Gypsy moth spraying: the question of the season

Gypsy moth egg masses on a young maple in my neighborhood
A few weeks ago I put an article about gypsy moth caterpillars up on my gardening website. Today I received this email from a reader in Oakville, Ontario:

I am interested in your article about the gypsy moth! We in Oakville have learned that some of our lovely forest areas are to be sprayed with Btk in May.

I was interested in your comments that this infestation doesn't last and in a few years is not present in that particular area where it was a problem. I would really appreciate any more thoughts you may have about this as my question to our local officials was to be: "What happens if you do nothing?" Thank you in advance for any information you may have for us.

This was my reply:

Thanks for contacting me about this issue, which is going to be of great interest and concern this summer. I have noticed that all the big oaks and maples at the local golf course across the road from me are covered in egg masses. If nothing is done, they are going to lose all their leaves this season.

We hand sprayed our own trees with Btk last May (they are not very large as yet, about 15 to 20 feet tall), and the control we got was amazing. I did a visual inspection of the deciduous trees on my property last week, and I found only two egg masses.

You are very fortunate that Oakville is willing to spray Btk in May. It is a safe, natural and effective control. It's true that the populations do crash after they get really bad, even if you don't spray Btk.

However, doing nothing this year could seriously endanger trees because they have been very stressed by almost a decade of drought, culminating in the worst drought in 49 years last year. If stressed trees get defoliated by the caterpillars, they will be in serious danger of further decline, and you could see a lot of large trees dying in Oakville in the next few years.

I would support spraying. It's a small price to pay for all the environmental and quality-of-life benefits of shade trees. Everything is a trade-off, but I believe the trees need us to help them now. With the drought stresses they have endured over the past decade, trees in southern Ontario don't have a lot of reserves. I hope that helps in deciding to support spraying.

The U.S. Department of Agriculture Forest Service has an excellent gypsy moth education website, from which the following information is taken:

The Effects of Defoliation on Trees

The effects of defoliation depend primarily on the amount of foliage that is removed, the condition of the tree at the time it is defoliated, the number of consecutive defoliations, available soil moisture, and the species of host.

If less than 50 percent of their crown is defoliated, most hardwoods will experience only a slight reduction (or loss) in radial growth.

If more than 50 percent of their crown is defoliated, most hardwoods will refoliate or produce a second flush of foliage by midsummer (figs. 11, 12). Healthy trees can usually withstand one or two consecutive defoliations of greater than 50 percent. Trees that have been weakened by previous defoliation or been subjected to other stresses such as drought are frequently killed after a single defoliation of more than 50 percent. (My emphasis, as this is the current situation of trees in southern Ontario.)

The entire article can be found here.
© Yvonne Cunnington, Country Gardener

Foiled the nasties: gypsy moth caterpillars, that is

I don't like dealing with pests and hate to spray anything, so mostly I tend to choose plants that are reputed to be pest-free. But, of course, when you're a gardener nothing is guaranteed, except that you will be plagued with all sorts of problems now and then.

This year one of the worries was a heavy gypsy moth caterpillar infestation. These beasties are voracious eaters that can defoliate entire forests, something we experienced in the early '90s, when we spent a lot of time in Ontario cottage country.

In previous years, we kept them under control by removing them from tree trunks, but this season, the caterpillar population has exploded.

Since we've been gardening here, we must have planted something in the range of 40 to 50 deciduous trees, mostly maples, oaks and ashes. When we checked the trees last week, there were so many caterpillars that we had to make a decision: to spray or not to spray.

The spray in question is the biological insecticide Bacillus thuringiensis subspecies kurstaki (Btk), a natural bacterial insecticide that works against gypsy moths.

The key thing is that Btk must be eaten by the caterpillars when they're very young, just as they start to feed. After they take it in, they get sick, stop feeding and eventually die. The great thing about Btk is that spiders, birds and bees are not affected at all, and neither are people or pets.

The indicator plant for the right time to apply Btk is bridal wreath spirea in bloom. If you don't spray at the right time, you may as well not bother. Btk isn't effective when the caterpillars are big (like the ones in the picture, above).

So we sprayed early last week with a backpack sprayer. It's hard work holding the spray nozzle above your head. Luckily, our trees are still young - 15 to 20 feet tall - and the spray reached the leaves of the lower part branches where most of the caterpillars were. My husband had to spell me off halfway through when my arm got sore.

Today, I looked at all of the trees and, tada, there were very few caterpillars. The ones I found I scraped off and killed if I could reach them. So the upshot of our spraying is that our trees get to keep their leaves this season. My arm was sore for a day or two after the effort, but it was worth it.

If you have gypsy moth caterpillars and the Btk spray window has closed, there are still things you can do:

• pick caterpillars off leaves and soak them in a pail of soapy water
• place sticky bands on tree trunks, where the caterpillers rest during the day (they feed mostly at night)
• tie burlap wraps around tree trunks and then collect and destroy the caterpillars

More information about gypsy moth caterpillars.

Featured comment:

Anonymous said...
We live in the Poconos in Northeastern PA. These disgusting gypsy moth caterpillars have literally invaded our area. Our houses are "moving/crawling structures". The caterpillars have even eaten the evergreens down to sticks. Our brilliant politicians have decided it was expensive to spray the BT and comments were "they will be gone in a few weeks". We are hostages in our own homes. Backyard picnics, graduation parties, swimming in pools, none existent in our area.
Our beautiful landscape has been eaten alive and hopefully will recover. Our area is known for the beauty, which is now nonexistent.
Thanks PA politicians for keeping our area safe. Spending our tax dollars wisely isn't a concern of yours.
Creepy-Crawly Disgusted Pennsylvanian