush growth by fertilizing over late autumn or winter. Use a balanced fertilizer rather than a highly nitrogenous one. Left untreated these wasps will kill the tree, it may take a few years but gradually it will die and possibly infect other trees in the process!! So treat now!