The best sprays for bagworms on evergreens are Bt (a naturally occurring bacterial insecticide), spinosad, and pyrethroid products like bifenthrin. But which one you choose matters far less than when you spray. Bagworm caterpillars are only vulnerable during a narrow window in late spring and early summer, and missing that window is the single most common reason treatments fail.
Why Timing Matters More Than the Product
Bagworm eggs overwinter inside the papery bags from the previous year and hatch in late spring, typically May through mid-June depending on your climate zone. The newly hatched caterpillars are tiny, exposed, and actively feeding on needles. This is when every type of spray works best.
As summer progresses, the caterpillars grow and build increasingly tough silk-and-needle cases around themselves. By August, they’re nearly full-sized and sealed inside bags that act as physical shields against sprays. At that point, even strong chemical insecticides struggle to reach the caterpillar inside. If you notice bags on your arborvitae or juniper in late summer and they already look like small pinecones dangling from branches, you’ve largely missed the spray window for that season.
Bt: The Go-To for Early-Season Control
Bt (Bacillus thuringiensis var. kurstaki) is a bacterial insecticide and one of the most effective options when applied at the right time. It works by being ingested: the caterpillar eats treated needles, the bacteria release a toxin inside its gut, and it dies within a few days. Bt is sold under brand names like Dipel and Thuricide, and it’s widely available at garden centers.
Because Bt breaks down in sunlight relatively quickly, repeat applications every 7 to 14 days during the hatching period give the best results. It targets caterpillars specifically and won’t harm bees, birds, or beneficial predatory insects, which makes it a strong first choice for most homeowners. The tradeoff is that it only works on young, actively feeding larvae. Once bagworms are larger than about an inch, Bt becomes much less reliable.
Spinosad: A Middle Ground
Spinosad is derived from a soil bacterium and sits between Bt and synthetic chemicals in terms of strength. It provides good control of bagworm caterpillars and is generally less harmful to pollinators and beneficial insects than broad-spectrum options, though it isn’t completely harmless to them. It takes about 3 to 7 days to kill the caterpillar after contact or ingestion, and you may need repeat applications for full control.
Look for spinosad on the label of products marketed for caterpillars, borers, and tent caterpillars. Like Bt, spinosad works best on smaller larvae caught early in the season. It’s a solid alternative if you want something slightly stronger than Bt without jumping to synthetic pyrethroids.
Pyrethroids: For Late Discovery or Heavy Infestations
If you don’t discover bagworms until later in the season, when caterpillars are already an inch or more, synthetic pyrethroids like bifenthrin and permethrin are your best remaining option. These provide fast knockdown and persist on plant surfaces longer than biological options, giving them a better chance of reaching larger larvae through their protective bags.
The downside is significant: pyrethroids are broad-spectrum insecticides that kill beneficial insects indiscriminately, including pollinators, parasitic wasps that naturally prey on bagworms, and predatory beetles. If you do use a pyrethroid, spray in the early evening when pollinators are less active. Avoid repeated applications to reduce the risk of the bagworms developing resistance over successive generations.
Carbaryl (sold as Sevin) is another rescue option for mid-to-late season infestations, with similar broad-spectrum concerns.
How to Spray Dense Evergreen Canopies
Bagworms feed throughout the canopy, not just on the outer tips, which means coverage is critical. For arborvitae, juniper, spruce, and other dense conifers, a light misting of the surface won’t cut it. You need to drench the foliage thoroughly, directing the spray into the interior of the plant where larvae hide and feed. A pump sprayer with an adjustable nozzle works well for shrub-sized plants. For tall trees, a hose-end sprayer or professional application may be necessary to reach the upper canopy.
Spray on a calm day with no rain forecast for at least 24 hours. Rain washes Bt and spinosad off the needles before caterpillars can ingest a lethal dose, and even pyrethroids lose effectiveness when diluted by a downpour shortly after application.
Handpicking Bags in Fall and Winter
Spraying is a summer strategy. In fall and winter, a completely different approach is available and surprisingly effective: physically removing the bags from your trees. Each female bag can contain 500 to 1,000 eggs that will hatch the following spring. Pulling the bags off by hand (or snipping them with pruners, since they’re attached by silk) and dropping them into a bucket of soapy water or a sealed trash bag eliminates next year’s population before it starts.
This works especially well on smaller ornamental evergreens where you can reach most of the branches. It’s tedious but free, requires no chemicals, and directly reduces the number of caterpillars you’ll face in the spring. On large trees with hundreds of bags, handpicking alone won’t solve the problem, but it meaningfully lowers the population heading into spray season.
Which Evergreens Are Most at Risk
Bagworms feed on over 100 species of trees and shrubs, but they hit evergreens the hardest because of how conifers grow. When a bagworm strips the needles from an arborvitae or juniper branch, that branch typically will not regrow foliage. Deciduous trees can push out new leaves the following season, but most evergreens cannot replace lost needles on defoliated sections. A severe infestation left untreated for a single season can kill an arborvitae outright or leave it permanently disfigured with large dead patches.
Arborvitae, eastern red cedar, juniper, and spruce are the most commonly attacked hosts. If you have a row of arborvitae acting as a privacy screen, bagworms can move through the entire planting in one season if unchecked. Inspect your evergreens in late May and June by looking closely for small, quarter-inch bags made of bits of needle and silk attached to branch tips. Catching them at that size is the difference between an easy fix and a serious problem.