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
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.
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.
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)
Trees are to be fertilized according to one of the following two regimes
- monthly Sep to May
- monthly in Dec and Jan only
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 28nd 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.
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.
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.
Update June 2018
June is the traditional time to prune out gall wasp. So trees being subjected to Treatment One and Two had their galls pruned out in the last week of June.
At the same time as undertaking pruning we noted lots of observations for each tree. These have been tabulated below.
The following points from this round of observations are most interesting:
- Trees that have been feed monthly from Sep to May had an average of 2.5 mature lemons per tree vs only 1 mature lemon on average for the trees fed Dec and Jan only. So reducing fertilizing seems to impact productivity significantly.
- Two treatments did not have any gall wasps present. They were treatment 4 (the reduced fertilizer and no gall removal) and treatment 5 (kaolin treatment)
- Feeding regimes did not seem to have an impact on susceptibility to Pink scale or induction of flowering.
- Feeding regimes did not seem to have an impact on susceptibility to Citrus Leaf Miner. In fact, the most severely affected tree was on the reduced fertilizing regime.
- The kaolin treated trees showed no impact of Citrus Leaf Miner whatsoever. I hope that some residual clay from the previous October/November has interrupted the life cycle of the moth (rather than it just being a coincidence). I am particularly keen to see what happens next autumn with the two trees undergoing the Kaolin treatment regime.
|Tree Number||Number of Citrus Galls present in tree||Leaf Miner evident in Tree||Number of mature, ripe lemons on tree||Number of immature, second crop lemons on tree||Leaf Condition||Other comments|
|1A||2 x small||Moderate||4||0||Dark leaves, new growth is lighter in colour||Flowers starting to form. Pink scale evident|
|1B||1 x small||Very little||1||1||Slight yellowing. New growth has been slightly burnt by frost||Flowers starting to form. Pink scale evident|
|2A||4 x small|
1 x large
|Severe||4||0||Light green coloured leaves||Pink Scale Evident|
|2B||4 small||Moderate||0||3||Slight yellowing but generally dark green leaves.||Pink Scale Evident. Ladybug present.|
|3A||5 small||Very little||5||0||Dark green leaves. Lots of new growth||Lady bugs present, NO scale.|
|3B||3 small||Very little||1||0||Several leaves are yellow, otherwise mostly dark green||Lots of flowers, NO scale|
|4A||none||Moderate||0||0||Slight yellowing of leaves, mostly light green||Pink Scale Evident|
|4B||none||Very little||0||0||Moderate amounts of yellowing in leaves||Lady Bug present, Pink scale evident|
|5A||none||None||3||0||Slight yellowing of leaves, mostly dark green||Pink Scale, Brown scale and Ant present|
|5B||none||None||2||0||Slight yellowing of leaves, mostly dark green||Flowers starting to form|
It is going to be interesting to continue observing the results of the trial. So far, the most promising results have been with the Kaolin treatment. However, we are only a year into the trial, so who knows what the coming years will bring?
Update December 2018
The gall wasps in our grapefruit, lemon and orange trees began to emerge in large numbers on 2nd December. This particular day was hot and windy. The high winds will help to disperse the wasps to infest other trees in the area. I suspect they may travel hundreds of metres, if not kilometres in the wind.
The 2018 peak emergence is three days later than 2017. Which shows just how much these wasps behave like clockwork. It also shows why “Prune in June” is such terrible timing for controlling the problem.
We noticed that wasps continued to emerge from local trees for most of December. Many galls stayed dormant and haven’t had any wasps emerge. We haven’t noticed this in previous years, but will look out for it more in the future. I suspect that this may be due to sterile eggs being laid under the bark. Perhaps this is enough to stimulate the tree to form a protective gall even though the eggs are unviable?
Update June 2019
Our gall wasp experiment is now two years old and we are starting to see some trends.
We recently moved all the trees into our warehouse to inspect them. Outcomes from these inspections are shown in the table below. During the inspection, we looked at the number of galls on each tree, the quantity of fruit and other observations such as incidence of pests.
After the initial inspection we performed the “prune in June” to the trees undergoing treatment one and two.
Fruit were then harvested from the tree. Interestingly some of the trees (especially treatment 3) had lots of fruit, but they were very small. So we also weighed the total quantity of fruit harvested for each treatment.
|Tree Number||Number of Citrus Galls present in tree||Leaf Miner evident in Tree||Number of mature, ripe lemons on tree||Total Weight of Crop for both trees (Kg)||Other comments|
|2B||3||Very little||8||1.10 kg|
|3A||10||None||7||1 x fruit smaller than most|
|3B||3||Very little||14||1.09 kg||All fruit small|
There were a few interesting observations that we made about our citrus gall wasp experiment this year:
- Fertilizing regimes had a small impact on the quantity of fruit harvested, but didn’t have the clear cut influence that was observed last year. Could this be due to biennial cropping beginning to occur in the trees on the high fertilizer regimes?
- The trees NOT undergoing the “Prune in June” approach are generally larger in size than those being subjected to prune in June (especially after this treatment was carried out). The trees not subject to prune in June also had slightly more fruit (by weight and total number
- All trees except for two (2A and 5B) had galls present. Treatments One and Three both had large numbers of galls compared to the other treatments. The Kaolin treatment (Number 5) only had one small gall between the two trees.
- All trees were suffering from scale regardless of their treatment
- The Kaolin treatment (Number 5) still continues to be the most positive treatment regime. It had the least number of galls (only one) and by far the highest yield of fruit (both in number and by total weight)
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.