Check Evergreens Now For
Bagworms
By
Larry Caplan, Extension Horticulture Educator, Vanderburgh County, IN
For
the Evansville Courier and Press, June 15, 2003
Evergreen plants, especially junipers,
arborvitae, and spruce, are among the favorite food of one of the Midwest's
more interesting insects: the
bagworm.
Bagworms are not really worms. They
are actually moth caterpillars. Bagworm caterpillars spend most of
their lives in silk-lined bags that are covered with bits of needles and leaves
that they cut from plants they feed on.
These camouflaged bags allow the caterpillars to hide from hungry birds
and other predators while also offering protection from the elements.
These bags can get to be about an inch-and-a-half long, and are usually
brown. Each spindle-shaped bag contains
one caterpillar.
In late August, male caterpillars become
adults and develop wings. They fly
around, looking for the females that are still in their bags, and mate. The female lays her eggs inside the bag, and
then dies. If you open up a female's bag
in the fall, you may find as many as 1500 pearly yellow eggs. The eggs overwinter in the bag, and begin to
hatch in early June.
Many of the young caterpillars immediately
begin to feed and spin the beginnings of their silken bag around
themselves. However, on over-crowded
plants, some of the caterpillars will crawl to the top of the plant and dangle
from a silk thread. The wind catches the
thread and blows the tiny insects to new trees, sort of like the way the baby
spiders did in the classic story "Charlotte's Web." (I had always wondered how a plant suddenly
developed an infestation, since the females don't move from their plant.)
Bagworms can, over a few years, develop
large populations on a bush or tree. I
remember observing a juniper that had so many bagworms on it, you could hear
them chewing! A healthy plant can regrow
leaves and needles, but 2 or 3 years of being defoliated will put the plant in
severe stress.
The best control for bagworms is to let
Mother Nature handle it. There are a
couple of parasitic wasps that lay their eggs into the living caterpillar; the
young larvae devour the caterpillar from the inside out, preventing it from
reproducing. Usually, your only evidence
that you have parasitic wasps helping you is you'll see some of last year's
bags with tiny holes in the side, where the wasps escaped from the bag.
You can't buy these parasitic wasps, but
you can encourage them to visit your evergreens by making your landscape more
attractive to them. The adult wasps feed
on flower pollen; by planting lots of flowers around your evergreens, you will
attract these beneficial insects. When
it comes time for them to lay their eggs, they won't have to go too far to find
a bagworm. While we're not sure what the
best types of flowers to plant are, Purdue University entomologists recommend
planting a wide variety of different annuals and perennials.
Another low-tech way to control bagworms is
to pick them off by hand and drop them in a bucket of soapy water. This works well if there are only a few
bagworms on the plant. This can be done
any time from mid summer to late spring; right now, with the little caterpillars
hatching, it's impossible to locate them.
If you have large numbers of bagworms,
you'll want to spray your trees and shrubs.
Two environmentally friendly products you can use include spinosad (made from a naturally occurring fungus) and B.t.
(from naturally occurring bacteria). Spinosad can be purchased as Conserve or Fertilome Borer, Bagworm, Leaf Miner and Tent Caterpillar
Spray; B.t. can be purchased as Dipel, Biotrol or Thuricide. Both of
these products are safe to the environment, and won't harm people, honeybees,
birds, or other wildlife if used according to label directions.
B.t. needs to be applied to young bagworms,
before they reach an inch in length. Spinosad should be applied by mid-summer, before the
bagworms reach one and a half inches.
Older stand-by foliar insecticides, such as
acephate (Orthene), carbaryl (Sevin), malathion, and some of the permethrin
products can also be used. All of these
products work better on younger bagworms, but they may also be used on slightly
larger caterpillars. Be aware that all
of these products may harm beneficial insects that also control spider mites;
we've seen lots of spider mite outbreaks the last several summers, due to all
the spraying people are doing for Japanese beetles. Read and follow all label directions before
buying, using, and disposing of these products.
For more information on bagworm control,
contact the Purdue Extension Service at (812) 435-5287 or e-mail me at LCaplan@purdue.edu
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