Those Ugly Tree Pests
Can Be Treated and Now Is the Time!
Besides picking bagworms off
infested trees and shrubs, the only way to kill them is to spray with an
ingestible insecticide while they are feeding.
The following article from OSU’s Entomology and
Plant Pathology Department details the life cycle of the bagworm and how best
to control the pest.
Description: The common bagworm, Thyridopteryx
ephemeraeformis, is discovered most frequently in its larval form, feeding
on trees from within a silken bag it constructs from foliage and other plant
tissues (hence, the common name of bagworm) (Fig. 1). Adult males are small
moths with a black, hairy body and clear wings with a wingspan of about 1 inch
(25 mm). Adult females are wingless, have no functional legs, eyes, or
antennae, and are almost maggot‐like in appearance. The female’s body is soft,
yellowish white, and practically naked except for a circle of woolly hairs at
the posterior end of the abdomen. Mature larvae are about 1 inch (25 mm) long
and have a dark brown abdomen, while the head and thorax are white with black
spots. Both larvae and adult females are found within bags on their host
Distribution: Bagworms are found in most states east of the
Rocky Mountains. It is most common from Pennsylvania to Nebraska and south to
Florida and Texas. It is common in all areas of Oklahoma.
Life Cycle: Overwintered eggs (contained within the bags of
1‐year‐old females) begin to hatch in late April or early May and young larvae
begin to feed and construct bags immediately. The first evidence of an
infestation is normally a small bag, about 1/4 inch (6.5 mm) long, standing
almost on end. As larvae grow, silk and fragments of the host plant foliage are
added to the bag until it reaches 1 1/2 to 2 inches (38 to 51 mm) long. When
larvae are mature they fasten the bag to a plant stem with silk. Pupation
occurs in the bag in late summer and males emerge in late summer to early fall.
They engage in a mating flight in search of the wingless females, who remain
inside their bags. After mating the female lays several hundred white eggs
inside her old pupal case, drops from the bag, and dies. There is one
generation per year.
Hosts: In Oklahoma, the most common hosts are eastern red cedar, other
junipers, and arborvitae. They sometimes damage pines, spruce, bald cypress,
maple, boxelder, sycamore, willow, black locust, oaks, and roses. The bagworm
has been recorded on 128 different plant species in various parts of the United
Damage: Bagworm larvae damage their hosts by feeding on
the foliage. Heavy infestations can completely defoliate small plants.
Defoliation usually kills hosts such as red cedar and other junipers. Broadleaf
hosts are not killed but are weakened and become more susceptible to woodboring
insects and pathogens.
Cultural control: Infestations can be reduced by handpicking bags
(and overwintering eggs within bags) during fall, winter, or spring before eggs
hatch. Eggs remain viable within bags so be sure to destroy bags upon removal
by burning them. When larvae become active, bagworms can still be removed by
hand if the numbers are small and the affected host plants are small enough to
reach the canopy. Again, take care to destroy the bags once they are removed.
Biological control: There are several naturally occurring parasitic
wasps and predatory insects that attack bagworms. The activity of these natural
enemies apparently explains the fluctuation in bagworm populations observed
from year to year.
Chemical control: Chemical controls are most effective if applied
early when larva are small. In Oklahoma, it is normally a good practice to make
applications of insecticide by early June. Bacillus thuringiensis var. kurstaki,
a bacterial insecticide (Thuricide –brand name), is reported
to provide good control of bagworms. Also effective are products that contain
the active ingredient spinosad, another microbial agent (Captain Jack’s Dead Bug Brew
–brand name). These insecticides must be ingested by the caterpillars in order
to achieve kill, so be patient as it will take some time to see results. Repeat
applications may be needed later in the summer in order to keep susceptible
plants free of bagworms. This is not due to the occurrence of multiple
generations. Rather, not all eggs will hatch at the same time in some years and
there may be migration of larvae between host plants. In most years, treatment
in early June will catch most of the emerging larvae and provide fairly good,
season‐long control. The larger, older larvae can be controlled with products
containing acephate (Orthene), carbaryl (Sevin), bifenthrin, cyfluthrin, and
J. Rebek, State Extension Specialist for Horticultural Insects -Entomology and Plant
Pathology, Oklahoma State University 127 Noble Research Center,
Stillwater, OK 74078 405.744.5527