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