Will a soapy water spray get rid or kill bag worms

Asked March 3, 2013, 6:05 PM EST

Will a soapy water spray get rid or kill bag worms


1 Response

Simple answer is NO! The typical bagworm case that you see on the evergreen or deciduous tree this time of year holds either the pupal case of the emerged male bagworm moth or the remains of the female body containing 300 - 1,000 eggs of very tiny caterpillars that will hatch and emerge this Spring (2013). The pupal case (bag worm bag) is very impervious to any and all liquid sprays. I usually tell people that it is "like their little yellow raincoat you wore to school in the thrid grade."

You could carefully remove as many of the bagworms that you can find on the tree. This will be a big job and require a lot of safety considerations. The biggest problem in removing the bags will be the small silk thread that the larvae used to tie off the bag and attach it permanently to the twig or branch. I have seen this silk thread remain and girdle the branch even though the bag itself was removed and sent away in the trash.

When the Ivory Silk Tree Lilac blooms or the Catalpa trees are in full bloom, the larvae are hatching and emerging from the small hole in the base of the bag. The larvae look like a "comma" on a standard typed page of a letter. They are attached to the bag with a thin silk thread that becomes a "parachute" or a part of a parasail like they use on the beach. The tiny larvae gets blown to other trees in the area and the infestation spreads from one tree to many. The ones on your tree hatch and spread to most parts of the tree. They then start to feed and soon grow into the small caterpillars. They make a small bag that they carry around as they eat and add parts of the leaves and twigs (even fecal matter) to the bag. This bag looks like the rest of the tree when the pieces attached remain greenish and blend in. When fall comes the bag turns brownish because the added material turns brown and then you notice how much they have eaten and how many have colonized your tree.

The best time to take care of the caterpillars is when they are young. If you can spray the entire tree two weeks after the tiny caterpillars emerge, and then a week to ten days later, you will have reduced the population significantly. The most environmentally friendly product to use is the bacterial toxin spray Bt -- Bacillus thuringiensis -- which is only toxic to caterpillars. If you can't spray the whole tree yourself, then find a reputable tree company that will use the Bt spray at the correct timing. Again, timing is everything in the control of the bagworm.

If you wait too long to spray the Bt, then you can use stronger insecticides in the pyrethroid category. Again, if you wait too long the bagworm will react to the insecticidal spray and close off the bag and begin to pupate early. That bag is impervious to the spray that you put on. Cold winter weather doesn't kill the eggs inside the bags. If we had several periods of 20 below zero, it might slow them down a little but with 300 to 1,000 eggs per bag there will still be enough survivors to cause damage.

To sum up:
1. Careful mechanical picking and removal to the trash is a good place to start.
2. Biological control with Bt spray at the proper time. Call a Certified Arborist to schedule your trees to be sprayed.
3. Chemical control with the pyrethroid insecticides at the proper time. Again, use a Certified Arborist to get the spray to all parts of the tree.

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