Based on questions I’ve received in the past week I think it’s time for an article on bagworms. I don’t believe I’ve covered the subject since June 2008, so it is way overdue.
Bagworms are brown, dangling bags that might make you think of Grinch-style Christmas tree ornaments. They come from a perennial moth-like insect that loves to frequent some evergreens. In the larval stage, there are worms inside those cocoons of juniper and cedar needles. And while their usual hosts are junipers, cedars, and arborvitaes, they have been known to show up on hollies. And if you don’t catch them early, they can cause extensive damage. So, as soon as you spot one, pluck it off and throw it away.
When I was an irresponsible youth growing up in Walnut Bend, it seemed as though EVERYONE had junipers. And, of course, everyone got bagworms. My mother would ask me to pluck them off and kill them, assuming I would simply mush ‘em or step on ‘em. But not me. We kept lots of empty metal coffee cans around, and I’d put as many bags as I could in one, grab my folks' cigarette lighter fluid (everybody smoked back then) and saturate the bags. Then I lit them on fire.
Little did I know back then, that was actually the best way to control them, except for the lighter fluid part. Instead, drop them in a bucket of soapy water. After an hour or so, dump the water, put all the soggy stuff in a resealable plastic bag, and throw it in a trash can.
You should probably treat the whole plant with a residual control, too, in case you miss one or two that could procreate.
Bagworms spend the winter as eggs in the bag of a female from the year before. The eggs hatch May through July in these parts. Individual larvae drop from the bag on a strand of silk and either swing onto a branch or sail in the wind to another tree. Immediately upon landing, they build a silk-lined bag, often decorating it with bits of foliage. Larvae remain in the bag feeding, with only their heads and thoracic segments extending out.
They are very small when they hatch, but as the larvae grow, the protective bag expands to accommodate them. In August, feeding will stop, and adults emerge in two or three weeks. Females remain in the bag looking like a slug. Males become winged moths and fly to a female’s bag where they mate. The male dies, the female lays up to 1,000 eggs in her bag, then dies as well. The bag will protect the eggs until the following spring.
It is best to use some method of control before they get to the egg-laying stage in September or October.
If you see worms and silk threads, you can fight them organically with liquid or powdered Bt (Bascillius thuringeinsis). I know of people who will both remove the bags and apply Bt. I’ve also seen successful treatment with products like Sevin, powdered or liquid.
I’m also okay with using systemic insecticides with acephate as the active ingredient. Normally, I wince at systemics that can affect the bee population, but since most evergreens don’t have flowers at this time of year, you have my tacit approval for this method.
Finally, no matter what tactic you use, be sure to rake away any debris at the base of the plants and dispose of it. It might contain fallen bags, worms and eggs.