There’s one word I’d use to best describe bagworms: “weird!” These are truly odd but fascinating insects that live throughout the eastern US, west to Nebraska and south to Texas. Unfortunately, they’re also very damaging to several species of trees, including juniper, arborvitae, cedar, pine, hemlock, spruce and honeylocust. Other trees such as cypress, willow, black locust, sycamore, apple, maple, elm, poplar, oak and birch can also be attacked.
In this close-up of a bagworm, it’s easy to understand how the pests can be mistaken for pinecones.
Photo Credit: ©2003 Buglady Consulting
This tree looks like it’s dying from the top down, but the damaged section is simply where bagworms have fed off the tree.
Photo Credit: ©2005 Buglady Consulting
Bagworm larvae feed on the foliage of these plants, creating open areas in the canopy. This can even cause tree death if bagworm populations are high enough over a long period of time.
So, why the name “bagworm?” Well, caterpillars that just hatch from their eggs secrete a silken bag around them. While the young larvae feed, bits of plant material are incorporated into the “bag.” As the caterpillars grow, their bags enlarge with them. What’s really weird about this is that the caterpillars move the bag around with them as they feed, and they can withdraw back into their bags when disturbed, much like a turtle. These bags are spindle-shaped and reach a length of 1½ to 2 inches.
Bagworms survive winter as eggs laid inside their mothers’ bags. In May or June, eggs hatch, and the young caterpillars crawl from these bags and start to feed. They continue to feed and grow throughout summer. In late summer, typically August, the caterpillars attach their bags firmly to the tree on which they’re feeding and enter the pupal stage, where they change to adults in early fall.
Males are black, furry and have clear wings. They are good fliers and seek the wingless, legless adult females. The females stay in their bags looking much like the caterpillar stage, but they release a sex pheromone that attracts males. The males mate with the females inside their bags. The female lays 300-1,000 eggs within her bag and then dies.
One of the simplest methods of managing bagworms is to harvest the bags from the tree or shrub they are on. They can be removed by hand or cut off with scissors. This is most effective from late fall through winter, or in early spring before eggs hatch. Be sure to throw away the bags so larvae don’t crawl back onto your plants.
If you haven’t noticed any bagworms, you may have various biological control agents at work keeping bagworm populations somewhat in check. Birds are known to tear bags open and feed on what’s inside. It addition, various predators and parasites attack bagworms. Unfortunately, these organisms can’t always keep the bagworm populations low.
If populations are too high, spraying is an option. The safest sprays are those containing the bacterium Bacillus thuringiensis (Bt). Bt is applied to the foliage, and it’s most effective when the larvae are small – typically after all the eggs have hatched in June. Insecticides can also be used. Like Bt, they’re applied to the foliage. Consult your local extension office or garden center for insecticide recommendations for this pest, and be sure to follow all label directions.
Though bagworms are fascinating don’t let these little pests go unnoticed and ruin your trees. With careful observation and a little know-how, you can easily bag your bagworms.