Saturday, May 26, 2012

Tent Caterpillars invade Gulf Islands

      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.
      According to Dr. Myers, the severity of outbreaks varies with a cycle of approximately eight to ten years, and different regions are not all in the same part of the cycle at the same time. Last year Saturna was experiencing a peak year with a lot of disease, whereas Galiano was quite healthy and will probably be on their way to peaking.
      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.
      Tent caterpillars have many native enemies including birds, yellow jackets and other predatory wasps, parasitic flies, tiny parasitic wasps and predatory bugs as well as viruses, bacteria and fungus diseases. Encouraging these native enemies is the most environmentally sound (and often the simplest and least expensive) method of suppressing tent caterpillars.


  1. As of June 21 or so, the apple trees were budding out with new leaves, and the tent caterpillars are few and far between......

  2. visiting Galiano June 29th - plenty of tent caterpillars about!

  3. It's Septemter 3, 2012 and in my yard on Gabriola Island I have an outbreak of tent caterpillars on my lilac. Is this a normal thing?