What is the Citrus Gall Wasp?
Around Melbourne, Citrus gall wasp (Bruchophagus fellis) has become a large problem for backyard citrus growers. It is a small, 3 mm in length, shiny, black wasp that is native to Australia. It originated in northern parts of Australia where the native finger lime is the normal host plant. However, the Citrus Gall Wasp has also adapted to use our introduced citrus trees as host plants.
It was first recorded as a pest of citrus in Queensland and New South Wales in the 1930s. During the last decade, it has spread to the Riverina and Sunraysia regions of New South Wales, Victoria, South Australia and the southern parts of Western Australia.
The citrus gall wasp causes unsightly lumps in citrus trees. This is the tree’s reaction to foreign bodies of the wasp larvae, that are incubating in the branch. Traditional thinking is that these galls are stressful to the trees. It is also believed to create weaker branches and lower the productivity of the trees.
There are two natural predators of the citrus gall wasp. The wasps Megastigmus brevivalvus and M. trisulcus insert their eggs directly into the citrus gall wasp egg. There it slowly develops in the host larva, eventually killing it. When well established, these wasps can parasitise more than 90% of gall wasp larvae. Neither of these natural predators has established populations in Melbourne, to make a significant impact on the gall wasp.
Citrus Gall Wasp Life Cycle
The citrus gall wasp has a life cycle that spans one calendar year. The adult wasp emerges from its woody gall in late spring or early summer. The emergence of the wasp is determined by the temperature. Most wasps emerge around the same time (within 20 days of each other). Warmer springs will see them emerge earlier than cooler springs.
Once the wasp has emerged, it has only 5 to 7 days to mate and then lay up to 100 eggs under the green bark of a citrus tree.
The eggs hatch after 2 to 3 weeks and feed within the stem for the next 9 to 10 months. During this time the tree will promote woody growth around the infected area. These galls become visible from about December and will gradually enlarge through autumn and winter.
After a short pupation period in spring, the adults emerge from little holes in the galls and the cycle continues.
Adult wasps normally do not move very far, but can be transported longer distances on prevailing winds or by movement of infested branches or trees. They usually re-infect the same tree, or another one nearby.
The NSW DPI has been trialling some different treatments in commercial citrus orchards. One organic treatment method is showing promise. It involves spraying calcined kaolin clay (Surround®) twice during the citrus gall wasp emergence period. This disrupts the egg laying and has shown to reduce gall occurrence significantly.
Observations that I’ve made when growing citrus
At Leaf, Root & Fruit we implement and maintain many edible foodscapes across Melbourne. Citrus trees are a popular fruit tree and many of our clients have them in their gardens. Over the years, this has given me plenty of opportunities to watch and observe the Citrus Gall Wasp in action. Here are a few observations:
- Citrus Gall Wasps tend to infect mainly lemons, grapefruit and to a lesser extent oranges.
- Whilst they will infect mandarin and Tahitian lime trees, they seem to prefer lemons and grapefruit as their host
- I’ve never seen an infected cumquat
- I’ve never noticed them on any of the native limes such as the finger lime or sunrise lime. This is ironic, because they are the original host plants for the citrus gall wasp. We’ve had a lower number of native citrus trees available to inspect for gall wasp, so this may not be accurate. UPDATE 07/07/16… this how now been disproved. Our gardening community report that they gall wasp does very much infect the native citrus and cause galls (see comments section below).
- The gall wasp always lays its eggs in very lush, new growth. This is evident as galls are most easily seen six months later at the base of light-green coloured shoots.
- Citrus trees are traditionally fertilized heavily in spring, summer and autumn. Heavy fertilizing in spring promotes a flush of new growth that is preferred by Citrus Gall Wasp.
- There is a public awareness campaign to “Prune in June”. Removing all citrus galls from a tree in June results in a flush of strong new growth in spring. This new growth is preferred by Citrus Gall Wasp.
- Slow growing, underfed trees are rarely infected by Citrus Gall Wasp in spring
- The Citrus Gall Wasp problem is now so widespread and established in Melbourne that eradication of the wasp from the area is going to be impossible through pruning or other mechanical methods.
- Citrus trees infested with Citrus Galls can still be quite productive. I’m not sure that the galls are as stressful to the tree as traditional theories have made out.
An alternative theory on controlling citrus gall wasp in Melbourne
Based on the observations above, I have come up with some theoretical practices that are worth trialling for growing citrus in Melbourne.
Avoid pruning out the gall wasps in winter. Doing so unbalances the tree so that it has a larger root system area than foliage area. This causes the tree to try to restore the balance by growing vigorously in spring. The vigorous spring growth results in a flush of new foliage that the gall wasp prefers. Yearly pruning regimes, such as this perpetuates the cycle. The “Prune in June” program is likely to result in a downward spiral of the tree over the course of several years until you are left with nothing but a stick. Most fruit trees are covered in fruit (and possibly blossom) in winter, so it is another good reason not to “Prune in June”.
Avoid heavily fertilizing trees in winter or spring. This also results in a flush of new growth that the gall wasp prefers and perpetuates the cycle. Unfortunately, citrus trees are heavy feeders and require a lot of nitrogen rich nutrients. Without these heavy feeds, the leaves will go yellow and they may not be as productive as heavily fed trees. In some cases, I’ve waited until December to feed my citrus trees and this has meant that they have remained uninfected by the gall wasps. Feeding citrus trees in February or March can result in a second flush of new growth that is preferred by the Citrus Leaf Miner. This means that to avoid pests the only time to feed citrus in Melbourne is Late December and all of January. This is probably not enough for supporting productive healthy citrus trees. So there is a conundrum for the Melbourne based citrus grower. It seems we can have a well-fed productive tree, or an underfed, yellowing, pest-free tree, but not both!
Grow a variety that is not preferred by the Citrus Gall Wasp. Most people know someone else with a lemon tree and in winter they are a staple of local food swaps. So why not grow a mandarin, native finger lime or cumquat instead? There are plenty of other varieties out there to try. Check out our citrus variety guide for more ideas.
The Leaf, Root & Fruit Citrus Gall Wasp Experiment
To test these theories, we are going to set up a trial to see which management practice is most beneficial for growing citrus.
We will purchase ten small lemon trees that are relatively identical in size, shape and stage of growth. The trees will be paired up and allocated to five different treatments:
- Citrus Galls pruned out in June every year, fertilized monthly Sep to May.
- Citrus Galls pruned out in June every year, fertilized monthly in Dec and Jan only.
- Citrus Galls not pruned out, fertilized monthly Sep to May.
- Citrus Galls not pruned out, fertilized monthly in Dec and Jan only
- Citrus Galls not pruned out, fertilized monthly Sep to May, treated with calcined kaolin clay (Surround®).
This experiment will run for several years. We will be regularly looking at the overall tree health as well as productivity. Further details on the experiment, as well as periodic updates will be found on our Citrus Gall Wasp Experiment Page. We will also keep you updated on the results as they develop and share them on our Facebook Page and our newsletter.
Want to know more about growing citrus in Melbourne? Check out our handy 5 part growing guide.
Have you made any similar observations to ours? Do you have any thoughts or great ideas on how to control citrus gall wasp in Melbourne? Please include them in the comments section below.