LDD moth update

LDD moth update

Ontario has once again been hit hard by the invasive LDD moth (gypsy moth). They’ve been nearly impossible to ignore in most areas of southern Ontario as large numbers of invasive caterpillars have been seen crawling over almost everything. As forecasted by the Ministry of Natural Resources and Forestry (MNRF) earlier this year, many areas have experienced severe defoliation on a variety of trees and shrubs.

Status of the Outbreak

By now most of the caterpillars are in their last stage of development before their transition into moths. We see mostly large caterpillars and in some areas the caterpillars are already pupating (forming a cocoon). They’ll remain in the pupa stage for 10-14 days before emerging as a moth.

Large LDD caterpillar seen in Caledon on June 30.

In late July, we can expect to see fewer caterpillars and more moths. This is a good thing for our trees as the moths focus their attention on breeding and egg laying, not feeding. As we reach this point it gives most healthy deciduous trees a chance to send out new leaves before they’re fully depleted of energy and resources. Unfortunately, if caterpillars have moved from your deciduous trees to the conifers, there is less of a chance that the conifers will survive a severe defoliation as they do not produce more needles this time of year.

LDD pupa with old egg mass beneath.

The good news is that we’re starting to see the effects of natural control methods including the nuclear polyhedrosis virus (NPV), which is harmless to people but lethal to LDD moths, spreads through contact between infected caterpillars. It can also be spread through the feces of birds that eat dead or dying caterpillars. The virus is more effective at spreading when there is a high density of caterpillars. The fungus Entomophaga maimaiga, also harmless to people and lethal to LDD moths, overwinters in soil and infects the caterpillars. It needs cool, wet weather to persist and be effective.

Dead caterpillar due to NPV in a V shape.

Dead caterpillar due to the fungus.

What can you do?

As the transition from caterpillar to pupa to moth happens, we can help limit future outbreaks on a local scale and reduce the chance of another year of severe defoliation.

Collect and dispose of pupa and egg masses in the coming weeks. Once the moths emerge, they start to lay egg masses about the size of a toonie that can hold up to 1,000 eggs. To remove them, use a strong edge to scrape the pupa and egg masses into a container of soapy water. A butter knife, coin or drywall trowel works great. Then let them sit for 48 hours in the solution before disposing of them. Make sure you use a lid to stop any escapees and stir the mixture well so that the soap doesn’t settle out.

Female LDD moth laying egg mass.

Learn more about the LDD moth, what CVC is doing and read answers to frequently asked questions on our LDD page.

9 Comment
  • Shane Dunworth Crompton says:

    Can the caterpillars cause skin irritation?

    • Hi Shane. Yes, the hairs on LDDs (called setae) contain histamine which for some people can cause an allergic reaction on the skin if touched. This rash is usually mild and will generally clear up on its own within a week or so. This is why we recommend using gloves and other protective wear if you are dealing with LDD moths (the egg masses also contain these hairs). If you experience more severe symptoms please contact a medical professional.

  • Ann says:

    Any particular type of soap? ingredient? I use biodegradable soaps – will these work?

    • Hi Ann. Great question! Biodegradable soap should work fine, the type of soap typically mentioned for LDD control is common dish detergent. There are no special ingredients that are required, some sources mention other substances being added to the water mixture such as a small amount of bleach or rubbing alcohol. However, soap alone is more often recommended because it is safer to handle and effective. The key thing to keep in mind is to mix them well into the soap mixture and let them sit (covered with a tight fitting lid if possible) for 1-2 days. If you are trying to kill the pupae then you can leave them in the mixture for 5-6 days or longer.

  • Libby says:

    I purchased some moth traps not sure when I should put them up. Don’t want to put them out to soon that the lure become ineffective or to late and the male moths can mate

    • Hi Libby. We have already seen the moths emerging in some parts of the watershed. Have a look around near the trees in your backyard, if you are seeing large (3cm) tan-brown moths that are energetically flying during the daytime, hang the traps at that time. If you are mainly seeing pupae (cocoons) that are still occupied then I would wait a few days and check again. Keep in mind that not all LDD moths pupate or emerge at the same time even in the same area. Also pheromone traps, while they do provide a certain amount of control are generally used more as monitoring tools. Unfortunately, no matter what there will still definitely be moths in your area that successfully mate so it is important to consider other methods of control such as egg mass scraping. Thanks so much for your question and your efforts!

  • David Clayton says:

    Have seen fewer caterpillars this year than last and have controlled by hand picking. Have seen some V-shaped as if by virus and am now catching a few in pheromone traps. Infestation seems to be highly localised – some trees bare but the same species a few metres away not touched.

    • Hi David. Thank you for your efforts and keen observations! And yes, while we don’t have exact figures, there are many reports of NPV infection killing LDD caterpillars in our watershed this season. This is a normal part of the LDD outbreak cycle, NPV can kill up to 75% of larvae in heavy outbreak years! It is also true that some trees will experience more defoliation than others even on the same species in the same area. Some trees just develop more natural resistance to insect attacks than others. Another reason may be that when they are very small the caterpillars are able to move on strands of silk in the wind, but as they get larger they are only able to crawl so they often do not move too far if they are already on a suitable host.

  • Lee K Tovey says:

    Thank you. Informative and the pictures are most helpful with theidentification and removal of egg masses.

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