Jul 302013
 

Citrus Gall Wasp

Many of our more persistent garden pests are not native to Australia but citrus gall wasp is definitely an Aussie grown garden variety pest. Originally, this native wasp was limited to Queensland and northern New South Wales and its preferred host was native finger limes.  But citrus gall wasp has rapidly adapted to the wider variety of citrus fruits now on offer.  Since the 1990’s it has successfully migrated from Queensland, through NSW and can now be found as far south as Melbourne where it is virtually endemic in the iconic back yard lemon tree. 

What: Adult citrus gall wasps are rarely seen as they less than 3mm in size. The adults are poor flyers but can be windblown from other citrus trees nearby. The adult wasps mate in early to late spring when the female implants her eggs in the citrus tree that she herself emerged from just days before. Each female can lay up to 100 eggs, usually under the bark of new shoots, and the larvae hatch after 2 – 4 weeks. Often the larvae are already present in newly purchased citrus trees in spring.  The wasp larvae grow within the soft stem tissue for 9 to 12 months until they too pupate and emerge as adult wasps the following year.  Now that the effects of climate change are being experienced, the life cycle may be repeated during a single year instead of just egg-laying in spring.

Plants affected: All citrus especially lemons and grapefruit.

Damage Caused: The wasp larvae grow within the citrus stems until late summer when gardeners start to notice unsightly galls appearing on their trees.  These galls or calluses are formed in response to the presence of the feeding larvae. Galls cannot be ‘cured’ or reversed.  Old galls are unsightly but are also empty as the adult wasp will have left through the tiny exit holes.  Developing galls can be removed but this may also mean the loss of developing fruit at the end of the infected stem. Citrus gall is more damaging to younger citrus trees than older trees.

Control Methods: Controlling citrus gall wasp can be difficult but damage can be minimised by:

  • Avoiding high nitrogen fertiliser in spring as this promotes soft sappy growth – just perfect for the egg laying stage.  Feed trees in late autumn and early winter instead.
  • Removing all newly formed galls that don’t show signs of exit holes before the end of winter.  Old galls have already been exited.  Prune only a maximum of 1/3 of the tree to avoid stressing it too much.
  • From mid-August, hanging yellow sticky traps with a chemical attractant inside to trap emerging adult wasps. The yellow is an attractant and the sticky coating makes it impossible for the wasp to escape.  Remember to twist the top so that the chemical is released. Since beneficial insects may also be attracted to this trap, do not leave on after November as the wasps are no longer about and even small birds may become trapped.
  • Destroying infected stems by burning or bagging.

Photographs:

Pic 1, 2 & 3: Elaine Shallue, SGA

  15 Responses to “Citrus Gall Wasp”

  1. I have gall wasp on my lemon tree & this morning I was having my morning lemon & water drink & as I was sucking it through a straw I noticed what looked like a worm. On closer inspection it was & I’m troubled now at the thought of having ingested these worm things before I saw this one & am wondering what happens if I have? Can you help?

    • It definitely had nothing to do with gall wasps which don’t enter the fruit. If it was, indeed, a worm (it could have been a bit of seed or something else quite harmless) it wouldn’t be a health problem because the extremely acid conditions in your stomach would kill it.

  2. Do the galls affect the thorns? I pruned my tree back to the trunk a few months ago, its since come back nicely but on two of the new branches there is one thorns with a thickened woody appearance that looks like it could be gall wasps, but I’m unable to find reference to them affecting thorns.

    • It would be very difficult to distinguish stem and thorn if the gall in the stem under the thorn.

  3. What is the best time to prune fall wasp in melbourne

    • Towards the end of winter is probably best, but because the wasp is becoming more widespread and the climate is changing, it would be wise to inspect trees moderately frequently and prune out any galls that DO NOT already have holes in them

  4. Hi guys.
    Just a quick one but I’m a consultant in the commercial citrus industry plus heavily involved in Citrus Gall Wasp control options.
    The only reason I’ve found this page is from a client who printed this page out and brought it into my work asking for pheromones.
    Firstly there are no pheromones currently available that work against Gall Wasp. Citrus Australia have spoken to an entomologist from Adelaide University recently to do research on Gall Wasp and pheromones however this work is yet start.
    They are also not attracted to yellow sticky traps. Insects that are attracted to flowers will be attracted to yellow sticky traps and Gall Wasp are not attracted to flowers.
    Currently there are only two proven chemical control options and neither are suitable for non commercial orchards or trees.
    I just thought I’d reply to this post to make a few corrections.
    SharronP has made the perfect recommendations for home trees and pruning galls off is the only non chemical way of controlling them. Spraying trees during the wasp emergence period with Bifenthrin is also effective but needs to be applied weekly and is extremely disruptive to beneficials.
    There are native parasitoids being bred up to counter Gall Wasp but in our region we are finding that by the time they become established the trees will near on be a write off.
    Hope some of that information helps. They are a serious pest of citrus and a number of orchards are at a point that they are no longer economically viable.

    • Thank you, Matt, for your comments. You are right that the attractant is not a pheromone but another chemical. I have changed the post accordingly. What you say about commercial orchards is absolutely correct and the wasps are not particularly attracted to yellow alone. However, the manufacturer of the yellow traps with added attractant has done research which shows that yellow was a more attractive colour for wasps than blue or green. The manufacturer also say that test were also conducted in commercial orchards and found that the traps were not effective in that situation, but were effective in home gardens where the number and density of lemon trees is VERY much lower. We would not recommend bifenthrin at all because of its non-specific effects.

      We look forward to the results of research on parasitoids and pheromones.

    • Ive heard that spraying an oil helps prevent some Citrus leaf miner wasps laying their eggs, could this be the same for a gall wasp?

      • Oil sprays will not help a tree with gall wasp. The only way to deal with gall wasp is to remove the galls if the wasps have not already left the gall. In some cases, a yellow sticky trap containing a wasp attractant, hung on the tree in late winter, might also help a bit. Oil sprays (white oil, or better, a homemade one with vegetable oil, will prevent scale, but not gall wasp.

  5. In the long run you have probably done the right thing – sometimes you need to “be cruel to be kind”.

  6. We’ve been advised that on mum’s lemon tree (she has 1 large & two smaller trees and full of lemons) that she has Citrus Gall Wasp. We are arranging for the pruning/removal of the citrus gall wasp. Once this has been done, is it ok to eat the lemons. I’ve been told that the wasp releases parasites that penetrates through to the fruit. Is this true?

    • Citrus gall wasp does not directly affect the fruit. Its effect on fruit is to decrease production. When the wasps emerge from infested galls, they mate and lay eggs under the bark of the tree where new galls will form after the eggs have hatched. The larvae stay in the new galls to pupate and eventually emerge as wasps. At this time of year you need to be checking the galls to see if there are signs that wasps are still in them or new ones developing and prune them off. There is little point in pruning galls where you can see small holes where the wasps have emerged.

  7. Hi. I have the gall wasp infections on my lemonade tree, here in the west of Brisbane. I have been unable to prune due to an injury, but now I find that the tree has a large second crop coming on. Would I be best to wait till next year to prune off the affected branches. Also, if the wasps have already hatched this year, is there any harm leaving the empty galls on the tree? Thanks for your help.
    Melissa

    • I’d be leaving pruning the galls out for a while. If you do it now you may be sacrificing many good lemons. Observe the tree carefully and if you see new galls appearing, then prune them off. Make sure you put the pruned galls into a plastic bag, seal it and dispose of in the rubbish (not compost or green waste bin).

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