September 14, 2022

Gypsy Moth Caterpillars – What to Expect This Summer 

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Gypsy Moth Caterpillars – What to Expect This Summer

Spongy moth caterpillars (formerly known as gypsy moths) plagued our area last year! If you were one of the unfortunate ones removing buckets of caterpillars off your trees, I feel your pain. 

Unfortunately, they’ll be interrupting our backyard BBQs again this year – but not quite as bad as last summer. Egg mass surveys in the area suggest a significant population decline this season. It looks like we’re on our way out of this infestation! 

Spongy Moth’s

What is the spongy moth? 

Spongy moths are an invasive species from Europe that devour leaves, wreaking havoc on our trees and gardens. They can also leave you with an itchy rash after skin contact. Tiny hairs on the caterpillar (called setae) cause a histamine reaction in some people, leading to small, red bumps. 

While spongy moths don’t pose a danger, they are certainly a huge inconvenience! Mother Nature has equipped most trees with the ability to withstand defoliation for a few years, but others will not fare as well. A severe infestation makes the trees more susceptible to drought and other pests, setting them up for a poor growing season. 

How do infestations happen? 

The spongy moth population is cyclical. They pop up every 7-10 years and harass us for a few years before dying down again. 

The population crash is often caused by a fungus called Entomophaha Maimaiga or the NPV virus (Nucleopolyhedrosis Virus). (Say that 3 times fast) 

NPV is a naturally occurring virus that spreads rapidly during population outbreaks. If you see dead caterpillars in a V shape on your trees, that’s the NPV virus at work!  

The fungus remains in the soil from year to year, effectively killing off the caterpillars. And, of course, some of them fall prey to birds and mammals (or gardeners with a scraper and bucket of soapy water).   

What do I do if spongy moths return this year?

Look for egg masses on your trees, furniture, and woodpiles to get an idea of how badly you’ll be impacted this season. Healthy egg masses are slightly larger than a quarter, either tan or brown and firm to the touch. These tiny masses can contain up to 1000 eggs!  

While your chances of getting every caterpillar off your property are slim, you can greatly reduce the number of these critters in your yard. 

1. Scrape off the egg masses. 

If you see healthy egg masses on your trees, scrape them off and either burn them or drop them into a bucket of soapy water (and leave them in the bucket for a few days). If you notice masses that are white and soft with small pinholes, those are from last year and probably don’t contain any live eggs. 

2. Tape your trees. 

The caterpillars’ hatch during April and May, fall to the ground, and then travel up your trees in search of leaves to snack on. Placing a sticky barrier around the trunk can trap them and end their travels! Grab a roll of duct tape (it really is great for everything) and twist a band around the tree, with the sticky side out. Replace it when it loses its stick. 

3. Create a burlap skirt. 

Adult caterpillars seek shade during the day in May and June and move back up to the canopy for a nighttime feast. Creating a burlap skirt around the trunk traps the caterpillars as they climb so you can then remove and discard them. 

To create your burlap skirt: 

  • Wrap a layer of burlap around the tree (approximately 18 inches wide)
  • Tie twine around the centre 
  • Fold down the top to make a ‘skirt.’

4. Spray heavily infected trees. 

If caterpillars have heavily infested your trees, and you can’t remove them by hand, spraying is an option. You can purchase BTK (Bacillus thuringiensis kurstaki), a biological insecticide designed specifically to kill caterpillars. The best time to spray is between mid-May and early June.

5. Take good care of your trees. 

Healthy trees will be able to withstand a spongy moth infestation. Water your trees during heavy dry spells and protect their roots with mulch. 

It looks like we will fare better this year. Start now to seek out signs of the spongy moth caterpillar in your yard, and know we are moving towards the end of this infestation!