Experiment: Control of Citrus Gall Wasp


Recently, I posted some observations and theories that I have been developing on citrus gall wasps. To test out the theories, we’re setting up a trial to see which treatment regime might produce the healthiest and most productive trees.

Setting up the Gall Wasp experiment

We used 10 x Meyer Lemon trees in the experiment. They seem very prone to Citrus Gall Wasp.

In August 2017 we purchased 10 Meyer lemon trees. At the time of purchase, they were relatively identical in size, shape and stage of growth. The trees were randomly paired up and allocated to five different treatments:

  • Treatment 1: Citrus Galls pruned out in June every year, fertilized monthly Sep to May.
  • Treatment 2: Citrus Galls pruned out in June every year, fertilized monthly in Dec and Jan only.
  • Treatment 3: Citrus Galls not pruned out, fertilized monthly Sep to May.
  • Treatment 4: Citrus Galls not pruned out, fertilized monthly in Dec and Jan only
  • Treatment 5: Citrus Galls not pruned out, fertilized monthly Sep to May and treated with calcined kaolin clay (Surround®).

The trees were potted up into 40 cm pots and filled with Fulton’s Bulk potting mix. We chose Fulton’s Bulk potting mix as it was an average performer in our recent potting mix trials. This meant that we could control the amount of nitrogen rich fertilizer, that we applied during the first few seasons of growth. A better performing potting mix would have promoted a flush of spring growth regardless of any extra fertilizer that we applied.

We potted up 10 lemon trees. Two trees are each going to be subjected to different treatments.

The plants were placed together to ensure that all experienced similar growing conditions, sunlight, microclimate and irrigation.

There are several large citrus trees growing less than 50 metres from where the experiment is set up. There are other citrus trees visible in nearby neighboring yards. They are all likely to have some Citrus Gall Wasp in them.

Citrus Tree Pruning Regime

Treatments subjected to the “Prune in June” regime (four trees in total) are to have all visible galls pruned from the tree in June. The galls are to be soaked in a bucket of water. None of the purchased trees had any galls present when purchased. We decided to mimic trees that have been infected to speed up the accumulation of results. So approximately one third of the foliage was removed from the trees subjected to the June pruning regimes (despite there being no galls present in the first year). The other six trees will not be pruned at all in June, in any year.

Our citrus trees following our simulated “prune in June” regime in the first year.

All ten trees will be tip pruned as required in/or around November and again in/or around February (if required). Whilst the trees are small, this will be done with secateurs or finger tips. As the trees grow, we may resort to using hedge trimmers. Pruning will aim to retain as much fruit and blossom on the tree as possible. This pruning is a typical regime that I use on most citrus trees (see our guide on citrus tree maintenance for further details)

Fertilizing Regime

Trees are to be fertilized according to one of the following two regimes

  • monthly Sep to May
  • monthly in Dec and Jan only

Trees are fertilized with Richgro Organic Fruit and Citrus Fertilizer, which was the best performing organically certified fertilizer in our recent experiment.

It will be applied at the rate of one handful per pot, per month. The fertilizer will be scattered around the surface and then watered in well.

Calcined Kaolin Clay (Surround®) Regime

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 laying of  eggs  and has shown to reduce gall occurrence significantly. We’ve decided to include this treatment as it’s relatively inexpensive, easy to apply and it’s organic.

Calcined Kaolin Clay (Surround®) is to be applied fortnightly for a total of three applications. The first application is to occur in spring at the start of each October. UPDATE 05/12/17: Going forward we will adjust the application of spraying the kaolin clay treatment so that we spray fortnightly applications four times, with the first application in the third week of October (see December 2017 update below)

It is to be applied at the “sunburn rate”. To do this we mixed 8 spoonfuls of power in a small jar of water. This was then tipped into a sprayer and made up to a final volume of 2 litres by adding more water. The trees are to be moved away from the other trees for treatment. The clay is applied using the sprayer and allowed to dry before being returned to their usual position.

Results of Our Citrus Gall Wasp Control Experiment

Update December 2017

The peak emergence for Citrus Gall Wasp in Melbourne looked to occur on (or around) the 22nd November 2017. We filmed the above video of wasps emerging from galls on our tree and mating. I monitored the developing galls and when emergence was imminent we moved all ten trees around the perimeter of our infested grapefruit tree.

Citrus Gall Wasp Melbourne

Our citrus gall wasp experimental trees were temporarily placed around the perimeter of our infested grapefruit tree.

The emergence date was about a fortnight later than we had expected. We had based our expected emergence date on NSW data. Future years’ emergence dates for Melbourne are also likely to be around the same time (late October-early December)  Going forward we will adjust the application of spraying the kaolin clay treatment so that we spray fortnightly applications four times, with the first application in the third week of October.

Citrus Gall Wssp Experiment Melbourne

Flushes of new growth evident on Koalin Treated Tree 5A. This tree is likely to show evidence of gall was infestation next autumn. We will adjust our application of the Kaolin treatment for future yers to better coincide with the gall wasp peak emergence.

The kaolin treatment from this year is likely to be ineffective against the gall wasp. Particularly with tree 5A, as it had a flush of new growth since the final Kaolin application. That new growth had no kaolin on it, leaving it susceptible to infection. I expect this tree to have at least a few galls evident next year.

The following trees had lots of new growth at the time of peak emergence. I expect that they will have a large amount of infestation next year:

1B, 2B, 4B, 5A

The following trees were absent of much new growth at the time of peak emergence. I expect that they will have very little infestation next year.

1A, 2A, 3A, 3B, 4A, 5B

At this stage there is no correlation between the amount of new growth and either fertilizing or pruning regimes. I expect this is because the experiment has only been set up in the last three months. Next spring, once the trees have settled into their growing cycles, a correlation will hopefully be more evident.

All trees have some small fruit set. Tree 2A has the biggest and strongest crop.

Citrus Gall Wasp Experiment Melbourne

All ten Meyer Lemon trees have some fruit set. The fruit on Tree 2A has the largest fruit.

Stay Tuned!

This experiment will run for several years. We will be regularly observing  overall tree health as well as  productivity. Periodic updates will be added to this page as the results develop. We will also share them on our Facebook Page and in our newsletter. Sign up to the newsletter so that you don’t miss out on seeing the results.

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