QUESTION: We have been seeing a number of woolybear
caterpillars this past week. What sort
of damage will they do to our landscape plants?
Should we be spraying them with an insecticide?
ANSWER: Wooly bear caterpillars are a common
sight this time of year. After a summer spent
munching on grass and leaves, the wooly bear caterpillar
is crawling now-----all over
sidewalks, roads, decks and patios. Although there are several species of fuzzy
caterpillars that range from blond to orange in color, the banded wooly bear is
the one most commonly found this time of year. They are easily recognized by
their fuzziness and characteristic color pattern. Each end is clothed in dense
black hairs and the middle third is rusty orange to yellow in
color. Supposedly the name of these little critters comes from
their similarity with bears---their hairy appearance and their wandering
habits. They are the caterpillar or larval stage of the Isabella moth (Isia isabella).
Although they feed on many plants, these caterpillars rarely cause major damage
because they feed so late in the season.
Some people have had reactions to
the hairs of these caterpillars ranging from allergies to rashes. In one
instance a hair got caught in the eyeball of a child who been playing with one
of the caterpillars. The hair had to be surgically removed. from the
eyeball. Its best to not allow children to play with caterpillars, but
wooly bears are otherwise harmless.
Now that fall has arrived,
these caterpillars begin wandering in search of a good place to cozy
up for the winter. They actually make their own antifreeze in order to survive
even the coldest temperatures. With the warmth of spring, they pupate
within a silken cocoon. The caterpillar form feeds on a variety of herbaceous
plants. Included among the hosts are grasses and some weeds. They
are particularly fond of narrow leafed plantain which is a common garden weed
that is found in lawns. It is not known to feed extensively on
ornamental landscape plants or vegetables.
Folklore says the wider the bands of
black hairs, the more severe the upcoming winter. According to
entomologists, the year-to-year variation in the size of the colored ban is due
to age and rainfall. Older caterpillars have more black than young ones
and caterpillars that feed and grow in an area where the fall weather is wetter
have more black hair than caterpillars from dry areas. The band width on
individual caterpillars make for interesting conversation and that’s about
QUESTION: The leaves on our trees are being completely eaten by caterpillars. Will they eventually kill the tree?
ANSWER: Tent caterpillar infestations are
being reported throughout our coastal area. They are easy to notice with
their unsightly tents and defoliation of deciduous trees. These notorious
caterpillars attack a wide variety of plants including alder, apple, ash,
birch, cherry, cottonwood, willow, fruit trees, and roses. During heavy
infestations the caterpillars will migrate and feed on many other plants.
Early in their development, tent
caterpillars tend to eat all of the leaves on one branch before moving on to
the next. Later, they split into smaller groups and attack several
branches. A single tent may result in 20% defoliation of a small
tree. Trees infested with several tents are often totally
defoliated. A single occurrence rarely kills a tree, but it does reduce
growth and makes the tree more susceptible to other hazards such as drought and
disease. A healthy tree which has been attacked will usually grow new
leaves by mid-summer.
There are two kinds of tent
caterpillars that occur here—forest and western. Forest tent caterpillars
do not form a true tent despite their name. Rather they spin silken mats
on tree branches or trunks. The mature caterpillar is about 2 inches
long. Its body is blue with black spatters and has white,
The western tent caterpillar is
often the most numerous in western Washington and has orange and black markings
which are familiar to many people. This species spins tents on the tips
of branches. The eggs hatch in early spring just as the new buds break in
April or May. The young larvae begin feeding in groups. The larvae of
both species molt (shed their skins) four times during their five to six week
As the caterpillars mature, they
begin to feed in small groups or singly. Just before they spin their cocoons in
mid-June, they crawl about looking for a protected place in plants or on
structures to attach their cocoons. The adult moths emerge in approximately 7
to 10 days. The moths are stout-bodied and light brown. They often fly in
clusters around street or porch lights on summer evenings. After the moths
mate, the females lay 100 to 350 eggs in a froth-covered band around small
twigs or branches of host trees. The eggs mature in about 3 weeks but do not
hatch until the following spring.
In most years, tent caterpillars are
primarily a nuisance. They do not transmit diseases to humans, do not
bite and are not poisonous.
Tent caterpillars have numerous
enemies. One is the tachnid fly which parasitizes the larvae by
depositing white eggs on the caterpillar’s body. When the egg hatches, a
small maggot burrows into the caterpillar and begins feeding. Tent caterpillars
are also subject to a virus called wilt.
An effective way to control tent
caterpillars is to remove their egg cases from trees. The egg masses are a
brown or gray frothy material which has hardened to look like Styrofoam.
Most egg cases are one-half inch long bands that appear on twigs. While
some are long bands found on twigs, others are flattened shapes found on tree
trunks or buildings. In both cases, egg cases can be picked off by hand
or pruned out.
Once the caterpillars have hatched,
the simplest way to control them is to remove and destroy the larvae and their
nests by stripping or pruning them from branches. Burning the nests, a
traditional method of control, is no longer recommended because of the fire
When tent caterpillars are numerous
or hard to reach, chemical controls are recommended. Infested trees may be
sprayed with a biological insecticide, Bacillus thuringiensis
(B.t.). The control is selective; it kills only caterpillars and is
relatively safe for other insects, fish, birds, and warm blooded animals.
Thorough coverage of foliage is necessary, and spraying should not begin until
early signs of leaf damage appear. The effects of B.t. are not immediately
apparent. Caterpillars sicken and stop feeding right away, but they do
not die for a few days.
QUESTION: Our tree is filled with what we believe to be
tent caterpillars. Should we spray them
ANSWER: Right on schedule, fall webworm infestations
are beginning to appear on deciduous trees and shrubs throughout our coastal
area. Their large, conspicuous tents are
often mistaken for those of the notorious tent-caterpillar which appears in
early spring. Although these critters
may look awful, they are generally not a threat to our trees and shrubs.
Fall webworms are usually found
in groups and feed together on the foliage of their host plant. They are unique from the standpoint that they
skeletonize and consume the leaves under the protection of a tent-like web
which they enlarge from time to time as they develop and more food is
needed. Large portions of a tree may be
covered by these webs.
The caterpillars feed
entirely within the tent, which protects them form predators and
parasites. The tents also help with
mechanical control. When the “tented”
branches are within reach, they can often simply be snipped off and destroyed.
While the webs and
accompanying defoliation caused by fall webworms are unsightly, trees do not
die as a result of being defoliated by caterpillar pests. For most gardeners, it is the unsightliness
of the webbing and defoliation that causes the greatest concern.
Out of reach webs can easily
be removed by using a hook fashioned from a coat hanger taped to the end of a
long pole or a large nail driven through a long pole (exercise appropriate caution
around power lines). Destroying the web
in this fashion also exposes the caterpillars to predation and parasitism. Yellow jackets, paper wasps, birds, predatory
stink bugs and parasitic flies all feed on webworms. Burning webs is not a good idea. Twigs and branches that are defoliated by
caterpillars will produce new leaves; twigs and branches killed by fire will
Chemical control should be
used if the infestation is heavy, or if tents are high in the trees and
difficult to reach. Bacterial insecticides
containing Bacillus thuringiensis are
formulated specifically to kill feeding caterpillars without harming other
insects. Thoroughly cover the leaves
next to the nests. As these leaves are
incorporated into the nest and eaten, the Bt will be ingested.
Silver Spotted Tiger Moths
can hardly believe it. We just found a branch on our spruce tree crawling with
little caterpillars. What are they? How do we get rid of them?
Based on your description, the insects most likely are Silver Spotted Tiger
Moths. These insects are sometimes referred to as the "polar bears of the
insect world" since they seem to thrive during very cold temperatures with
no apparent injury. It is an occasional pest of conifers here in our coastal
area. Its main host is Douglas fir, but it will sometimes feed on spruce, pine
and other conifers. This insect is often confused with tent caterpillars
because both make tents; however, they do not feed on the same kinds of trees.
The larva or caterpillar,
which is now visible, is the damaging stage of this insect. Mature caterpillars
are about one and a half inches in length and covered with a combination of
dense, black, reddish brown and yellowish hairs. It is said that these hairs
are poisonous and upon contact may cause rash, itching, or a burning sensation.
In mid-June, the caterpillars
leave the trees to seek sheltered pupation sites, like under the eaves of
houses and in tree bark crevices. The brown colored cocoons are constructed
from silk and body hairs. Inside is a shiny reddish-brown pupa. The adult moth
emerges and lays pea green eggs in clusters on needles and twigs of host trees
in mid to late summer. The eggs hatch in 10 to 14 days.
The small, furry, rusty brown
or blackish caterpillars feed on the needles in large numbers. They feed well
into fall until cold weather begins. Young caterpillars hibernate during winter
in dense webs which they spin. They resume their feeding in early spring. On
warm days you may even find them feeding as early as January.
When they are about
two-thirds grown, they become less gregarious and begin to disperse and feed
throughout the tree singly or in small groups of two or three. Feeding during
this period is usually not very harmful to trees or noticeable since the damage
is widely scattered.
Feeding is restricted to the
needles at the ends of the lateral branches. Whole branches are often stripped
of their leaves, making individual trees appear quite unsightly at times. Cases
of total defoliation are rare. Most often, damage occurs as defoliation in one
of a few limbs of a tree. Since the buds are not harmed, the new growth will
cover the damaged area by early next summer.
In general, the caterpillar
is not considered to be a serious pest. It is generally reduced by the same
parasitic Tachinind flies that attack tent caterpillars and these parasites
keep populations under control. However, in some years the caterpillars are
more numerous and may not be adequately reduced by the parasites requiring the
application of insecticidal sprays. Home garden formulations of Bacillus
thuringiensis, a bacterial insecticide, are available for use against
Another option is to simply
remove the infested limb, tent and caterpillars .
QUESTION: We just found a bunch of caterpillars in our
Douglas fir tree. What are they? Will they crawl onto our other plants and eat
ANSWER: More than likely, the caterpillars you are
describing are the larvae of Silver-spotted tiger moths. The main host for this pest is Douglas fir,
but it sometimes feeds on spruce, pine, and other conifers. The chewing damage caused by the
silver-spotted tiger moth larvae is limited and usually not very serious. Home gardeners who discover this pest can
prevent additional feeding damage by simply pruning out the infested branch and
destroying the insects. If left
uncontrolled, the caterpillars will form cocoons and pupate in early June. Adult moths emerge later in the summer and
mate. Females lay pea green oval eggs in
clusters on needles and twigs of host trees in mid to late summer. The eggs hatch in 10 to 14 days. Larvae over-winter in dense webbing. A single generation appears each year.
QUESTION: We heard on the television this week (May
2000) that the Washington State Department of Agriculture is spraying for moths
in Seattle. Do we have
those same moths here? If we do, should
we be concerned?
ANSWER: The Washington State Department of
Agriculture is planning to undertake aerial pesticide spraying in portions of
Ballard and Magnolia in Seattle
in late April and May to combat a potential Gypsy Moth infestation. At this time, there have been no other
infestations reported. The WSDA has
placed Gypsy Moth traps throughout Western Washington
in order to detect any new infestations.
The traps are monitored on a weekly basis. The gypsy moth has a voracious appetite for a
wide range of plant material including conifers. Anyone who would like additional information
may contact the WSDA’S GYPSY MOTH HOTLINE at 1-800-443-6684.