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Who among us is guilty of not noticing something until it’s too late? Yes, all of a sudden there is nothing left of your blue spruce or arborvitae. Bagworms have been munching on the needles for weeks, and we wonder how it all happened.
Well, they are at work right now, so go outside and take inventory of your evergreens because that’s what the bagworm likes the most. Now is the time they do their damage unless we put a stop to it.
Ten years ago or so I saw the worst bagworm infestation I have ever seen in my life. I can honestly say that it was somewhat life altering because now I have a permanent image that I can refer to when people ask about their bagworm problems.
This blue spruce was teeming with little chewing bags that, although the air was still, it seemed as though a breeze was passing through the boughs of the tree. In short, it was freaky.
In the case of the mostly-eaten-bagworm-covered blue spruce, the homeowner will need to use a systemic insecticide in order to control the infestation. And because these worms are clearly still actively feeding, it will be an effective control.
The systemic insecticide will be taken up into the tree, and as the caterpillars feed, they will ingest the insecticide and die. This will notwork on the problem once feeding has stopped.
And it wouldn’t surprise me if the spruce is dead by now, anyway, because of heat, drought and the stress of near complete defoliation. This is a case of too little and too late!
Usually hand removing the bagworms is a sufficient control measure, but in cases where the infestation is severe, reoccurring and on an already stressed tree, you may need to take more heroic measures.
If we understand the life cycle of a bagworm, we can responsibly control it.
Bagworms begin their lives in late May to early June, when hundreds of larvae emerge from the almond shaped bag that has been affixed to a twig all winter long. As the larvae hatch, they begin to feed immediately on the foliage of the plant. As the individual larva feed, they also begin to construct protective bags of available plant material around their hind ends.
The bagworm will drag the bag around everywhere it goes until it reaches maturity, at which point it affixes the bag to a twig. The male bagworm leaves its bag, flies to the female, who never leaves her bag, and mates.
Once mating is complete the male dies. Once the female lays her eggs inside her bag, she dies, leaving her eggs inside the protective bag. The period of active feeding usually lasts to about early August, then mating and the eggs are in place by late summer.
So now we have a bag that used to be the protective home for one larva that by late summer is home to hundreds of eggs. These eggs will winter over in the bag, affixed to the twig, only to hatch out the following May.
This is when most control measures can be taken by simply removing the bags with the eggs in them in order to prevent the eggs from hatching.
Be aware of reoccurring infestations, because if you catch them early, you can use biological controls like Spinosad and Bt to kill the newly hatched and feeding larvae.
The bio insecticides typically work on caterpillars that are about a half-inch long but anything larger than three quarters of an inch may need something more.
So, alas, try to pick off the egg bags before the problem gets to that point. You can use a plastic grocery bag like a glove to pick them off and then turn it inside out , tie it up and step on it to smash the sacks.