Jeff Dodson, MG 2014
Hello Everyone from the Clinic Desk,
Teaching the plant pathology section of the Master Gardener
Training Course is always a learning experience for me. Last week was no exception as the questions
by the students made me think over points in the subject matter. One of these that stuck with me in relation
to the clinic is the idea of controlling the critters that attack our
plants. Of the control measures
available to us, I favor all cultural and biological controls over chemicals, as
do the vast majority of those reading this newsletter.
Every year the clinic gets phone calls, emails and walk-ins
from clients concerned with the presence of web nests and their caterpillar
inhabitants that appear on bushes and trees as the insects voraciously devour
foliage. By-and-large the causal agent, the
Western tent caterpillar, is seen every year and so control becomes a yearly
task for us all with susceptible plants.
As they are a native species, this lepidopteran will always be around
and so we must first search ourselves individually to know how much of their
feedings we can live with before losing our sanity… or plants. Ordinarily, damage to plants is seasonal and
the plant recovers, but if the environment is favorable to the insect, it can
have rapid sequential life cycles that likely will set plants back severely,
cause fruit damage or even kill plants.
One-hundred-percent control is not likely to be accomplished, but cultural and biological control measures will definitely bring a level of
management into the picture that you may find acceptable. As always, knowing your enemy and its
habits/life cycle will allow you to assess where best you can apply measures
that will eliminate the threat. As spring
approaches, daily inspections of your plants will yield an early detection of
the pest. This is when it is most
vulnerable. Egg cases that hold up to
400 eggs are laid in the fall as an overwintering mechanism for the
insect. They look like a gray, hard
styrofoam mass clinging closely to twigs, usually in the apical regions where
the newest plant growth will first appear.
These can be removed while pruning by simply rubbing them
off the stem and placing them in a bag that can be disposed of later. VOILA!
You have instantly gotten rid of hundreds of the little rascals. Continue your daily inspections as you will
probably not get all the egg cases. Once
they hatch the tiny caterpillars begin webbing and feeding immediately and,
although they start small, their colonies and associated damage expands rapidly
(1-2 days) and become much more difficult to deal with.
In the morning they leave their nest and venture out to find
foliage to feed upon, laying down a trail of silk as they go. This they use to
find their way back to their nest as the day ends and temperatures drop. They are safest from their many natural
enemies and from any liquid control measures you might apply while in their
nest. However, they are all in one place
and so susceptible to you pruning out the nest, placing it in a bag and disposing
of it.
Also, while small (under 1 centimeter) they are vulnerable
to Bacillus thurigensis (B.t.), a
control measure that you can buy as a liquid and apply to the foliage (not the
insect) which the larvae eat and die. Older
caterpillars seem impervious to B.t. The
young caterpillars stop eating upon ingesting B.t. and within a day or two are
dead. Only effective upon moth and
butterfly larvae, it is a very safe material to apply, however, please be aware
that it will kill any other caterpillar that ingests it, too. In addition, dormant oils have been shown
repeatedly by WSU and other universities to be very effective at Western tent
caterpillar control. As always, implicitly
follow the label instructions on anything applied to your plants.
There are certainly other, more potent, chemicals you can
use to control these insects but I have found the above methods to be
completely acceptable. Many a client
have utilized these control measure, also, and been very pleased with the
results. If you use them, please let me
know your assessment.
Best regards,
Jeff