Treating Citrus Gall Wasp in Melbourne: An alternative approach to “Prune in June” 23


What is the Citrus Gall Wasp?

Controlling Citrus Gall Wasp Melbourne

Citrus Gall Wasp (Bruchophagus fellis) emerging from a gall.

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.

Observation’s 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.

Citrus Pests and Diseases Melbourne

Citrus Galls caused by the Citrus Gall Wasp are traditionally pruned out in June or July to prevent reinfection in spring.

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 general timing of events related to growing citrus in Melbourne. Please note, some of these events may vary slightly for different citrus varieties. This is especially the case for lemons which may have multiple crops per year.

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.


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23 thoughts on “Treating Citrus Gall Wasp in Melbourne: An alternative approach to “Prune in June”

    • Leaf, Root & Fruit Post author

      Well there you go… I was wondering if the infected native limes just didn’t react by producing a gall, but I’m glad to now know that they do, and that it means my trees are gall free. Thanks for clearing that up Joelle!

  • Nancy

    1. In my garden in Brunswick the wasps have got into lemon, lime and lemonade trees. I also think they may have been in a native lime that wasn’t growing at all until I moved it and cut off a suspect looking area.
    2. Kevin the Brunswick plumber says that you should put the prunings in a bucket of water or cut the gall with a blade, to kill the wasps before sending to landfill in a bag.
    3. There are trees around here that have been neglected which are covered in huge amounts of gall and really have become unhealthy and unproductive (except from the wasps’ point of view).
    4. Generally I just cut them off every couple of months and this works to keep under control.

    • Leaf, Root & Fruit Post author

      Hi Nancy,
      “Kevin the Brunswick Plumber” is right on the money in terms of soaking the prunings to kill the larvae.

      Thanks for sharing your observations and tips with us.
      Duncan

  • Barbara Austin

    Thank you for the information. I have never had the gall wasp until I moved to Mooroolbark – now I know why.
    I bought a small lime tree 12 months ago don’t know what it was but in that short time it developed gall wasp. so disappointed as it was only very small. Can let you know if you would like.

    Looking forward to hearing the results of your experiments and doing some of my own.

    • Leaf, Root & Fruit Post author

      Hi Barbara,

      Thanks for sharing your experience. Gall Wasps can be a frustrating issue . Please report back on progress, we’d really appreciate it.

      Good Luck and Happy Gardening

      Duncan

    • Leaf, Root & Fruit Post author

      Hi Heather, sounds like an interesting idea and it may work. I’d think that you’d still end up removing a lot more of the plant than you might using some of the other methods. I’ve also noticed that the galls seem to form at the base of new growth. So you would probably need to prune back all of the spring growth, plus some of the older wood at the same time. Thanks for sharing your idea. If you give it a go, please let us all know how you get on.

      Good Luck and Happy Gardening!

  • Ruth Williams

    It’s good to read your thoughts and experience on the best way to manage the citrus gall wasps. We have been cutting the galls out on a more leisurely schedule, where we would notice one while walking past the tree, cutting a few out at the time. It seems to have worked for us for a few years now and the tree, which is probably 60 years old has a full yield of lemons. This year I thought I should follow the ‘Prune in June’ advice more religiously but my more ‘lazy gardener’ ways have won out, so I’m glad to have read your article as I will go back to our old ways!

    • Leaf, Root & Fruit Post author

      I’m a big advocate for “Do Nothing Gardening”, and this is just an extension of this. Sounds like you’ve got the right take on things.

      Good Luck and Happy Gardening!

  • Olwyn Smiley

    I look forward to seeing the results of your trial.An excellent idea!
    A couple of comments: a neighbour’s lemon tree, uncared for, unfertilised, and with minimal soft new spring growth, was covered in galls – whole branches were thick masses of gall. Very few fruit. It struck me that the huge number and density of galls was probably quite disruptive to the structure of the tree. We pruned it heavily last year; haven’t checked recently to see how it is growing. I will be interested to see.
    Observing other well-fed and cared for trees, fruit production hasn’t suffered despite heavy loads of galls.
    Re different citrus varieties, my observations are similar to yours: lemons, limes and grapefruit are particularly prone to gall wasp infestation. Also Japanese seedless mandarins, Makrut limes and oranges. Cumquats, Imperial and Murcott mandarins have never been never affected here.
    Penny Woodward suggested on the 3CR radio garden program that spraying with eco oil or similar affects the ability of the wasps to successfully implant their eggs on the trees. I plan to try this.
    I have never had the citrus leaf miner until the past season – all the trees have had some of their new growth affected, pruned trees and unpruned trees! Don’t know where they came from…
    I like your lateral thinking!

    • Leaf, Root & Fruit Post author

      Interesting observation about well fed vs weak trees Olwyn. The kaolin clay is supposed to fulfill a similar role to the eco oil. I’m pretty excited about this experiment and looking forward to sharing the results with you all.

      Thanks for sharing your experience with us!

      Duncan

    • Laura

      I had a similar experience pruning a friend’s neglected lemon tree – all parts of the tree were severely infested, and the older wood had gall upon gall upon gall so that it was thick, warped, lumpy, (remember the face of the elephant man?). Some of these lumped up branches had feeble new growth coming out of it, but many also had dead sticks coming out – either new growth that had died off or older branches that had been choked by galls closer to the trunk. Many of the lumpy areas were black and rotting. I must have cut 90% off the tree, despite trying to save any less infected branches so the tree would still have some canopy. It was a brutal pruning.
      The traditional method of pruning might not be right for every situation, but it would have saved this tree from years of damage.

  • Chris Newman

    Maybe Slice the Gall open in June, this would be much better than letting them develop. The larvae dry up an die I think with this method.

    • Leaf, Root & Fruit Post author

      Yes, this is another method of dealing with the galls. Recently someone recommended a potato peeler as a great (and safe) tool for doing this. Thanks for sharing your tip Chris!

  • christopher torr

    Counter Observation…
    Noted above you made the following comment, “Whilst they will infect mandarin and Tahitian lime trees, they seem to prefer lemons and grapefruit as their host”.
    I have 3 citrus, Tahitian Lime, Meyer Lemon & Lisbon Lemon in close proximity and all are equally infected. I will trial the following method over the next season and advise.
    * Jun-July citrus galls not pruned out, but tree lightly pruned to shape & improve airflow,
    * Dec-Feb proposed time to fertilise,
    * Jan-Feb galls that appear cut one side of gall to expose larvae
    Cheers

    • Leaf, Root & Fruit Post author

      Thanks for sharing your experiences with us Christopher. Your trials sounds great and we look forward to hearing the outcomes. Please keep us all updated.

      Good Luck and Happy Gardening!

  • Gabriella Hont

    Look forward to your results. I moved here 4 years ago and have been pruning gall wasp from established lemon as traditionally recommended. Problem seems to have progressively worsened. Also have an established lime which wasn’t infected, until this year.

  • Ruthie

    I have a very big well established lemon tree which is highly productive. The tree had the gall wasp lumps on it when we moved into the property 10 years ago. If I tried to prune it all out, i’d only have a tiny tree left over as there as so many. Therefore, I just leave it. The leaves hide the branches well and you cannot see the lumps unless you really go up close and look. The lemons on the tree are still bountiful and so many I can’t even give them away. They’re also very large lemons, and I never even fertilise it. So I don’t think its affected the productivity or quality of the tree much at all. Therefore, i’m going to continue to just leave it and see how it all goes.

    • Leaf, Root & Fruit Post author

      Hi Ruthie,

      Thanks for sharing your experience with us. Glad to hear that your lemon tree is thriving despite the gall wasp.

      Good Luck and Happy Gardening!

      Duncan

  • Barry Mackay

    My lemon tree has been in full production all year round for 30 years, until two years ago, when it was infested by the gall wasp.
    The galls on some parts of the tree were over a foot long, the crown of the tree was leafless, and the fruit was down to 3 lemons.
    So, I gave it a radical prune and took it back to bare branches during the last week of October this year with the prunings being immediately burnt.
    There is no new growth at this stage, but fingers crossed for the next few months.

    • Leaf, Root & Fruit Post author

      Thanks for sharing your experience Barry. Please keep us all updated on your outcomes.

      Good Luck and Happy Gardening!

      Duncan