Gall Wasp Control

In most parts of Australia, if you’ve grown citrus, you’re more than likely to have come across swollen galls on your grapefruit, lemon, lime or orange. 
They are caused by citrus gall wasps (Bruchophagus fellis) – native Australian insects that were originally found only in northern NSW and Queensland where native citrus grow, but they are now found in almost all states apart from Tasmania.  

These small (about 2mm – 3mm) shiny black wasps are most active in spring and summer, with the female laying a clutch of eggs into the bark of the soft growth of citrus trees. As the eggs hatch, the larvae burrow into soft stems. As the larvae grow, the plant forms woody tissue around them, causing swelling and the characteristic gall. Once the weather warms, the adults hatch and the cycle starts again…with the eggs being laid almost immediately, and often in the same tree!
While the galls generally won’t kill trees, they can seriously impact their vigour, reducing the size and yield of fruit, and causing growth to be weak and spindly. 
Here’s how to tackle the problem: 
First identify ‘active’ galls. If the gall is covered in tiny pin holes, the wasps inside have already hatched and gone. Look for swellings where the bark is still intact.   
Traditionally, galls were pruned them off, cutting the affected limb well behind the gall, then bagging the prunings, solarising and placing in the bin, to prevent reinfestation. However, in smaller trees you can lose all the most productive branches this way. 
Instead, grab some secateurs, a sharp knife or even a potato peeler and, facing the blade away from your body, slice the top off one side of the active gall, or slice the gall open (like a hot dog bun). This exposes the contents of the gall (the larvae) to air and kills them, without causing lasting damage to the tree. The best time to do this is mid to late winter.  
Just make sure you don’t ring-bark the stem, as this will kill it. 
To prevent reinfestation: 
• Don’t prune citrus in winter  
• Avoid overfeeding your citrus tree, particularly in winter and spring.    
• Spray vulnerable new growth of citrus trees with a horticultural oil in spring 
• Keep in mind that sticky traps can kill some of the ‘useful’ predatory insects too. 
• Encourage your neighbours to treat their trees too! 

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