Citrus Gall Wasp in the Home Garden
Citrus Gall Wasp has been causing havoc in gardens across Adelaide in recent years. Left to it's own devices this tiny insect will reduce the vigor of citrus trees, along with their ability to fruit abundantly. If your citrus has lumpy, deformed, stems it's a sure sign they have been infected by gall wasps. Lemon and grape fruit trees seem to be infected at a rate greater then oranges, but do be on the look out across all your citrus trees for the 'warty' looking deformities on stem growth.
Understanding the life cycle of this tiny wasp will help you to control it. Treatment for the home gardener is all about minimising damage caused by the wasp, and is most effective when a number of methods are incorporated together.
Citrus Gall Wasps have one life cycle a year, with adult wasps hatching and mating in early to late spring (depending on seasonal temperatures). The female wasp then implants her eggs into soft, fresh shoots on the citrus tree from which she has just emerged. Amazingly each female can lay up to 100 eggs ... now that is a lot of baby gall wasps.
Essentially, the eggs hatch into larvae which then grow inside the stem of your citrus, with the tree forming a 'gall' - this is the lumpy, bumpy bit - which grows ever larger over a period of about 9-12 months.
If you have galls on your tree then NOW is the time to remove them. Old galls - those from last season - will have tiny holes in from which the wasps have already emerged. It is perfectly ok to leave these galls on your tree, but do be sure to remove all new galls (those without holes), and place the plant material in your waste bin (not the green bin). Alternatively, now is a good time you could 'paint' each new gall with a sticky, horticultural glue - this is said to prevent the wasps from emerging from the gall, and is best applied just prior to hatching.
It also pays to thoroughly spray your citrus trees with a horticultural oil such as Eco-Oil every two weeks during the same period. The pungent aroma of the oil, together with the oily coating on the leaves and stems are said to deter the female wasp from laying eggs on your tree.
Citrus gall wasp traps are available in store, and should be hung from trees from late August through to late October. Gall Wasps are attracted to the yellow colour, and to the pheromone. Do be sure to ensure you remove these traps in November so that you do not inadvertently trap beneficial insects or small birds.
It is best to avoid applications of high nitrogen fertilisers in spring. These fertilisers promote soft, sappy growth .... perfect for gall wasps to lay eggs into (also perfect for Citrus Leaf Miner - so by reducing nitrogen fertilisers in spring you will help minimise two citrus pests). Instead feed your citrus trees with a balanced, slow release citrus fertiliser.
If you are on good terms with your neighbours, and they have citrus in their backyard too, talk to them about this nuisance pest, and encourage them to treat their trees as well. The more gardeners and home owners work together, the better control we will have on this tiny pest.
The following links contains useful information on the lifecycle of the wasp and control methods used in commercial orchards.