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University
of Rhode Island GreenShare Factsheets
Cankerworms
Lepidoptera:
Geometridae
There
are numerous cankerworms, more commonly known as inchworms, which
attack deciduous trees and shrubs.The spring cankerworm, Paleacrita
vernata (Peck), and fall cankerworm, Alsophila pometaria
(Harris), tend to be the most common cankerworms, but the elm spanworm,
Ennomos subsignaria (Hubner), and linden looper, Erannis
tiliaria (Harris), also have periodic population surges. 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 oak, as well as a variety
of other broadleaf trees. The linden looper is common on basswood,
linden, apple, maple, oak and other trees.
Young larvae chew small irregular holes in young leaves, skeletonizing
the leaves. As they mature, the larvae begin eating larger irregular
holes and finally entire leaves, leaving only the major leaf veins.
Although low populations do not damage healthy trees, high populations
can defoliate whole trees, causing them to expend considerable resources
to refoliate. 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 and are sometimes
considered nuisance pests, although they cause no harm to mammals.
Cankerworms move by arching the middle part of their bodies to pull
the hind prolegs up to meet the anterior true legs. Many caterpillars
move in a similar fashion. Fall cankerworm larvae grow to 19 to
25 mm (1/4 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 prolegs
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 have only two fleshy prolegs
at the end of the abdomen. 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. Linden
loopers have a bright yellow band of color down each side, with
brown to black lines running down the back.
Cankerworm larvae feed on tree leaves from late April to mid-June.
Adult female cankerworms are wingless and emerge to lay eggs in
the late fall (fall cankerworms) or early spring (spring cankerworms).
Fall cankerworms emerge as adults during warmer periods in October
through early December. The wingless females are a dull gray color
and crawl up on tree trunks to await a winged male. The males are
about 25 mm (1 inch) long, dull gray 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 overwinter and hatch in late April to early May. Adults
occasionally emerge in March, especially in more northern areas.
Spring
cankerworms emerge as adults 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; irregular clusters of about 50 eggs are attached
under flaky bark or in cracks and crevices of tree trunks. Upon
hatching in April and May, the young larvae rapidly feed on the
fresh tender spring leaves of various trees. By late June to early
July the larvae of both species have matured and they descend to
the ground on silk threads. The larvae then burrow into the ground
to a depth of 2.5 to 10 cm (1 to 4 inches), spin a silken cocoon
and pupate. The pupae remain in the soil until the late fall or
early spring. - Trapping
the adults is an important strategy. The trunks of susceptible trees
can be banded with sticky adhesives, such as tanglefoot, in order
to 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. - Horticultural
oil sprays can also be quite effective. The 2-3% dormant horticultural
oil spray is most 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. - Spraying
with Bacillus thuringiensis (Bt), a biological control, is
quite effective against young cankerworm and looper larvae. Wait
until all the eggs have hatched but spray before the larvae grow
to more than 25 mm (1 inch) in length. - If insecticides
must be used to control cankerworms and loopers, best results are
obtained if the spray is applied after all the eggs have hatched
and the larvae are still small. Be sure to check for currently registered
insecticides. Adapted
fromDavid J. Shetlar, Ohio State University Extension, 1999
Pesticides
are poisonous! Read and follow all safety precautions on labels.
Handle carefully and store in original containers out of reach
of children, pets or livestock. Dispose of empty containers
immediately, in a safe manner and place. Pesticides should never
be stored with foods or in areas where people eat.
When trade names are used for identification, no product endorsement
is implied, nor is discrimination intended against similar materials.
Be sure that the pesticide you intend to use is registered for
the state of use.
The user of this information assumes all risk for personal injury
or property damage.
For more
information, call the URI CE Gardening and Food Safety Hotline
at 1-800-448-1011 or (401)874-2929 from outside Rhode Island;
Monday-Thursday between 9 am and 2 pm.
University
of Rhode Island Cooperative Extension provides equal program
opportunities.
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