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Insect and Disease Fact Sheet Compliments of
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1-877-79TREES
Spring And Fall Cankerworms (and
other inchworms and loopers)
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There
are numerous cankerworms, or inchworms, that attack deciduous trees and
shrubs. The spring cankerworm, and fall cankerworm, are the most common pests
encountered but the elm spanworm, and linden looper, also have periodic
outbreaks. These
cankerworms, inchworms, loopers and spanworms occur throughout North America.
The fall cankerworm is also known from Montana, Colorado, Utah and California
while the spring cankerworm territory extends southwards into Arkansas and
Texas. Other inchworms, loopers and spanworms occur in western states. |
Fall Cankerworm |
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Linden Looper |
Plants Attacked Both
fall and spring cankerworms feed on a wide variety of trees including apple,
ash, beech, elm, hickory, linden, maples and oaks. The elm spanworm attacks
elms, hickory, ash and oaks as well as a variety of other broadleaf trees.
The linden looper is common on basswood, linden, apple, maples and oaks and
other trees. Damage Young
larvae chew small irregular holes in young leaves or skeletonize
leaves. As they mature, the larvae begin eating larger irregular holes and
finally entire leaves except for the major leaf veins. Low populations do not
damage healthy trees but high populations can defoliate trees causing them to
expend considerable resources to re-foliate. |
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Many
of the cankerworms and loopers spin down from the trees on a strand of silk
when they are ready to pupate. These larvae drop onto people, cars and picnic
tables, resulting in them being nuisance pests. |
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Description and Life Cycle
"Cankerworm"
is an older name used for what we commonly call inchworms, loopers, measuring
worms or spanworms today. They move by arching their mid-body to pull the hind
pro-legs up to meet the anterior true legs. There are numerous caterpillars
which use this inchworm behavior. Cankerworm larvae feed on tree leaves from late
April to mid-June. The adult females of cankerworms are wingless and emerge to
lay eggs in the late fall (fall cankerworms) or early spring (spring
cankerworms).
Fall
cankerworm larvae grow to 3/4 to 1 inch long and are usually apple green to
brownish green in color with a dark middle stripe and three narrow white lines
on each side. Fall cankerworms have three pairs of fleshy pro-legs at the- end
of the abdomen.
Spring
cankerworm larvae reach the same size but are green to reddish brown in color
and have a single yellowish strip on each side. Spring cankerworms only have
two fleshy pro-legs at the end of the abdomen.
Fall
cankerworms emerge as adults in late fall, often during warmer periods in
October through early December. The wingless females are a dull grey color and
crawl up on tree trunks to await a winged male. The males are about one inch
long, dull grey in color and often have two light, wavy stripes on the
forewings. After mating the female lays a cluster of barrel shaped eggs, often
encircling small branches. The eggs over winter and hatch in late April to
early May. Occasionally adults emerge in
March, especially in more northern areas.
Spring
cankerworms emerge as adults in late winter, usually during warm spells in
February or March. The wingless females are often mottled with grey or brown
color and have a darker stripe down the back. The winged males are
brownish-grey with three dark, irregular stripes across the front wings. The
eggs are oval shaped and irregular clusters of about 50 eggs are attached under
flaky bark or in cracks and crevices of the tree trunk.
Upon
hatching in April and May the young larvae rapidly feed on the fresh tender
spring leaves of various trees. Periodic outbreaks of large numbers are
especially annoying as the crowded larvae often hang from trees on a strand of
silk.
By
late June to early July the larvae of both species have matured and they then
descend to the ground on silk threads. The larvae then burrow into the ground
to a depth of one to four inches, spin a silken cocoon and pupate. The pupae
remain in the soil until the late fall or early spring.
Elm
spanworms are generally gray-brown in color and are often confused with spring
cankerworms. The adult spanworm is a completely white moth which flies in
August. The linden loopers have a bright yellow band of color down each side
with brown to black lines running down the back.
Control
Hints
Most
controls are directed towards trapping the wingless females or spraying the
active larvae after they start feeding on the tree leaves.
Strategy 1: Adult Trapping - Banding the trunks of
susceptible trees with sticky adhesives such tanglefoot
will trap females as they crawl up the trunk to mate and lay eggs. Trapped
females may remain attractive to the males which also get stuck in the sticky
band.
Strategy 2: Horticultural Oil Sprays - The 2-3% dormant horticultural
oil spray is quite effective against the eggs. Be sure to thoroughly wet the
trunk bark if spring cankerworms are present. Reduce the oil rate, especially
on maples, if the trees seem to be active in the spring.
Strategy 3: Spray with Bacillus thuringiensis (Bt) - This biological control is
quite effective against young cankerworm and looper larvae. Wait until all the
eggs have hatched but spray before the larvae get to be over inch long.
Strategy 4: Insecticide Sprays - Most common insecticides are
effective for control of cankerworms and loopers. Best results are obtained if
the spray is applied after all the eggs have hatched and the larvae are still
small.
Information obtained through the Ohio State Extension Factsheet HYG-2558-95
Insect and Disease Fact Sheet Compliments of
New Century
1-877-79TREES