Friday, June 19, 2009

Bagworms Made It a Successful Day (...and how often can you say that?)

It's been a successful day, as Prairiewolf likes to say, because I've learned something new.

In reading our local paper this afternoon, I came across an article by one of the area Extension agents on bagworms (Thyridopteryx ephemeraeformis). Normally I might have scanned it at least briefly, because I'm generally interested in learning about insects, but we are in the middle of a major bagworm outbreak. They are everywhere, and have been "decorating" trees all over the place for several years now. So I read the article quite closely, looking for hints about what I can do besides spray insecticides.

(The photo below was taken last August, behind the draw. It's obviously a fresh bagworm bag, given the bright color of the cedar berries that the larva attached to his/her bag for camoflage. In fact, the bagworm inside this bag was probably pupating as I took this photo!)

Although I knew a fair amount of the material presented, I did learn some important new information. For example, I learned how bagworms disperse. I've wondered about this, since I've only seen the larvae crawl around, quite laboriously, and I know that only the males ever attain adult form and fly. Yet the redcedars out in the middle of grassy fields will be festooned with obviously they must be more mobile than I've given them credit for in the past.

(Here is a photo, taken last September 15th, of a male bagworm moth resting on the side of our house.)

Which, I learned, they are. After hatching, the tiny little caterpillars (larvae) will spin a long thread of silk, catching a breeze (of which we have plenty in Kansas), and ballooning to a new site. By the way, "ballooning" is the actual technical term - a lovely scientific concept. Only when the larvae have found a suitable feeding site will they settle down, start producing their characteristic bags...and eat their way to our attention.

Knowing that bagworms have hatched out in the last month or so, I started wondering about what SHOULD be keeping their population under control naturally. Multiple website visits later, I can report that...

* Ichneumon wasps, especially the species Itoplectis conquisitor, parasitize the larvae. There are many other insect predators as well, with tachinid flies mentioned by name.

* Altogether at least 19 species of insects have been recovered from bagworm cases, including parasitoids like the ichneumons, hyperparasites, and scavengers.

* Bacteria, at least one virus, and numerous fungal species can also cause bagworm larvae to die.

* Finally, birds can be significant predators of the bagworm larvae as well. Species mentioned by name were chickadees, nuthatches, titmice, sapsuckers, sparrows and finches.

So why are the bagworms so out of control around here these days? In my research, I found the normal explanation that their numbers are cyclical, with the population climbing rapidly at times until predator/parasite populations can follow suit and bring the bagworms back "under control" again. For now, I'll assume that's the entire explanation...although with weather disruptions, chemical interference and other environmental changes, I also find myself thinking about more permanent and long-lasting bagworm issues.

Perhaps the most interesting parts of the research I looked at today involved non-chemical means of decreasing bagworm populations. You've probably heard about picking the bags off and destroying them. But how do you do that in 20' tall cedars, like the 50 or so that I have lining my driveway, parts of the draw, and the north side of the house?

A suggested idea that I loved was to plant flowers who are members of the Aster family. These daisy-shaped flowers attract and feed the adult ichneumon wasps and tachinid flies that parasitize the bagworms, helping to bring them into your area and increase their numbers. I already have quite a few asters and their relatives, especially in my front flower garden. I just need to plant a lot more, especially around the base of infested evergreens.

I'm already feeding birds and providing lots of habitat (as well as nesting boxes) to support their presence in the yard. Judging by the activity in the yard, I've got nesting Carolina wrens, chickadees, goldfinch, and house finch - all of which are likely bagworm predators.

Hopefully climate conditions will increase fungal or bacterial infestations of bagworms, too. I can't imagine my yard without its many big red cedars, but time will tell. Meanwhile, I'll be out planting and transplanting perennials to provide sumptuous nectar and pollen feasts for ichneumons and tachinids. Bagworms, watch out!


Anonymous said...

Thanks for the info! Will try the asters next year...this year have hand picked over 1100 bagworms off a single arbor vitae!

Gaia Gardener: said...

Arbor vitae seem to be especially attractive to bagworms, unfortunately. I've had a couple killed by them over the years, so it's a good thing you picked yours clean.

I hope the asters will help!