The bagworm, Thyridopteryx ephemeraeformis, is one of the most common and destructive pests of ornamental trees and shrubs, particularly conifers. These insects cause rapid and severe defoliation. Evergreen species like juniper and arborvitae are especially vulnerable since they cannot easily replace lost foliage. Effective management depends entirely on precise timing, requiring a grower to act decisively during a very narrow window.
Understanding the Bagworm Life Cycle
The bagworm spends winter as an egg mass, with hundreds of eggs housed inside the tough, silk-lined bag of the deceased female from the previous season. Hatching typically begins when temperatures warm consistently in the late spring, usually between late May and early June. The tiny, blackish larvae emerge from the base of the old bag.
Newly hatched larvae disperse in two ways: by crawling to new feeding sites on the same plant or by “ballooning.” Ballooning involves dropping from the tree on a fine strand of silk, allowing the wind to carry them to nearby host plants. This dispersal phase lasts only a short time before the larva immediately begins spinning a miniature silk bag, camouflaged with bits of foliage and debris from the host plant.
The larva remains sealed within this portable shelter for its entire feeding period, only protruding its head and thorax to eat and move. Once a bag reaches approximately a half-inch in length, the insect becomes increasingly shielded from chemical treatments. Chemical control is highly effective only during this short period when the larvae are small, mobile, and actively feeding.
Identifying the Critical Spray Window in Kansas
In Kansas, the critical period for effective bagworm control runs from late May through early July. Hatching is temperature-dependent, but generally begins in mid-to-late May. The ideal window for treatment is when all eggs have hatched and the larvae are actively feeding, yet still very small.
Experts often recommend scouting and treatment from mid-June to early July, as the larvae are still tiny and have not yet caused noticeable damage. The newly constructed bags will be less than half an inch long and may be difficult to spot unless closely inspected. Applying a targeted spray too early, before the hatch is complete, risks missing later-emerging larvae.
In years with cooler spring temperatures, the hatch may be delayed, pushing the effective spray window later into June. Conversely, a warmer spring may mean control measures are needed closer to the start of June. A single, thorough application in late June or early July is often sufficient for minor infestations, but two applications—one in early June and a second three to four weeks later—are recommended for trees with a history of heavy damage.
Selecting the Right Control Method
The most effective treatments are stomach poisons that the bagworm must ingest while feeding on the foliage. One of the most common and safest options is a biological insecticide utilizing the bacterium Bacillus thuringiensis (Bt), specifically the kurstaki strain. When ingested by the caterpillar, Bt releases a toxin that paralyzes the digestive system, causing the larva to stop feeding within hours and eventually die.
Because Bt must be eaten, it is only effective on very young, actively feeding bagworms and has minimal impact on beneficial insects or pollinators. An alternative effective treatment is Spinosad, a neurotoxin derived from soil bacteria. Spinosad is also ingested and generally remains effective for a longer period than Bt, providing better control against slightly older or larger larvae.
The insecticide must completely coat all the foliage, including the interior branches and the tops of the plants. Any foliage missed provides a safe refuge for the bagworms, allowing them to continue feeding and growing their protective bags.
Dealing with Late-Season Bagworm Problems
If the effective spray window of late May through early July is missed, chemical control becomes largely ineffective. By late summer (typically mid-to-late August), the larvae have finished feeding, anchored their bags to a branch, and sealed themselves inside to pupate. At this point, they are protected by the thick bag and are no longer eating treated foliage.
Once feeding has stopped, the only reliable method to manage the population is mechanical hand removal. This involves physically cutting the bags from the branches using shears or a knife. It is necessary to destroy the removed bags immediately by crushing them or placing them in a bucket of soapy water. Hand removal, particularly in the fall or winter, directly reduces the number of overwintering eggs, preventing a massive re-infestation the following spring.