Bagworms (Thyridopteryxephemeraeformis) infest many
shrubs and trees, but conifers (evergreens) are the preferred hosts. Damage to
plants results from feeding by the caterpillars, which causes loss of needles.
Mild infestations of this pest slow the growth of Leyland cypress. Heavy
infestations can kill a plant.
DESCRIPTION OF THE PEST
The adult male bagworm is a dark-colored, hairy moth with a
1-inch wingspan and clear wings. The adult female does not have wings or legs,
is yellow, and appears almost maggot-like. The larvae (immature forms, aka
caterpillars) range in size from about 1/8 inch to 2 inches depending on
Each larva produces a carrot- or cone-shaped bag that it carries
as it feeds. The bag is formed from silk that the larva produces. As it feeds,
the larva adds bits of plant material to the bag for camouflage. The bag is
about 2 inches long when complete. Home gardeners sometimes mistakenly identify
it as a pine cone.
COMMENTS on disease
bagworms survive the winter as eggs in a bag. The larvae hatch
during May. Each one produces a strand of silk that allows it to be blown by
the wind to a new location on the same plant or to a new plant. They soon begin
to spin their cases. When mature, each larva pupates (transforms to an adult)
within its bag. An adult male moth emerges from its bag in late summer
(August/September). It locates an adult female in her bag. After mating, the
female lays 500-1,000 eggs in her bag and dies.
Several parasites and predators
feed on bagworms, generally keeping their numbers under control so that damage
is not noticed. Removal of the egg-containing bags during winter and early
spring is a very effective method for preventing problems before the next
growing season. Once removed, the bags should be destroyed or placed in a deep
container (5-gallon bucket), which allows beneficial parasites that may also be
present in the bags to escape while retaining the bagworm larvae.
If an infestation is severe or the bags are out of reach, spray
with the bacterial insecticide, B.t.
This insecticide contains spores of the bacterium,Bacillusthuringiensis,
which when eaten, kill the caterpillar. Young larvae are much more susceptible
to B.t. than are older larvae. As such, apply
this pesticide in the spring as soon as bagworms are seen (usually in May) and
repeat two weeks later. Control is most effective when spraying is done in late
afternoon or early evening. This insecticide is very safe to use. Once the bags
have reached ¾ inch long, the efficacy of B.t. sprays decreases rapidly.
Sprays applied later in the season (May and June), when bagworms
are larger must be with a contact insecticide, such as permethrin, cyfluthrin,
lambda cyhalothrin, carbaryl, malathion or acephate. Note that these
insecticides will also reduce populations of beneficial insects (predators and
parasitoids) that help control spruce spider mites, which can result in an
outbreak of this occasional Leyland cypress pest. Soil application of
neonicotinoid insecticides, such as imidacloprid or dinotefuran only give
minimal (less than 10%) control from bagworm damage, and should not be
substituted for spray control.