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 parasitize 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.
Control methods for citrus gall wasp
There are several approaches to controlling gall wasp – notice I said controlling and not completely eradicating them, because it can’t be done! The best results are achieved when multiple control methods are employed.
Firstly, we can use cultural controls – practices that reduce pest establishment, reproduction, dispersal, and survival.
Fertilizing – If we consider that adult gall wasps are present in spring and early summer and only attack soft new spring growth, we can reduce the amount of viable growth for them by reducing an excessive spring flush, and we achieve this by not over-fertilising citrus in early spring. The gall wasp problem is exacerbated by the use of high-nitrogen synthetic fertilizers which push out a lot of new soft and sappy growth very fast. Fertilizing later will produce a summer flush of new growth which will miss the peak citrus gall wasp period.
Pruning – In trees that have a few galls only, the galls should be pruned off, put in a plastic bag, tied shut and disposed of in the garbage bin. the other disposal option is to burn any gall-infected twigs and branches before the end of winter. If a tree is heavily infested with citrus gall, it can be pruned hard (do not remove more than one-third of the canopy in any one year as it can weaken the tree) at least one month before the citrus gall wasps emerge. 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.
Watering – 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. Sprinkler irrigation below trees creates more humidity below the canopy, favouring the pest, while drip irrigation, which is a much better choice, creates a drier environment.
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 a lot of 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.
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 small cage to house the trap in, which will let only smaller insects through, as pictured below. You can find instructions on how to build this gall wasp trap enclosure on the Bulleen Art & Garden Nursery website.
Biological controls such as the parasitic wasps Megastigmus brevivalvus and Megastigmus trisulcus discussed earlier, which parasitize 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.
Chemical control is the last resort in integrated pest management (IPM), because they kill beneficial insects and make the pest problem worse, and they poison the environment and people.
According to the NSW Department of Primary Industries Citrus Research and Development Team, “Foliar insecticides are disruptive to IPM and do not always achieve effective CGW control due to the short residual periods of most foliar insecticides.” Similarly, from Government of Western Australia, Department of Agriculture and Food, we receive a similar warning, “Note that pesticides are extremely disruptive to integrated pest management”
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. I’m a toxicologist and a horticulturist, so I’ll answer that.
Why can’t I just use the chemicals farmers use?
Current research by NSW DPI shows that a single application of a soil-applied systemic insecticide provides good control of citrus gall wasp in small to medium sized trees when applied in late spring to target the larvae that hatch from eggs inside the citrus branches. This method will not work satisfactorily for large mature trees. Systemic pesticides are ones that are absorbed by a plant or tree so that the poison circulates through the plant’s tissues, poisoning any insect that eats it. The only soil-applied systemic chemical insecticide registered (permitted) agriculturally for gall wasp control is trademarked Samurai®, and the active ingredient is 500 g/kg clothianidin.
Why can’t you use this in your home garden? Let’s have a look at some information in the Sumitomo Samurai Systemic Insecticide Safety Data Sheet:
SECTION 2: Hazard(s) identification
- Health hazards Carc. 1B – H350 STOT RE 1 – H372
- Environmental hazards Aquatic Acute 1 – H400 Aquatic Chronic 1 – H410
- H350 May cause cancer.
- H372 Causes damage to organs through prolonged or repeated exposure.
- H400 Very toxic to aquatic life.
- H410 Very toxic to aquatic life with long lasting effects.
The rating “Health hazards Carc. 1B” indicates it’s a class 1B carcinogen, which according to the International Agency for Research on Cancer (IARC) rating, makes it a presumed human carcinogen. To better explain this, the IB category is defined as having carcinogenic potential for humans, largely based on animal evidence (that it does cause cancer in animals). That doesn’t mean that humans aren’t animals and aren’t affected, just like every other animal, it means that this chemical hasn’t been tested on human, for obvious ethical reasons, and there haven’t been enough careless human accidental exposures to provide sufficient data.
On top of being a class IB carcinogen, clothianidin is also a neonicotinoid, the class of insecticides highly toxic to bees, implicated in wide-scale bee poisoning and banned in parts of Europe. The safety data sheets lists the LC50 Honey bees (acute oral toxicity) as 0.00379 μg/bee and the LC50 Honey bees (48-hour acute contact) as 0.04426 μg/bee. To put this into perspective, compare this to the bee toxicity ratings below.
- If the LD50 Honey bees (acute contact) is less than or equal to 2 micrograms per bee, the pesticide is classified as Toxicity Category I, “highly toxic to bees.”
- If the LD50 Honey bees (acute contact) is less than 11 but greater than 2 micrograms per bee, the pesticide is classified as Toxicity Category II, “toxic to bees.”
- If the LD50 Honey bees (acute contact) is greater than 11 micrograms per bee, the pesticide is classified as Toxicity Category III), “relatively nontoxic”, and no bee caution statement is required on the label.
The bee toxicity of clothianidin is over 45 times more toxic than the level required to classify a pesticide as “highly toxic to bees.”
According to the organization Beyond Pesticides, clothianidin is a member of the the neonicotinoid family of systemic pesticides, which are taken up by a plant’s vascular system and expressed through pollen, nectar and guttation droplets from which bees then forage and drink. That means that every part of the plant becomes poisonous, posing a high risk to bees.
Looking at data from United Nations Food and Agriculture Organization (FAO), we see that Clothianidin is also highly persistent.
- An aerobic soil degradation study was conducted with three different soils.Calculated half lives (DT50) were 143, 227,and 490 days for silt, silt loam, and loamy sand, respectively.
- A second aerobic soil degradation study was conducted with two silt loam soils.Calculated half lives (DT50) for the silt loam soils were 541 and 808 days.
- A photolysis study was conducted on a soil surface.The half live was calculated as 8.2 days.
What this means is that clothianidin stays in the soil, and stays toxic for a very long time, and any plants growing in the contaminated soil will take up the pesticide.
The US EPA conforms the FAO findings and also tells us that Clothianidin is highly mobile, gets carried with water to non-target locations and can end up leaching into groundwater. Here is an extract from the USA EPA document on Clothianidin:
“Clothianidin appears to be a persistent compound under most field conditions. Based on analysis of the laboratory studies alone, the major route of dissipation for clothianidin would appear to be photolysis if exposure to sunlight occurs (e.g., the measured aqueous photolysis half-life was <1 day and aerobic half-lives were 148 to 1155 days). Although photolysis appears to be much more rapid than other avenues of degradation/dissipation of clothianidin in the laboratory studies, the very slow rate of dissipation that was observed in field studies suggests that photolysis probably is not significant under most actual-use conditions. Photolysis may be quite important in surface waters if residues have reached clear bodies of water and are in solution rather than bound to sediment. Clothianidin is stable to hydrolysis at environmental pHs and temperatures. Degradation is also relatively rapid under anaerobic aquatic conditions (overall half-life of 27 days); however, metabolic degradation occurs very slowly in aerobic soil. Clothianidin is mobile to highly mobile in the laboratory [soil organic carbon partition coefficients (Koc) values were 84 to 129 for all test soils except for a sandy loam soil which had a Koc value of 3451, although only a modest amount of leaching was observed in the submitted field studies. Previous studies have confirmed that compounds with a similar combination of mobility and persistence characteristics have a potential to leach to ground water at some use sites. Volatilization is not expected to be a significant dissipation process.”
This pesticide is insidious in its capacity to contaminate produce. If we look at the “Sumitomo Chemical TECH UPDATE – SPRING 2016, Samurai® for control of citrus gall wasp (Bruchophagus fellis) in grapefruit, lemons, limes, mandarins and oranges” media release, we see what happens when it’s used on citrus.
“Withholding period – Harvest: DO NOT harvest for 19 weeks after application. Grazing: DO NOT graze treated area or cut treated area for stock feed.”
The citrus becomes toxic and can’t be eaten for 19 weeks, or nearly 5 months! That doesn’t mean that residue levels are zero after that, there are warnings in the same document regarding residual pesticide levels in produce destined for export.
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 this stuff. 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. So, do you still want something to spray? Want some more chemicals?
The Government of Western Australia, Department of Agriculture and Food, tells us that “The only insecticide currently registered for control of citrus gall wasp is methidathion (Supracide®). Supracide® is a non-systemic pesticide. It kills adults on contact and needs to be correctly timed to be effective. Apply Supracide® to the spring flush of infested trees four weeks after gall wasp emergence is completed. As this pesticide is currently under review and may be removed from use, consult Infopest or the Agricultural Pesticides and Veterinary Medicines Authority (APVMA) prior to use.”
It’s no surprise that it’s under review and may be removed from sale! Methidathion is an organophosphate pesticide, this class of pesticides was derived from nerve gas developed during World War II. Its use is banned in the European Community. It interferes with the transmission of nerve impulses and has been implicated in the poisoning of farm workers and children in rural areas, leading to neurological and developmental problems. The EPA has also classified the pesticide as a possible human carcinogen.
Methidathion is among the top pesticides associated with pesticide poisonings, and in 2001, the Canadian Pest Management Regulatory Agency cancelled all methidathion registrations, noting the high risk it posed to workers and the environment, especially when safer alternatives were available. The California Department of Pesticide Regulation listed methidathion as a toxic air contaminant in 2008, because of its neurotoxic effects, its carcinogenic properties, and its capacity to drift far from the farm fields where it is used, being found in the air in distant places such as in Sequoia National Park.
It should be obvious by now declaring chemical warfare on pests with extremely poisonous synthetic chemicals is a recipe for disaster.
Now that we’ve got that out of the way, there’s one more cultural control I want to discuss.
The problem with dwarf citrus – when NOT to prune
Dwarf citrus are regular citrus grafted onto a dwarfing rootstock, suck as ‘Flying Dragon’ rootstock. There are very few ungrafted genetic dwarf citrus varieties around. The thing to consider with dwarf citrus is that they 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 you do the maths, you 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 you prune off 30cm off the top, you’ve lost a years growth! Is this a problem? I’ve seen far too many cases where I’ve been shown tiny, sorry looking citrus trees in pots and told they were a few years old, that had to be pruned for gall.
If we don’t prune dwarf citrus, what can you do to prevent the spread of gall wasps? We can use Nature to help us out, especially the cold winter winds.
Some new gardeners mistakenly plant evergreen plants and trees in winter (spring or autumn as second preference is the time to plant evergreens) and wonder why they die. They assume there’s plenty of water in winter, but they forget that there’s no root growth, the roots are no larger than the pot they came in, and that cold winter winds can really dry plants out. When the pot-sized root ball uses up the small amount of water it can reach, and there’s no rain for a few days, the evergreen tree or plant dries out and dies from lack of water – in winter.
We can use the drying effect of cold winter winds (or warmer season winds if we spot the galls that are beginning to form early enough) 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. 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!
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 shave the sides of the citrus galls of 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.
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.
In case you’re wondering, if your neighbours have an unmanaged citrus tree that is badly infested with gall wasp, if it overhands your fence, as a preventive measure you can hang a gall wasp trap on their tree. I’ve done that with my neighbours tree, which is more gall than lemon tree but somehow manages to produce more lemons than anyone can use.
- NSW Department of Primary Industries – Citrus gall wasp in Southern Australia, October 2016 Primefact 2010, 3rd edition, NSW DPI Citrus Research and Development Team
- Government of Western Australia, Department of Agriculture and Food, Citrus gall wasp in Western Australia
- Beyond Pesticides – Clothianidin & CCD Fact Sheet
- United Nations FAO – Clothianidin Toxicology
- US EPA-Pesticides – Clothianidin
- Beyond Pesticides – Lawsuit Challenges EPA on Four Deadly Pesticides, April 7, 2008