Tent caterpillars are
upon us again this year, with a vengeance. We watched the tents
forming on our apple trees and the surrounding alders and hawthorns,
and with a few warm days we saw an explosion of caterpillars. They
love the heat. Even though the damage to individual trees can be
severe, they usually grow back foliage by the summer and rarely are
trees killed, so there is no need for panic and drastic measures,
like cutting your trees down.
Still, it is hard to
stand idly by and watch the drastic defoliation that occurs in a tent
caterpillar outbreak.
I thought last year was
pretty bad, and asked Judith Myers, Saturna resident and UBC
researcher, some questions about tent caterpillars. Judith has
studied western tent caterpillars and their biological controls for
several years, and has several study areas, including Saturna,
Westham, Galiano and Mandarte Islands and the Cyress Mountain area.
The caterpillars hatch
into moths, which lay many eggs so have the potential to increase.
When they get very dense they get a viral disease that is specific to
them. That kills many of them. As parasites build up in the tent
caterpillars, and they defoliate the trees, their numbers begin to
decline. They are a native insect and their natural targets are the
deciduous trees, especially red alders, and the hawthorns and wild
roses. Their preferred hosts are alder, apple, ash, birch, cherry,
cottonwood, willow, fruit trees, and roses. During heavy
infestations, the tent caterpillars will migrate and feed on many
other plants.
Dr. Myers says that the
tent caterpillars are probably doing particularly well because humans
create disturbances which increase the number of red alder trees and
fruit trees that they can feed on. If they are just left alone,
they will naturally decline and continue their cycle, but most people
want to prevent or treat the outbreaks on their fruit trees. With a
few trees, hand picking and cutting off nests, in the evenings when
the caterpillars return to their nests, can help to reduce damage.
The cut-off nests are burned or put into bags and sealed for
disposal. Some orchards use Btk spray (Bacillus thuringiensis
kurstaki ) which is good for hard to reach areas. The
microbial sprays based on this bacterium work only after a
caterpillar eats a piece of leaf with Btk crystal proteins and spores
on it. The proteins dissolve in the highly alkaline conditions found
in a caterpillar gut and this paralyses their digestive tract. This
causes the caterpillar to stop feeding and eventually to starve. Btk
is non-toxic to humans, other mammals, birds, snakes, fish,
earthworms and most other insects. It is an excellent choice for
caterpillar control because it does not harm the beneficial insects
and other animals that keep caterpillar numbers low. Btk must be
eaten by caterpillars to have an effect, therefore it should only be
used when caterpillars are actively feeding. It does not work on
eggs, pupae or adult stages. For best results, spray in the evening,
when no rain is expected. Use a fine spray and ensure that both sides
of leaves are thoroughly covered. However, if only the apple trees
are protected, caterpillars can move on to them later in the season
from other host plants nearby.
Removing eggs and small
tents, spraying, and continual vigilance can reduce the problem and
protect the trees. The BT sprays have been widely used for years,
are safe, and don't kill off the natural enemies of the moths.
