Woolybear Caterpillars


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 all. 

Tent Caterpillars

June 2012

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, footprint-shaped markings.

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 growing period.

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 hazard.

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.

Fall Webworms


QUESTION:  Our tree is filled with what we believe to be tent caterpillars.  Should we spray them with something?

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 not.

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


QUESTION: We 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?

ANSWER: 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 caterpillar pests.

Another option is to simply remove the infested limb, tent and caterpillars .

Early MAY

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 the leaves?

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.

Gypsy Moth


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.