Bagworm season is upon us in Kentucky. Depending on where you live, the hatching could have occurred anywhere from just in the last few days to a couple of weeks ago. If you want to monitor for their emergence, you can use this handy emergence map made through the National Phenology Network here: https://www.usanpn.org/data/forecasts/Bagworm
Description, Damage & Hosts
An adult bagworm is a moth, but the destructive stage is the caterpillar. Unlike other caterpillars that you can see out in the open and feeding, the bagworm wears a tactical stealth sleeping bag that helps to camouflage it.
The caterpillar creates the bag from silk it can produce itself and from the materials they find in their surroundings. This means that they can have pine needles, cut off chunks of leaves, even fruiting bodies attached to the bag (Figure 1). Some clients that spot the bag are confused at the number of pinecones or seed pods in their tree, not knowing their plant has been invaded.
Bagworms are most commonly found in evergreen trees and shrubs but also can infest deciduous trees. Some favorite hosts are arborvitae, juniper, pine, red cedar, and locust trees. Their damage causes defoliation but also bronzing in evergreens. While bagworm damage in one season is not usually enough to kill a plant, successive years of infestation and damage can prove deadly.
Bagworms overwinter as eggs inside of their mother's bag. As the season progresses, the eggs accumulate degree days. Every day at or above a certain temperature counts as a degree day or days towards hatching.
Once they reach their magic number (between 600 and 900 growing degree days), the eggs will hatch. After this, the caterpillars might re-infest the same plant that they were born in or they may "balloon" away to a new site. This means the caterpillar will crawl to the tip of a leaf and release a silk strand that catches the wind to help it find a new place to settle. Kite surfing baby caterpillar pioneers could have whizzed by you very recently! Their bags start small as you can see in Figure 2, but if you are paying close attention in spring, you can spot them.
As the summer progresses, bagworms will consume leaves around them to expand their bag. Eventually the bag will reach about 1.5 to 2 inches long. The caterpillar can be seen poking its head out of the bag and feeding (Figure 3). If disturbed, it will withdraw inside the bag and are difficult to extract.
By the end of the summer, the caterpillar will pupate and males will emerge as moths that flutter around looking for females. Females never fully become what we recognize as a moth. They are wingless and never leave the bag. They release a pheromone that attracts the males to them so mating can occur through the opening of their bag. Then females lay their eggs and perish.
Managing bagworms is easiest usually around early June.
If you notice the bags or know that you have a tree that was infested last year, you should consider purchasing a Bt-based product to control the young caterpillars. This is an organic option and very effective against caterpillar pests if used against the early instars.
Other options include Spinosad and neem. If you miss this early window for control, the problem will be harder to deal with later in the season.
If you catch this in late July or August then you will have to resort to using a bifenthrin, carbaryl, or cyfluthrin product. These will kill beneficial insects whereas Bt products are much less harmful to our good bugs.
You can also wait until fall/winter and remove the bags by cutting them off the twig with a pair of shears and destroying the bagworm in soapy water. This will reduce the number of caterpillars that emerge the next year.
For more information, contact the Pulaski County Extension Service at 606-679-6361. Become a fan of Pulaski County Horticulture on Facebook and follow @hortagentbeth on Twitter, kyplants on Instagram, and follow us on YouTube at Pulaski County Horticulture.
The Pulaski Co Extension office is open to the public on a regular basis, Monday through Friday 8am to 4:30pm.
The Lake Cumberland Master Gardeners have pine straw mulch for sale at the Pulaski County Extension office. It is sold in bales for $7 per bale (50 and over, $6 per bale). It can be purchased during office hours 8am to 4:30pm Monday - Friday, although Tuesdays are preferred.
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