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Citrus Gall Wasp
(Bruchophagus fellis)
A native wasp (Bruchophagus fellis) has taken a liking to exotic citrus trees. Originally this little critter (the adult is only about 3mm long) was native to coastal NSW and Queensland. It utilised the native finger lime (Microcitrus australiasica) for development. Now it has many other citrus to choose from.
The female wasp lays her eggs inside the stem in spring. Usually young stems only a few weeks old are chosen but leaf midribs, petioles (leaf stems) and fruit stems can also be infested.
Heavily flecked young twigs in spring can indicate that the wasp is laying large number of eggs in the current spring growth. These twigs may die and be replaced by weaker growth.
Usually galls develop and if the infestation is heavy enough the galling can weaken older trees and reduce fruiting.
Gall Formation
In response to the mechanical damage inflicted by the female wasp as she pierces the stem and inserts eggs, and the subsequent feeding of larvae, young plant tissue grows around and encloses the insects in 'tumor-like formations'.
Inside the stem the larvae hatch and feed on the plant tissue. Inside any one gall, which can be as large as 25cm long and 3cm thick, there can be hundreds of larvae.
Life cycle
The Citrus Gall Wasp has complete metamorphosis, that is: egg, larva (four stages), pupa, and then adult. The adults emerge from the galls through tiny exit holes, which are clearly visible and can cover the gall.
Immediately after they emerge they mate and the females lay eggs. About 100 eggs are laid. There is one generation per year and the adult wasps only live for about a week after emergence.
The illustration shown here is courtesy of NSW Agriculture.
1. Eggs (X24)
2. Larvae
3. Pupa
4. Adult wasp
5. Emergence holes of adults
6. Galled lemon twig showing emergence holes
7. Gall cut open to show cells in which wasp develops
8. Adult wasps laying eggs.
Control
There are no practical chemical controls for home gardeners, although there are registered chemicals for commercial growers that are mostly highly toxic.
In the home garden, remove all infected stems (cutting back to well behind the galls) by the end of August at the latest, before the adults emerge. The stems must be destroyed by burning, or wrap in a plastic bag and put in the garbage.
The use of insect traps such as the 'Insectrap' can be a useful adjunct to cultural practices. This is a non-toxic sticky trap that attracts the wasp, which is then stuck fast and dies. Click here for more on the sticky trap.
The Citrus Gall Wasp does have natural predators. Native wasps (Megastigmus spp.) parasitise the gall wasp larvae. These predatory wasps are known to lay eggs in over 90% of gall wasp eggs, eventually resulting in smaller and fewer galls.
The Autumn 2007 issue of Coastal Fruitgrowers' News, produced by NSW Department of Primary Industries (DPI) reports on a study that mentions that Citrus Gall Wasp was controlled in the citrus growing region of Emerald, Queensland, by the introduction of over one million parasitoids that were relocated there from a coastal orchard.
In NSW some areas are Declared Quarantine Areas and any trees in this area may have to be treated in a prescribed manner.
In Queensland the Citrus Gall Wasp is a target pest of the Cape York Peninsula quarantine, and as such, Queensland Department of Primary Industries and Fisheries state that,
'A targeted pest or a plant infested with a targeted pest must not be moved within or out of the Cape York Peninsula quarantine area without an inspector's approval.' www2.dpi.qld.gov.au/health/14775.html
Information sources:
Cloyd, R.A., Nixon, P.L., & Pataky, N.R., 2004, IPM for Gardeners, published by Timber Press, USA.
Kerruish, R.M. & Unger, P.W., 2003, Third Edition, Plant Protection 1 - Pests, Diseases and Weeds, published by RootRot Press, ACT.
Yates Garden Guide, 2006, 42nd Edition, published by HarperCollins.
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The information contained on this page is Copyright © SGA and intended for personal use only.
You may not use the information for any other purpose without the express written permission of SGA.
