Bagworms were late hatching this year, which resulted in a
later feeding season than normal.
Usually bagworms pupate in mid- to late August, but we
are still hearing reports of feeding bagworms around
the state and recently saw some in central Illinois
that were only 3/4 inch long. Bagworms usually
pupate when they are about 1-1/2 inches long.
Bagworms are whitish caterpillars that
construct individual silk cases around themselves. They
cover these silk cases with bits of leaves from the tree
or shrub they are feeding on. These bits of foliage
turn brown within a couple of days, resulting in
brown bags moving across the trees and eating leaves.
Throughout its entire life as a caterpillar,
insect places new foliage at the top of the bag. Thus,
an actively feeding caterpillar has green foliage at the
top of its bag. If the caterpillar pupates, it ties off the
top of the bag to a branch, and any green foliage
quickly turns brown. If the caterpillar dies, the foliage on
the bag will be completely brown. Bagworms that
are alive and actively feeding can be controlled
with trichlorfon (Proxol, Dylox), cyfluthrin (Tempo),
and other synthetic pyrethroids. Bacillus
thuringiensis var. kurstaki (Dipel, Thuricide) is
more spotty in
its control of large larvae but is the only option for
Bagworms start to feed at the top of the tree
and work their way down. A pair of binoculars is
useful for detecting feeding bagworms on large
trees. Bagworms most commonly attack eastern red
cedar and other junipers, as well as spruces,
arborvitae, Douglas-fir, honey locust, pin oak, red oak,
and tallhedge. Stripped branches of coniferous
evergreens will probably die.
Bagworms will pupate later this summer.
Male bagworms emerge as black, one-inch-long moths
with clear wings. They mate in early fall with the
adult females that stay in the bag. Adult female
bagworms are brownish and larval in appearance. Mated
females fill their bodies with up to 1,000 eggs in the
fall before they die. These eggs hatch in June of
the following year.
Old bags that housed males will have dark
brown pupal cases (about 1/2 inch long) sticking out of
the end. The other bags are likely female bags that can
be picked off trees from late fall through spring to
reduce the number of caterpillars that are present next
Do not just toss the picked bags to the ground under
the tree because young bagworm larvae crawl long
distances and will probably crawl up onto the tree.
Picking off all of the old bags will not totally eliminate
bagworms from the tree because young bagworm
larvae are blown on strands of silk from tree to tree for
a couple weeks after they hatch in June.