I have several arborvitae and pine trees that are covered with one-
to two-inch “cocoons” that are killing them. They are even on my
mailbox! Can you tell me what they are and what I can do to get rid
of them before my trees are completely dead?
ephemeraeformis) are the insects that appear as “cocoons” on many
different species of trees – and inanimate objects – at this time of
year. They have been feeding since late May, but do not become
noticeable until they cause considerable damage and become quite
large. They are very difficult to control at this point because they
are well protected from insecticides by the dense bags they
construct and because they are not feeding heavily – if at all - at
this point in their life cycle.
Bagworms are 'generalists'
While many insects are very host-specific, bagworms
are generalists. They feed on over 100 species of trees and shrubs,
including arborvitae, crabapple, honeylocust, juniper, maple, oak,
pine, spruce, sweet gum, and sycamore. The damage to conifers takes
longer to heal because most of them do not produce new growth from
old wood. Recovery happens as they continue to grow from their tips,
and eventually new growth will cover the damage. It can take years
for them to regain their appearance.
Dense bags or 'cocoons' protect a bagworm from
insecticidal sprays and need to be hand removed
at this advanced stage of development
These native insects overwinter as eggs in the bags
of female adults. The larvae hatch out from mid-May to early June
and immediately begin feeding and constructing bags from silk they
produce and bits of leaves from their host plants. They are quite
tiny when they first hatch, and carry their bags upright, making
them look as though they are wearing dunce caps.
Peek a Boo!
Cocoons are difficult to see since they blend
in so well with evergreen foliage.
Photo: Robert Donnan
Larvae continue to feed and grow through the summer
months, sticking their heads out of their bags to feed and move
about on host plants. They begin to pupate in August by securely
attaching themselves to twigs and/or inanimate objects. Then they
seal up their bags and re-orient themselves so they are facing
downward. They are no longer feeding when they pupate.
Adult bagworms are moths, although females lack wings and remain
grub-like; they never leave their bags. Adult males are
non-descript, charcoal-gray moths with clear wings that hatch out of
their bags and fly to mate with the females in late summer and early
fall. Males die after mating and females die after laying 500-1,000
eggs in each bag.
This late stage is often when the damage becomes
all too apparent on arborvitae.
They fact that females do not fly allows large
populations to build up on host plants in a short period of time.
Very tiny larvae can be blown in the wind, and they can crawl from
tree to tree when plants are relatively close together. They are
also spread on infested nursery stock.
Controls include removing the cases from infested
plants by hand, especially between now and when they hatch next
spring. Male cases can be left to weather off the plants since all
the eggs are in female bags. It can be easy to tell the difference:
male bags tend to be smaller than female bags, and the pupal case
often extends out of the bottom of the bag where the male emerged as
an adult. If in doubt, pick it. Hand removal is the only effective
option at this time of year. The silk bagworms use to attach
themselves to twigs is very strong and
usually has to be cut with scissors or hand pruners
in order to remove it without damaging the plant.
Circled Area: This is the best stage (early) to treat
bagworms on this Douglas fir
If plants are too large for hand removal, spray
applications are best directed at very small larvae. Since they are
caterpillars, small larvae are well controlled with least toxic
products such as Bacillus thuringiensis or Bt (Dipel, Thuricide,
others) and spinosad (Captain Jack’s Deadbug Brew, Monterey Garden
Insect Spray, others). Larger bagworms can be controlled with
carbaryl (Seven, others); cyfluthrin (Bayer Advanced PowerForce
Multi Insect Killer); and malathion.
If your trees are
very large, consider hiring a tree service to make the application
for you. Many have certified arborists on staff, and have the
equipment and training to effectively and safely spray large trees.
Bagworms on Spruce
Safe pesticide use