This archival publication may not reflect current scientific knowledge or recommendations. Current information available from the University of Minnesota Extension: https://www.extension.umn.edu

Ascerno, Mark E.

University of Minnesota Digital Conservancy

N OCJd 
Extension Folder 625-1982 
CANKERWORMS 
lfNIVE"RSITY OF MIN"JESO'T; 
r ''F•· TS 
Mark E. Ascerno, extension entomologist 
Agricultural Extension Service 
University of Minnesota 
The cankerworm, or inchworm, is the most common 
shade tree defoliator in areas of Minnesota where elm 
predominates. The slender, green or brown caterpil-
lars (figures 1 and 2) prefer elm and apple but will 
also feed on hackberry, basswood, oak, box elder, 
maple, and ash as well as on shrubs that are beneath 
heavily infested trees. Cankerworms, native to the 
United States, go through natural cycles, with periods 
of abundance ranging from two to seven years (with 
an average of four years), followed by periods of low 
populations lasting 13 to 18 years. 
LIFE CYCLES 
There are two species, the fall cankerworm and the 
spring cankerworm. Although similar in the kind of 
damage they do, they have different egg-laying habits 
and somewhat different body shapes. Both feed at 
the same time of year, often on the same tree, and 
spend the summer as nonfeeding pupae 'in the soil. 
Fall Cankerworm. This species emerges from the soil 
as an adult moth, generally in late October after a 
hard freeze. The gray-brown male moths are winged, 
Figure 1. Fall cankerworm larva. Note the three pairs or pro-
legs at the back of the body. 
Figure 2. Spring cankerworm larva. Note the two pairs of pro-
legs at the back of the body. 
about½ inch long (figure 3), and emerge a few days 
before the wingless, spiderlike female (figure 4). 
Mating takes place as the females crawl up tree 
trunks in search of small twigs on which to glue clus-
ters of eggs (figure 5). The males and females die 
shortly after the eggs are deposited. Some people are 
bothered by large numbers of moths crawling up ex-
terior walls, but chemical control at this time is not 
effective and therefore is not recommended. 
The eggs stay on the tree throughout the winter and 
hatch in the spring. Egg hatch varies from late April 
to mid-May with the exact time dependent on weather 
conditions. It normally coincides with the opening of 
elm buds. Knowing the time of egg hatch in a given 
year is important for successful chemical control of 
the cankerworm. 
Newly hatched fall cankerworm larvae are less than 
1/16 inch long and spin silk threads that allow them 
to be readily blown from tree to tree. They reach 1 
inch in length when fully grown, which takes about 
four weeks. Fall cankerworm larvae have three pairs 
of legs called prolegs on the back half of the body 
(figure 1). 
Figure 3. Winged male cankerworm moth. 
Spring Cankerwonn. The spring cankerwonn differs 
from the fall cankerwonn in that the pupae in the soil 
do not emerge as adult moths until early spring. Emer-
gence of the winged male and wingless female typical-
ly occurs during the third week of March in the Twin 
Cities area. Adult spring cankerworm males (figure 3) 
and females (figure 4) are similar to fall cankerwonn 
males and females in color, size, and appearance and 
can create the same nuisance of crawling on houses. 
As with fall cankerwonns, chemical control when 
adult moths are present is not effective and is not rec-
ommended. Eggs are deposited in loose clusters (fig-
ure 6) in bark cracks and under bark scales. The 
eggs are not easily seen. Spring and fall cankerworm 
eggs hatch at about the same time in the spring. The 
larvae are similar except that spring cankerworms 
have two pairs of prolegs instead of three (figure 2) 
on the back half of the body. 
DAMAGE 
A large, mature, vigorous deciduous tree is able to 
withstand a single season of complete defoliation 
with no damage; a tree that has been completely de-
foliated usually leafs out three weeks after the worms 
stop feeding. Even two seasons of defoliation produce 
only a slowing of the growth. However, limb dieback 
and loss of vigor can result if the same tree is com-
pletely defoliated for three or more consecutive 
years. Young, newly transplanted, or weakened trees 
are more susceptible to the detrimental effects of 
defoliation. Therefore, a tree's age, size, vigor, and 
Figure 4. Wingless female cankerworm moth. 
previous history of defoliation should be considered 
before deciding on control. For example, a young or 
weak tree should be treated but an older, vigorous 
tree need not be protected. 
Many people are not as concerned about the health of 
their tree as they are about the nuisance that mature 
cankerworms cause when they spin to the ground on 
silken threads, crawling or falling on picnic tables, 
doors, walks, and house siding. Large numbers can 
Figure 5. Egg cluster of fall cankerworm. It is recommended 
that a few such egg masses be monitored early in the 
spring to determine time of egg hatch. Notice 
hatched eggs with hollow centers. 
Figure 6. Egg mass of spring cankerworm with female moth. 
make being outside extremely unpleasant. Unfortun-
ately, control measures are not effective at this stage 
of the worms' life cycle, but, fortunately, this behav-
ior usually lasts only one week. 
CONTROL 
Whether the purpose is to protect trees or to elimin-
ate the nuisance of cankerworms, control needs to be 
accomplished when the worms are relatively small 
and inconspicuous. Cankerworm larvae begin to feed 
on buds or expanding leaves immediately after egg 
hatch. Normally, feeding during the first two weeks 
is not noticed by the casual observer. However, a 
close look will reveal small BB-size holes in the leaves. 
The best time for chemical control is in the second 
week of feeding when the damage is minor and the 
worms are still small enough (less than ½ inch) to be 
easily killed. 
The worms rapidly increase in size during the third 
and fourth week after egg hatch. During this period 
damage becomes extensive and very noticeable. In 
many cases large portions of the leaves have been 
eaten and only the veins remain. Unfortunately, 
though the damage is easily noticed, this is the worst 
time for control because the chemicals are not very 
effective, the damage to the tree is already done, and 
the chemical irritates the worms and causes them to 
drop from the trees in large numbers, creating an even 
greater nuisance. 
Several chemicals are effective for controlling canker-
worms when applied at the correct time. These mater-
ials are listed in table 1. Bacillus thuringiensis has the 
advantage of being specific for the caterpillars of 
moths and butterflies and will not harm beneficial 
insects, wildlife, or humans. 
BANDING 
One method of controlling cankerworms that has re-
ceived wide publicity is the use of sticky barriers 
applied in bands to tree trunks. As the wingless fe-
males crawl up the trunk to lay eggs they are trapped 
in these sticky bands. Despite the publicity, there is 
little evidence that this method is worthwhile, even 
in areas where most trees have been banded. The pos-
sible exception would be on a tree that is isolated 
from other cankerworm-susceptible trees. 
Although the practice is fine in theory, it generally 
fails because of one or more of the following reasons: 
(1) people do not clean and renew the sticky material 
as often as the bands become crowded with moths; 
(2) young cankerworms readily blow from tree to 
tree on silk threads and therefore avoid the band; and 
( 3) people fail to recognize and band for the two 
species ( one moving up the tree in the fall, the other 
in the spring). Furthermore, the sticky material is 
expensive, messy to work with, and tends to mar the 
appearance of the trunk for a long time. Although 
banding has a certain appeal in that it captures num-
erous adults, providing a sense of immediate gratifica-
tion, it does not, as practiced, affect defoliation or 
the spinning behavior of the larger worms. Its use is 
not recommended. 
STEPS FOR EFFECTIVE CONTROL 
Before making a decision to use control measures, be 
sure to review the preceding information because 
cankerworm control can be complex. The following 
steps are suggested when the decision to control cank-
erworms has been reached. 
1. Determine the egg-hatch date by either: 
a. Finding fall cankerworm egg masses ( figure 5) 
and observing them during early spring for signs of 
hatching. Hatched eggs will have a hole in the cen-
ter. 
b. Examining foliage of several trees for the first 
signs of the young cankerworms or their feeding. 
2. Add 10 days to the date of hatch as determined in 
step 1. This will be the best date for spraying. 
3. Examine representative trees during the 10-day 
post-hatch period to determine if development has 
slowed or quickened because of weather conditions. 
4. Modify the spray date as necessary, keeping in 
mind that the worms should be less than ½ inch in 
length and that at least 50% of the leaf surface 
should be intact. 
5. Spray with one of the suggested materials. 
Table 1. Insecticide suggestions to control the larval 
stage of cankerworms* 
Insecticide t 
Bacillus thuringiensis 
(Dipel, Thuricide, Biotrol) 
carbaryl 80% W.P. 
(Sevin) 50% W.P. 
carbaryl (Sevimol) 
malathion 57% E.C. 
25 % W.P. 
methoxychlor 50% W.P. 
25% E.C. 
acephate 15.6% E.C. 
(Orthene) 
75 % S.P. 
Amount 
per gallon of water 
Use as directed on 
the label. 
4 tsp 
2 tbsp 
1 tbsp 
2 tsp 
2 tbsp 
2 tbsp 
4 tsp 
1½ tbsp 
1 tsp 
*Spray timing is critical; if done at the correct time, 
only a single application will be needed. 
t W.P. = wettable powder; E.C. = emulsifiable concen-
trate; S.P. = soluble powder 
The information given in this publication is for educational purposes 
only. Reference to commercial products or trade names is made with 
the understanding that no discrimination is intended and no endorse-
ment by the Minnesota Agricultural Extension Service is implied. 
Issued in furtherance of cooperative extension work in agriculture and 
home economics, acts of May 8 and June 30, 1914, in cooperation with 
the U.S. Department of Agriculture. Norman A. Brown, Director of 
Agricultural Extension Service, University of Minnesota, St. Paul, Min-
nesota 55108. The University of Minnesota, including the Agricultural 
Extension Service, is committed to the policy that all persons shall have 
equal access to its programs, facilities, and employment without regard 
to race, creed, color, sex, national origin, or handicap. 10 cents 
111111~1~11m11ii1m1111111i1i1i1i1mim11111 
3 1951 D01 921 146 B 


Cankerworms

1 online resource (PDF,  4 pages)This archival publication may not reflect current scientific knowledge or recommendations. Current information available from the University of Minnesota Extension: https://www.extension.umn.edu.Ascerno, Mark E.. (1982). Cankerworms. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/207345

MAJ d-500 AGFO - J'71,o Extension Folder 625-1982 • CANKERWORMS --Mark E. Ascerno, extension entomologist Agricultural Extension Service University of Minnesota DOCLif~ ~ 'l 1987 The cankerworm, or inchworm, is the most common shade tree defoliator in areas of Minnesota where elm predominates. The slender, green or brown caterpil-lars (figures 1 and 2) prefer elm and apple but will also feed on hackberry, basswood, oak, box elder, maple, and ash as well as on shrubs that are beneath heavily infested trees. Cankerworms, native to the United States, go through natural cycles, with periods of abundance ranging from two to seven years (with an average of four years), followed by periods of low populations lasting 13 to 18 years. LIFE CYCLES There are two species, the fall cankerworm and the spring cankerworm. Although similar in the kind of damage they do, they have different egg-laying habits and somewhat different body shapes. Both feed at the same time of year, often on the same tree, and spend the summer as nonfeeding pupae 'in the soil. Fall Cankerworm. This species emerges from the soil as an adult moth, generally in late October after a hard freeze. The gray-brown male moths are winged, Figure 1. Fall cankerworm larva. Note the three pairs or pro-legs at the back of the body. Figure 2. Spring cankerworm larva. Note the two pairs of pro-legs at the back of the body. about½ inch long (figure 3), and emerge a few days before the wingless, spiderlike female (figure 4). Mating takes place as the females crawl up tree trunks in search of small twigs on which to glue clus- A ters of eggs (figure 5). The males and females die W shortly after the eggs are deposited. Some people are bothered by large numbers of moths crawling up ex-terior walls, but chemical control at this time is not effective and therefore is not recommended. The eggs stay on the tree throughout the winter and hatch in the spring. Egg hatch varies from late April to mid-May with the exact time dependent on weather conditions. It normally coincides with the opening of elm buds. Knowing the time of egg hatch in a given year is important for successful chemical control of the cankerworm. Newly hatched fall cankerworm larvae are less than 1/16 inch long and spin silk threads that allow them to be readily blown from tree to tree. They reach 1 inch in length when fully grown, which takes about four weeks. Fall cankerworm larvae have three pairs of legs called prolegs on the back half of the body (figure 1). Figure 3. Winged male cankerworm moth. ---Spring Cankerworm. The spring cankerworm differs from the fall cankerworm in that the pupae in the soil do not emerge as adult moths until early spring. Emer-gence of the winged male and wingless female typical-ly occurs during the third week of March in the Twin Cities area. Adult spring cankerworm males (figure 3) and females (figure 4) are similar to fall cankerworm males and females in color, size, and appearance and can create the same nuisance of crawling on houses. As with fall cankerworms, chemical control when adult moths are present is not effective and is not rec-ommended. Eggs are deposited in loose clusters (fig-ure 6) in bark cracks and under bark scales. The eggs are not easily seen. Spring and fall cankerworm eggs hatch at about the same time in the spring. The larvae are similar except that spring cankerworms have two pairs of prolegs instead of three (figure 2) on the back half of the body. DAMAGE A large, mature, vigorous deciduous tree is able to withstand a single season of complete defoliation with no damage; a tree that has been completely de-foliated usually leafs out three weeks after the worms stop feeding. Even two seasons of defoliation produce only a slowing of the growth. However, limb dieback and loss of vigor can result if the same tree is com-pletely defoliated for three or more consecutive years. Young, newly transplanted, or weakened trees are more susceptible to the detrimental effects of defoliation. Therefore, a tree's age, size, vigor, and Figure 4. Wingless female cankerworm moth. previous history of defoliation should be considered before deciding on control. For example, a young or weak tree should be treated but an older, vigorous tree need not be protected. Many people are not as concerned about the health of their tree as they are about the nuisance that mature cankerworms cause when they spin to the ground on silken threads, crawling or falling on picnic tables, doors, walks, and house siding. Large numbers can Figure 5. Egg cluster of fall cankerworm. It is recommended that a few such egg masses be monitored early in the spring to determine time of egg hatch. Notice hatched eggs with hollow centers. Figure 6. Egg mass of spring cankerworm with female moth. make being outside extremely unpleasant. Unfortun-ately, control measures are not effective at this stage of the worms' life cycle, but, fortunately, this behav-ior usually lasts only one week. CONTROL Whether the purpose is to protect trees or to elimin-ate the nuisance of cankerworms, control needs to be accomplished when the worms are relatively small and inconspicuous. Cankerworm larvae begin to feed on buds or expanding leaves immediately after egg hatch. Normally, feeding during the first two weeks is not noticed by the casual observer. However, a close look will reveal small BB-size holes in the leaves. The best time for chemical control is in the second week of feeding when the damage is minor and the worms are still small enough (less than ½ inch) to be easily killed. The worms rapidly increase in size during the third and fourth week after egg hatch. During this period damage becomes extensive and very noticeable. In many cases large portions of the leaves have been eaten and only the veins remain. Unfortunately, though the damage is easily noticed, this is the worst time for control because the chemicals are not very effective, the damage to the tree is already done, and the chemical irritates the worms and causes them to drop from the trees in large numbers, creating an even greater nuisance. Several chemicals are effective for controlling canker-worms when applied at the correct time. These mater-ials are listed in table 1. Bacillus thuringiensis has the advantage of being specific for the caterpillars of moths and butterflies and will not harm beneficial insects, wildlife, or humans. BANDING One method of controlling cankerworms that has re-ceived wide publicity is the use of sticky barriers applied in bands to tree trunks. As the wingless fe-males crawl up the trunk to lay eggs they are trapped in these sticky bands. Despite the publicity, there is little evidence that this method is worthwhile, even in areas where most trees have been banded. The pos-sible exception would be on a tree that is isolated from other cankerworm-susceptible trees. Although the practice is fine in theory, it generally fails because of one or more of the following reasons: (1) people do not clean and renew the sticky material as often as the bands become crowded with moths; (2) young cankerworms readily blow from tree to tree on silk threads and therefore avoid the band; and (3) people fail to recognize and band for the two species ( one moving up the tree in the fall, the other in the spring). Furthermore, the sticky material is expensive, messy to work with, and tends to mar the appearance of the trunk for a long time. Although banding has a certain appeal in that it captures num-erous adults, providing a sense of immediate gratifica-tion, it does not, as practiced, affect defoliation or the spinning behavior of the larger worms. Its use is not recommended. STEPS FOR EFFECTIVE CONTROL Before making a decision to use control measures, be sure to review the preceding information because cankerworm control can be complex. The following A steps are suggested when the decision to control cank- W erworms has been reached. 1. Determine the egg-hatch date by either: a. Finding fall cankerworm egg masses (figure 5) and observing them during early spring for signs of hatching. Hatched eggs will have a hole in the cen-ter. b. Examining foliage of several trees for the first signs of the young cankerworms or their feeding. 2. Add 10 days to the date of hatch as determined in step 1. This will be the best date for spraying. 3. Examine representative trees during the 10-day post-hatch period to determine if development has slowed or quickened because of weather conditions. 4. Modify the spray date as necessary, keeping in mind that the worms should be less than ½ inch in length and that at least 50% of the leaf surface should be intact. 5. Spray with one of the suggested materials. Table 1. Insecticide suggestions to control the larval stage of cankerworms* Amount Insecticide t per gallon of water Bacillus thuringiensis Use as directed on (Dipel, Thuricide, Biotrol) the label. carbaryl 80% W.P. 4 tsp (Sevin) 50% W.P. 2 tbsp carbaryl (Sevimol) 1 tbsp malathion 57% E.C. 2 tsp 25%W.P. 2 tbsp methoxychlor 50% W.P. 2 tbsp 25% E.C. 4 tsp acephate 15.6% E.C. 1½ tbsp (Orthene) 75 % S.P. 1 tsp *Spray timing is critical; if done at the correct time, only a single application will be needed. tw.P. = wettable powder; E.C. = emulsifiable concen-trate; S.P. = soluble powder The information given in this publication is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorse-ment by the Minnesota Agricultural Extension Service is implied. Issued in furtherance of cooperative extension work in agriculture and home economics, acts of May 8 and June 30, 1914, in cooperation with -the U.S. Department of Agriculture. Norman A. Brown, Director of Agricultural Extension Service, University of Minnesota, St. Paul, Min-nesota 55108. The University of Minnesota, including the Agricultural Extension Service, is committed to the policy that all persons shall have A equal access to its programs, facilities, and employment without regard W' to race, creed, color, sex, national origin, or handicap. 10 cents 

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