Lumpy growths on young leaf stems, leaf mid-ribs and fruit stalks of citrus trees are caused by a pest called the citrus gall wasp (Bruchophagus fellis). All citrus trees are attacked, but grapefruit and rough lemons are the most susceptible.
The adult is a black wasp about 3mm long, which lays eggs in young citrus growth in spring. The larvae hatch within three weeks and start feeding inside the stems, causing galls to form. As the larvae grow, the galls become larger. The larvae feed throughout the following summer, autumn and winter, and emerge as adult wasps around September to October. There is only one generation per year.
Citrus gall wasp is usually found in coastal Queensland and New South Wales, and some inland areas (see map). However, this pest has recently been seen as far south as Melbourne.
Although citrus gall wasp does not kill citrus trees the damage it causes is unsightly, and repeated attacks can weaken trees and make them unproductive. To control this pest, remove all galls from the trees by the end of August. Place the galls in a plastic bag, then seal the bag and put it in the garbage. After removing the galls, it is a good idea to fertilise your tree to encourage strong new growth in the spring.
Further reading
To find out more about pests and diseases on citrus or plants in general, you may like to refer to Judy McMaugh’s book ‘What Garden Pest or Disease is That?’ (New Holland Publishers, ISBN 1864366990, $50). It is available from book stores or from Florilegium, 145 St Johns Road, Glebe, NSW, 2037. Phone: (02) 9571 8222.