Jul 23 2013
When Steve Tirone learned the identity of these pine cone look-alikes he had his work cut out for him.
Early this month he sent me this photo (below) and asked: “Any idea what this is? I had a few on my house last year. They are near my cedar tree.”
I emailed Monica Miller who replied: bagworm moth caterpillar.
Bagworms adorn themselves with disguising vegetation that eventually becomes their pupating bag. Until then they chow down on their favorite foods.
Steve looked closely at his cedar tree and discovered it was covered in caterpillars that were eating it alive. No wonder it looked sick! He also learned that pesticides don’t work on these bag-covered bugs. The only way save his tree was to pull off each one by hand, as in…
[I picked them off] for 2 hours today, along with chopping out large parts of the tree. Ugly work, ugly results, and there are still tons left. Every time I go by the tree I say “How did I miss that one, and that one?” Easy, because they look just like pine cones!
If any remain on the tree, what will happen next?
Monica did not specify the bagworm species — there are over 1,000 of them worldwide — but I’ll tell you the story of the evergreen bagworm moth (Thyridopteryx ephemeraeformis (Haworth)) because it’s a particular pest of cedars (Thuja occidentalis also called arborvitae).
From May to August the caterpillars eat and mature through seven instars. In August the mature caterpillars prepare to pupate by hanging their bags from host plants by a strong silken thread. Then they turn around inside the bag to face downward.
Four weeks later, in September and early October, the males emerge. They’re about an inch long and are easy to overlook because they’re small and black, about an inch long. They eat nothing.
The females never emerge. They’re wingless, legless and have no functioning mouth parts. They’re just a bag of eggs and pheromones waiting for a male to land on their bag and mate with them.
As soon as the females have mated they shut off their pheromones and lay 500-1,000 eggs inside the pupal sack inside the bag. They live a couple of weeks, crawl out of the bag to die or become mummified inside. The males are long gone, having died within a day or two of their emergence.
Over the winter the bag hangs on the tree with 500 to 1,000 potential caterpillars waiting to hatch next spring.
If you pull these bags off your cedars before April you’ll save your trees a lot trouble.
For more information see this fact sheet from Penn State.