I have several young blue spruce trees that seem to be dying
from the top down. Please look at the enclosed picture and see
if you can tell me why this happening and what I can do about
picture shows a blue spruce tree (Picea pungens ‘Glauca’) that
appears to be 8-10 feet tall, and the top fourth of the tree is
bare of needles. The bottom of the Spruce looks normal. The bare
top branches have what appear to be “pinecones” hanging from
them. The "pinecones" are actually bag cases that last year's
generation of bagworm larvae constructed.
Adult bagworms are moths. This pest has a wide host range that
includes arborvitae, spruce, pine, Douglas-fir, true firs,
hemlock, juniper, larch, birch, black locust, honeylocust,
maple, oak, sweetgum, sycamore and others.
"Pinecones" near top are actually bagworm cases
Bagworms overwinter as eggs laid inside the adult female's bag
case. In our area, the eggs usually hatch from mid-May to
early June, depending on moisture and temperature. The small
larvae begin feeding on the host plant, and start to construct a
bag case that is made up of silken strands spun by the insect
and bits of the host plant. Consequently, the bag cases are
well camouflaged, especially when they are still small. The
larvae move the bag cases along with them, and emerge from the
top as needed to feed.
They continue to feed and grow, and enlarge their bag cases as
needed through the remainder of spring and most of the summer.
They pupate in late summer, and hatch out as adults in seven to
ten days. The male bagworm adult is a small black moth with
almost clear wings. The female bagworm adult does not look like
a moth at all - she lacks eyes, antennae, legs, wings, and
functional mouthparts. She never leaves the bag case she
constructed as a larva. The male leaves his bag case, flies to
the female and mates, and then dies shortly thereafter. The
female lays 500 - 1000 eggs inside her bag case, and then dies.
Since their mobility is limited, a large population of bagworms
can build up on a single host plant. You can easily miss them
when their numbers are limited, but once a population builds up
on a host plant, you cannot miss the spectacular defoliation
caused by the larvae feeding.
Bagworms are often seen on Arborvitae after
it's too late
Handpicking the bag cases from the trees in winter and early
spring provides good control because you remove the eggs
before they hatch. You may need to use a pair of hand pruners to
cut the bag cases off of twigs to avoid damaging the plant - the
silk these insects produce to tie the bag cases to twigs is
surprisingly strong. Your spruce trees appear to be small enough
for that approach to be practical. Put the bag cases into a
plastic bag, seal it up tightly, and send it out with the trash.
My only concern with this approach is that we are very close to
time for them to hatch.
SPRAYS FOR BAGWORMS
If they have started to hatch and you miss some with the
handpicking, you can spray to control the escapees. While
they are still very small – under ˝ inch - Bacillus thuringiensis (Bt) provides good control. Otherwise, Sevin (carbaryl),
Bayer Multi-Force Insect Killer (cyfluthrin), Bonide Caterpillar
Killer (lambda-cyhalothrin), pyrethrins plus piperonyl butoxide,
and Captain Jack’s Deadbug Brew (spinosad) are labeled to
control bagworms on spruce.
Insects on trees and
Tree sprout elimination