Wednesday, June 06, 2007

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


  1. Oh, shudder...I have not-fond memories of burlap-banding when I was a middle-schooler and my Dad sending me out to pick the caterpillars off the trees. Ick. I'm sorry that you had to spray, but it does sound like you did such a thoughtful job in picking your poison and figuring out how best to use it. I hope the infestation doesn't return next year!

  2. I am west of Toronto Ont. and our neiborhood has an infestation!! My neighbor alerted me as her birches and others are attacked and sure enough both my white and river birch are covered!!!I absolutely have no patience for pests on our trees and gardens! Thank you for this nature friendly insecticide..I'll look out for it at my local garden centre .I was just out there looking at them and I feel they are crawling all over me!!!! NO NO I cannot pick them off! :(NG

  3. Anonymous9:45 PM

    oh no these are horrible,
    we studied these for a while in college Tree damage is caused by the insect larvae, or caterpillars, which emerge from their eggs beginning in early spring and continuing through mid-May. The larvae move to the leaves of trees and begin to eat, mostly at night. During daylight hours, larvae generally seek shade from the sun but feeding can occur in daytime in heavy infestations. Gypsy moth larvae grow by moulting, five moults for males and six for females. Feeding occurs in the “instar” stage or period between each moult. As might be expected, a caterpillar’s appetite increases with each moult. Feeding continues until mid-June or early July when the caterpillar enters the pupal stage emerging, finally, as a moth. Both male and female moths exist only to reproduce once with the male moths flying to find the females who are too heavy to fly. After the females lay their eggs from July to September, depending on location, moths of both sexes then die.

    Great post i really enjoed!


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