Elm leaf beetle
Whilst this pest is not a problem at Sophie’s Patch, I see people all over struggling with it and I feel I want to help point out the options available to control or at least manage it. You may not have this problem in your garden, however you may struggle with another pest problem, and my advice to you is to understand what is being recommended and even question treatment options, as they too may be harmful.
This insect pest was first identified in Adelaide by an arborist in 2011 and is now found throughout Adelaide on Europeans elms (Ulmus species) such as golden elms and Japanese elms (Zelkova species), although it does not affect the common form of Chinese elms (Ulmus parvifolium). A minor infestation of elm leaf beetle will not kill a tree, however a heavy infestation may cause a tree to defoliate. If healthy it will put out fresh new leaves the following spring however repeated infestations can cause the tree to decline and die. Having said that, as with most pest and disease problems, healthy trees are less likely to be severely affected in the first place.
Neonicotinoids – the conventional treatment
The conventional treatment being offered to home gardeners involves tree injection with neonicotinoids, a the family of supposedly ‘low toxic’ insecticides which is under worldwide scrutiny due to their effect on pollinators such as bees and beneficial insects, and they are banned in a number of countries in Europe due to their link to worldwide bee decline. Overseas studies have also linked this group of with aquatic and bird life decline. Some are also suggesting elm leaf beetle control, can be achieved with soil drenches of neonicotinoids or tablets of neonicotinoids pushed into the soil, however caution should be used here as there is little thought given to the effect of these chemicals on the soil biota.
I should point out that many of my colleagues in the horticultural media have different opinions to me about neonicotinoids and don’t see these chemicals as being a problem, however I feel we should be investigating alternatives. Worldwide bee decline is a scary thing and while it is quite a complicated issue, insecticide use is part of it. If some countries have banned them, I think we should think things through – this is the wrong thing to be wrong about!
Lifecycle of the elm leaf beetle
Before we look at possible treatments to control the problem, we must first understand the lifecycle of the elm leaf beetle. Beetles emerge from sheltered places such as leaf litter and debris, or wood piles in spring and fly up to the leaves where they chew ‘shot holes’ in them and lay their eggs on the underside of the leaves. When the beetle larvae hatch they also feed on the leaves, skeletonising them by eating all the leaf apart from the veins. When they are mature, they crawl down the trunk of the tree and pupate somewhere sheltered, before emerging as an adult beetle, starting the whole process over again. Two or even three generations of beetles may occur between spring and autumn when they go dormant till the following spring. Fact sheet.
Integrated pest management options
I would love to say there is an easy organic alternative to using neonicotinoids, but there is not! As with any Integrated Pest Management approach there are a number of factors to consider:
- Tree health – a healthy elm tree is less likely to be affected than a tree under stress so first and foremost look at your tree and consider how well it is watered and nourished. Many gardeners around Adelaide found that the problem is particularly bad after a dry year, for example we had a very dry spring in 2015 and by mid-October the soil moisture level in many areas was as a level we wouldn’t expect till February. So if your tree wasn’t being given extra supplementary watering, of course it was under stress and the beetle would be likely to be a big problems by summer. Also be aware of the negative effect of soil compaction over the tree’s root zone, from driveways, carparks or similar. I would also suggest that there could be benefits from looking at soil health by encouraging the soil biota with probiotics for soil or compost teas. Even though we would usually recommend mulching a tree, it may even be worth keeping the mulch away from an elm tree so there is no shelter for the larvae to pupate.
- Tree banding with ‘fruit tree grease band’ (available from garden centres and hardware stores). This band is a plastic strip smeared with non-drying glue and is wrapped around the tree trunk at several points with the sticky side out. Then larvae that head down the tree to pupate will get caught however this band will need to be changed monthly or as soon as it is covered in larvae. Some people have been doing this using gaffer tape however this does not last as well and needs to be changed weekly. Due to the ridged structure of elm bark it would be worthwhile making several bands as some larvae will be able to sneak past underneath the band. Once you have applied this band you can simply top it up with an additional smear of non-drying glue such as ‘Pest Barrier Tree Guard’ on top of the plastic tape. These methods will not necessarily reduce damage from the first generation of beetles to emerge in spring, however they should reduce beetle numbers and damage in subsequent generations in the first and subsequent years.
- Predators. Last summer I trialled the release of lacewing larvae which are general insect predators on a client’s large golden elm at a commercial premises. This tree is a magnificent specimen however it is planted next to a bitumen car parking area, so half of its roots system would not have direct access to water and could be suffering from soil compaction. Under the guidance of Stuart Pettigrew, an Integrated Pest Management specialist who runs Bug Central (www.bugcentral.com.au), we released lacewing larvae up into the crown of this golden elm. T here have been six releases so far, about 2 to three weeks apart since we first noticed a few holes in the leaves in mid-November. We monitored the pest regularly and while the pest was still present, the tree remained healthy looking and there was evidence of the pest being eaten by something. There was still some minor leaf damage from the pest however it was significantly better than the previous year with no treatment. The aim of releasing the lacewing larvae is to set up a resident population, so that in the long term the pest problem may reduce as we see a build-up of resident lacewings. We limited our lacewing releases to just three, which was about the same cost as doing tree injection. Now neither lacewing release or tree injection are a cheap option … however what price can we put upon bee decline? If it turns out that lacewings are not the answer, the let’s investigate other biological control.
If you have never read anything about neonicotinoids please read these links Where have all the ladybirds gone? Part 1 and Where have all the ladybirds gone? Part 2 written by two Australian IPM guys Stephen Goodwin and Marilyn Steiner. You can also read two articles from The Australasian Beekeeper at Neonicotinoids in Australia – part 2 which only came out in August 2016 or the first article Neonicotinoids in Australia written in 2011.
Looking forward, what can I suggest to home gardeners? Do not plant elm trees (or just stick with the common Chinese elm), be aware of the general health of your elm tree and investigate alternatives. Some gardeners have told me that they are trialling chooks or guinea fowl free ranging under their trees, as a way to manage the pests when they pupate on the ground. I am not sure what the best solution for elm leaf beetle will be, but let’s at least look at alternatives to using a chemical linked to worldwide bee decline.
Be aware that few arborists will tell you there are alternatives to tree injection, as that is a big part of their business.