What are Citrus Galls?
Citrus galls are unsightly swellings caused by a tiny female citrus gall wasp (Bruchophagus fellis) which lays her eggs in the soft new growth of citrus trees in spring. As the new growth hardens off, a woody gall forms around the growing wasp larvae. Each gall contains many wasp larvae, each in their own separate cell within the gall. Citrus gall wasps affect all citrus varieties, but lemons and grapefruits are more susceptible to attack, while mandarins are the least susceptible. .
Here’s what the culprits look like, they’re a shiny little black wasp about 2-3mm long.
Do Citrus Galls Harm the Tree?
Citrus gall wasp does not kill citrus trees, but it damages the tree nevertheless. Yes, the galls are ugly and unsightly, but the harm is more than just cosmetic. When a citrus tree is infested with galls, it weakens the tree, reducing fruit size, tree vigour and yield. Branches with citrus gall become thin, spindly and brittle, and produce shoots growing in strange directions, often into the tree, ruining the shape if left unpruned. In extreme cases, the galls can cause dieback of the branches.
Where Do Citrus Gall Wasps Come From?
The citrus gall wasp Bruchophagus fellis is a Australian native pest of citrus trees in Queensland and mid to north NSW. The natural host of this pest wasp is the Australian finger lime (Citrus australasica). This pest has found its way down as far south as Melbourne, and they’ve been travelling south on the back of a truck, I’m not joking! Most of citrus trees produced commercially in Australia for the nursery industry are grown in the northern states, and growers ship citrus trees to the southern states via road transport.
In the northern states, citrus gall wasps have natural predators such as the parasitic wasps Megastigmus brevivalvus and Megastigmus trisulcus, which parasitise the pest’s eggs to control the pest populations. These natural predators don’t travel by truck to the southern states with citrus trees, and they can’t tolerate the colder weather in the south of Australia either. Unfortunately the citrus gall wasps can handle the colder weather and have established themselves very well in the cooler Australian regions.
When are Gall Wasps Active?
In the southern regions of Australia, citrus gall wasps are active after they emerge from galls formed the previous year, from late September to mid December. Within this period, their numbers are greatest from mid October to mid November when they are laying eggs. The tiny adult wasps only live for 3–14 days depending on the temperature. Once the females mate, they lay their eggs under the green bark of soft new spring citrus growth, and each female can lay up to 100 eggs.
The larvae (wasp grubs) hatch from the eggs after 14–28 days, and begin feeding inside the young citrus branches for 9–10 months, causing the galls to form. As the larvae get bigger, the galls swell and grows in size. The wasp larvae will feed inside the gall right through the summer, autumn and winter, then they’ll briefly pupate in spring to turn into adults. These new adult wasps chew their way out of the gall, emerging in spring to repeat the cycle once again. As we can see, there is only one generation per year.
Adult citrus gall wasps do not normally fly very far, but can be carried longer distances by winds or by human transportation of infested branches or trees.
How to Control Citrus Gall Wasp – All the Methods that Really Work
There are several approaches to controlling gall wasp – notice I said controlling and not completely eradicating them, because that’s not possible! The best results are achieved when multiple control methods are combined, as the overall effectiveness of the control measures add up, they’re cumulative – i.e. if one control is 60% effective, and another is 30% effective, neither is outstanding on its own, but together they offer 90% effectiveness. This approach to pest control is known as IPM, or integrated pest management.
The following controls are discussed in this article:
- Cultural Controls
- Mechanical and Physical Controls
- Biological Controls
- Chemical Controls
Gall Wasp Cultural Controls
Firstly, we can use cultural controls – practices that reduce pest establishment, reproduction, dispersal, and survival.
Adult gall wasps are present in spring and early summer and only attack soft new spring growth. One way to control the pest is by reducing excessive new spring growth flushes which they prefer – to achieve this don’t over-fertilise citrus in winter or early spring.
Timing also makes a difference, as fertilizing later in spring will produce a summer flush of new growth which will miss the peak citrus gall wasp period.
Also, avoid using high-nitrogen synthetic fertilizers as they force feed plants and push out a lot of new soft and sappy growth very fast, which exacerbates the gall wasp problem. Using natural slow-release organic fertilisers is a far better option.
The lower canopy of citrus trees areas appear to be favoured by citrus gall wasps, possibly because of shading and the more humid microclimate that such a location provides.
Don’t use sprinkler irrigation below trees as this creates more humidity below the canopy, favouring the pest.
Use drip irrigation, which is a much better choice, as it creates a drier environment, which is less ideal for the pest.
3. Horticultural Oils
Horticultural oil sprays such as synthetic petroleum-based white oil and natural organic-certified horticultural oils are commonly used against citrus leafminer. Spraying horticultural oil on new growth flushes of leaves deters them from laying their eggs, as they avoid surfaces sprayed with oil. Similarly, they can also be used as a deterrent against adult citrus gall wasp to discourage them from laying their eggs into the soft green stems of new spring growth. Spray new growth as soon as it emerges.
In citrus trees which have only a few galls , prune out all galls before the end of winter. The galls contain the live larvae (grubs) of the citrus gall wasp, and need to be destroyed, otherwise they’ll hatch out in spring and spread further!
The following disposal methods are recommended by Australian government agriculture departments:
- Newly formed galls pruned out from mid-autumn to early winter (April-June in southern hemisphere, i.e. Australia or October-December in the northern hemisphere. i.e. US), can be destroyed by drying them out in the sun to kill the larvae.
After that, the galls can be disposed into green waste, compost or general rubbish. (To be on the safe side, I prefer to put them in a plastic bag which has been tied shut, doubled over, and tied again, or double-bagged and tied, and then put into the garbage bin for general rubbish).
- Galls removed after the first month of winter (after June in southern hemisphere, i.e. Australia or after December in the northern hemisphere. i.e. US), can be destroyed by burning, deep burial or shredding and solarising.
To solarise galls, place them in plastic bag that has been tied shut, and left to bake in the sun for at least four weeks. It’s preferably to use a black plastic bag, and to double-bag the galls to prevent any pests escaping. After that, dispose of the solarised bags into the garbage bin.
Please don’t put gall offcuts in normal household waste bins, green waste recycle bins or plastic garden bags as the pest will survive and spread to other areas. Citrus gall wasp can still emerge from cut galls if they’re left in the shade for less than six weeks, which is why they have to be disposed of properly.
If a tree is heavily infested with citrus gall, it can be pruned hard at least one month or more before the citrus gall wasps emerge in spring. Do not remove more than one-third of the tree’s canopy in any one year as it can weaken or kill the tree. Pruning will stimulate regrowth, and since the new growth can be attacked by citrus gall wasp it must be protected with a gall wasp trap.
The problem with pruning regularly to control gall wasp is that the tree can eventually be whittled back to nothing over a few years if it doesn’t regrow fast enough, which is particularly the case for dwarf citrus in pots!
Dwarf citrus trees are regular citrus trees grafted onto a dwarfing rootstock, such as the ‘Flying Dragon’ rootstock. There are very few ungrafted genetic dwarf citrus varieties around. The thing to consider with dwarf citrus is that they only grow to 1.5m (5’) tall by 1.5m (5’) wide… after 5 years! They’re great for containers, but at that growth rate you need some measure of patience. When we do the math, we realize that they grow approximately 30cm (1’) per year. So what, you may say?
Well consider the advice to prune out galls from citrus. If we prune off 30cm off the top of a dwarf citrus tree, we’ve lost a year’s growth! Is this a problem? I’ve seen far too many cases where I’ve been shown sorry-looking, tiny citrus trees in pots and told they were a few years old, and had to be pruned for gall.
If we don’t want to prune dwarf citrus, what can you do to prevent the spread of gall wasps? We can use winter’s cold winds to help us out.
5. Slicing Citrus Galls to Dry Them Out
Pruning galls out can be destructive to a citrus tree, as a lot of branches can be removed in the process. If we want to retain branches but remove galls, a far better option is slicing galls to dry them out. If this technique is performed properly, the branch can heal and and continue growing.
We can use the drying effect of cold winter winds or warmer autumn winds to kill the citrus gall wasp larvae without pruning off the branch.
To kill the gall wasp larvae, I use sharp secateurs to shave one side off the gall, opening up a ‘window’, which will dry out the pests inside, and possibly heal over in time if done early enough. Only cut off one side of the gall to expose the larvae, don’t cut right around and ring-bark the branch, as removing a ring of bark all around a branch will cause it to die. You can use a pruning knife or any other blade for the task, just remember safety first when handling sharp tools!
NOTE: for safe blade handling, always cut away from your body or other hand, that way if the blade encounters resistance then cuts through, the sharp blade will not be moving towards your hands or body at high speed!
There’s no need to go too deep, just enough to get the wood to dry out a bit, which will lead to the demise of the pests no longer protected inside. In this picture I’ve used large galls quite late in the season to make it easier to see the pests. Ideally you should slice off the side of the citrus galls when they first appear, this will minimise the cosmetic damage and make it easier for the tree to heal over. When galls are advanced, the little grubs have eaten a lot of the wood inside, making the branch brittle and prone to breakage, and are best pruned off.
If the citrus galls are covered in little holes, then it’s too late, the young wasps have already hatched, chewed their way out of the gall, and flown off to infect new citrus spring growth.
Here’s a close-up of an old citrus gall from which the gall wasps have already emerged. If this has happened and the branch becomes unproductive, then it may be a better option in the long term to prune the gall off to get stronger new growth.
Gall Wasp Mechanical and Physical Controls
The next category of controls, mechanical and physical controls block pests out, make the environment unsuitable for them, or kill them directly.
1. Insect Traps
The reproductive cycle of the pest can be broken by trapping the adult pest. Citrus gall wasps traps can be purchased and hung in trees to provide another cultural control method against this pest.
A citrus gall wasp trap is a long plastic cylinder covered with horticultural glue, a very stick pest-trap glue similar to the type used for glue-banding trees to stop pests climbing up the trunk. The yellow colour is attractive to many pest insects. The trap also contains a lure, a chemical inside it, which draws the pest in by scent.
One of the biggest mistakes people make with gall wasp traps is forgetting to twist the top to open the hole at the side so the lure scent can waft out, and then they complain that these traps don’t work. When the hole for the lure chemical is closed shut, the trap is just a sticky yellow plastic cylinder!
These traps last for about 3-4 months, after which the lure chemical runs out and the sticky surface gets completely covered in insects. They’re hung in the tree in August, the last month of winter, before the pests emerge in spring.
People mistakenly call these traps pheromone traps, but the lure is a general pest attractant, and not a pheromone. There’s an important difference here, a pheromone is a specific substance that acts as a chemical messenger to attract a specific species only and elicit particular behaviour.
The lure in these traps is food grade ammonium bicarbonate, which releases a faint ammonia smell as it slowly decomposes into ammonia, carbon dioxide, and water. Since ammonia is naturally released during the decomposition of proteins and related substances, some insects use the smell of ammonia to guide them to food and to stimulate eating, so ammonium bicarbonate acts as a general feeding attractant for insects.
These traps will catch some flies and a few other insects too, I’ve been told by an entomologist that they’ll trap some beneficial wasps, but I’ve never seen other beneficial insects such as bees, hoverflies, lacewing, ladybirds, spiders or praying mantises on these traps.
Citrus gall wasp trap ready to be replaced, this one’s been out for a while and completely covered in bugs.
The glue on these citrus gall wasp traps is very strong, so it’s best to hang them out of reach of curious pets and children.
If you’re worried about larger non-target insects or small birds making contact with the trap, you can build a gall wasp trap enclosure, small cage to house the trap in, which will let only smaller insects through, as pictured below.
Some citrus gall wasp traps can also be purchased with protective cages, which are removable and reusable, like the one pictured below. At the end of winter, when gall wasp traps are replaced, I unclip the cages off the old traps and clip them onto any new traps that I’ve purchased which didn’t come with one.
If your neighbours have an unmanaged citrus tree that is badly infested with gall wasp which overhangs your fence, you can hang a gall wasp trap on their tree as a preventive measure. I’ve done that with my neighbours tree, and it helps!
2. Calcined Kaolin
You can also spray a citrus tree with calcined kaolin, to protect it against gall wasp. This product is sold as Surround WP in the agricultural sector, and under various other names in the consumer gardening industry.
Calcined kaolin is a form of clay, and is certified for use in organic gardening. It forms a white film over stems and leaves, reflecting the sun’s rays to prevent fruit sunburn, which is it’s traditional agricultural use.
This natural substance also acts as a barrier to pests, and has been used for this purpose by European organic farmers for quite a while now. The layer of calcined kaolin makes it hard for insects to walk on and presents an unnatural surface for them to interact with, causing irritation, confusion and an obstacle to feeding and egg-laying. It may also create a barrier which prevents fungal spores establishing on leaves.
Calcined kaolin is sprayed on leaves, stems and and fruit – avoid spraying on flowers, as they don’t function to well with a layer of clay over them! It’s effective in control of:
- Gall Wasp
- Codling Moth and Oriental Fruit Moth
- Powdery Mildew
- Stink Bugs
For control of gall wasp, spray on new green shoots and branches which the pests lay their eggs into, they can’t attack branches when they have hardened off and become woody.
Calcined kaolin can be combined with a scent-lure yellow sticky gall wasp trap for more effective pest control levels.
How well does it work? According to the New South Wales Government, Department of Primary Industries article, Clay spray drives wasps away, published 6 Sep 2016:
“DPI entomologist, Jianhua Mo, said results from Sunraysia trials in 2015-2016 showed spray treatments with calcined kaolin clay could reduce the amount of damaging galls by more than 90 per cent.
“Two applications, one just before and another during CWG emergence, of the commercially available clay reduced the size of large galls, those more than 100 millimetres long, by 99 per cent and average gall size by 35 per cent,” Dr Mo said.”
That’s quite effective!
How to Use Calcined Kaolin Spray to Control Citrus Gall Wasp
The recommended application rate by the government agricultural agencies is 2X Surround sprays at 2.5‐5 kg/100 L at CGW emergence, which reduced galls by >90%.
- 2.5‐5 kg of calcined kaolin to 100 L of water, or
- 25-50g of calcined kaolin to 1 L of water
Spray at all angles to get nice, even coverage all over the new citrus growth, spraying to point of run off, or ‘near drip’.
Only two applications are required each year, approximately one month apart.
- For the first application, use 50g (or 200ml) of calcined kaolin to 1 L of water , spray early spring (September-October in southern hemisphere, i.e. Australia or March-April in the northern hemisphere, i.e. US).
- The second application is made to maintain coverage, so you can use a half-strength spray of 25g (or 100 ml) of calcined kaolin to 1 L of water, making sure that trees are well covered in late spring when the gall wasp populations are at their highest (mid to late November in southern hemisphere, i.e. Australia or mid to late May in the northern hemisphere, i.e. US).
Calcined kaoiln does not wash off very easily, even under heavy rain, but if needed, spraying can be repeat monthly using half-strength just like the second spray.
Most citrus flower in early spring, but Eureka lemons tend to flower and fruit throughout the year, so when spraying calcined kaoiln, try to avoid spraying the flowers, pollinator insects can do their job when the flowers are covered in a protective film of clay.
I have worked out the conversion of weight to volume for gardeners who don’t have scales to weigh the calcined kaolin.
- Calcined Kaolin: 25g = 100ml into 0.5 L of water, or
- Calcined Kaolin: 50g = 200ml into 1 L of water
Only mix as much as you need, as calcined kaolin can’t be stored once it’s mixed.
Remember, only the new green growing shoots and branches need to be sprayed, citrus gall wasp can’t attack the branches once they’ve hardened off and become woody.
I use a small 500ml (0.5 L) spray bottle to avoid mixing up too much. if there is any left over, it can be sprayed on young developing apples to deter codling moth, or sprayed on other fruit to prevent sunburn.
Gall Wasp Biological Controls
Pests can also be controlled by using biological control agents, their natural enemies, to keep their populations in check.
Biological controls such as the parasitic wasps Megastigmus brevivalvus and Megastigmus trisulcus discussed earlier, which parasitise the pest wasps eggs, are not viable in the much cooler southern states of Australia. In geographical locations where these natural predators can naturally establish themselves, they will offer some biological control, but in areas too cold to support them, they cannot be introduced as a biological control agent.
Gall Wasp Chemical Controls
Chemical controls are always the last resort option in integrated pest management (IPM), because most chemicals will kill beneficial insects and consequently make the pest problem worse. Synthetic chemical controls also tend to be very toxic, they usually have a significant negative environmental impact and present an unnecessary risk to human health.
Chemical controls are nowhere near as effective as people imagine, and toxic synthetic chemicals tend to cause as many problems as they solve. Some people just don’t understand how spreading poisons in our environment eventually poisons us and everything else in the end, and they’ll always ask what the farmers use and whether they can use the same.
The pesticides used in agriculture for gall wasp control are Schedule 7 (Dangerous Poison) pesticides in Australia. These have high to very high toxicity, are extremely hazardous and dangerous to health and have a high potential for causing harm at low exposures. They require special labelling, handling and use and are not available to the general public. A restricted chemical permit is required for agricultural use in Australia, and the pesticide is only permitted to be used only as specified on APVMA approved label or permit.
These are the very reasons why only farmers with restricted chemical permits are allowed to use these extremely toxic pesticides. Imagine what would happen in urban areas if the irresponsible public could use these pesticides. They would poison themselves, their children, pets and neighbours in no time. The fact that farmers can use pesticides this poisonous on food is a real concern.
Now, to put pest management into perspective, consider the problem that citrus gall creates for a backyard gardener, and then compare this to the shocking risk of spraying something that causes cancer in people, hangs around the soil for years and makes edible plants toxic from top to bottom, and is extremely toxic bees – that’s what the chemicals which are used, or were used and banned do! Still want something to spray?
Calcined kaolin works if we want be be spraying a control for gall wasps!
It should be obvious by now declaring chemical warfare on pests with extremely poisonous synthetic chemicals is a recipe for disaster.
In conclusion, by using an IPM approach and combining various safe control methods, gall wasps can be effectively controlled and pest populations can be significantly reduced over time.
Other articles on citrus problems and how to fix them:
- Citrus Problems – Why Citrus Fruit Drops and Flowers Fail to Develop
- Citrus Nutrient Deficiency – Yellow Leaves
- Citrus Problems – Why Is My Citrus Tree Dying?
- Citrus Problems – Why Citrus Fruit Splitting Occurs and How To Prevent It
- Citrus Nutrient Deficiency – Yellow Leaf with Green Veins
- Citrus Problems – Citrus Yellow Veins on Green Leaf in Winter
- Citrus Problems – Citrus Fruit Has Thick Peel and Hollow Core
- New South Wales Government, Department of Primary Industries – Citrus gall wasp in Southern Australia, October 2016 Primefact 2010, 3rd edition, NSW DPI Citrus Research and Development Team
- New South Wales Government, Department of Industry and Investment – Primefact 1010, Citrus Gall Wasp, May 2010
- New South Wales Government, Department of Primary Industries – Gall wasp project update – 2018 03 :Autumn chemical control trial results, Andrew Creek and Dr Jianhua Mo (NSW DPI): March 2018
- New South Wales Government, Department of Primary Industries – Clay spray drives wasps away, 6 Sep 2016
- New South Wales Government, Department of Primary Industries – Gall wasp project update – 2017 08 :counting and kaolin clay, Aug 2017
- Government of Western Australia, Department of Agriculture and Food – Citrus gall wasp in Western Australia
- Government of Western Australia, Department of Primary Industries and Regional Development, Agriculture and Food division – Citrus gall wasp control, 14 June 2019