When Do You Spray For Bagworms

If you’ve noticed strange, spindle-shaped bags hanging from your evergreen trees, you’re likely wondering when do you spray for bagworms. Timing your treatment correctly is the single most important factor in controlling these destructive pests and protecting your landscape.

Bagworms are caterpillars that construct protective cases from plant material, making them look like small pinecones or seed pods. They primarily target arborvitae, juniper, spruce, pine, and cedar, but they can also decimate deciduous trees. Left unchecked, a severe infestation can completely defoliate and kill a tree. The key to stopping them is understanding their life cycle, as spraying at the wrong time is simply ineffective.

Understanding the Bagworm Life Cycle

To know when to spray, you need to know what the bagworms are doing throughout the year. Their life cycle dictates everything.

Late Spring (May – Early June): This is the critical window. Eggs hatch inside the mother’s old bag, and the tiny larvae emerge. They immediately begin to spin their own tiny bags and start feeding. They are most vulnerable at this stage.
Summer (June – August): The larvae continue to feed ravenously, enlarging their bags as they grow. They move around the tree, sometimes dangling on silk threads to reach new foliage.
Late Summer (August – September): The mature larvae stop feeding and permanently attach their bags to a branch. They then pupate inside the bag.
Fall & Winter: Adult male moths emerge in early fall to mate with flightless females, who remain in their bags. After laying 500-1000 eggs inside her bag, the female dies. The eggs overwinter inside that durable bag, ready to hatch the following spring.

The Best Time to Spray for Bagworms

The absolute best time to spray for bagworms is in late spring to early summer, precisely when the young larvae have just hatched and are actively feeding. This period is your golden opportunity.

Here’s a simple seasonal guide:

Optimal Window: Late May through mid-June. This can vary slightly by your climate zone. A good rule is to begin monitoring your trees in mid-May.
Target: You are targeting the small, young caterpillars that have recently emerged. Their bodies are soft, and they have not yet built substantial protective bags.
Why This Timing Works: The larvae are tiny and vulnerable. Insecticides make direct contact with their bodies or are consumed by them as they eat treated foliage. The bags are also still small, so spray penetration is better.

What if You Miss the Spring Window?

Don’t panic if you find an infestation later in the summer. You still have options, though they require more effort.

Mid-Summer (July): Larger larvae are harder to kill with spray, as their bags offer more protection. You may need a stronger systemic insecticide, and thorough coverage is even more crucial.
Fall & Winter: Spraying is completely ineffective once the larvae have pupated and the eggs are dormant. This is, however, the perfect time for manual removal.

Step-by-Step Guide to Effective Spraying

Follow these steps to ensure your treatment is successful.

Confirm the Infestation: Look for the distinctive 1-2 inch bags. In early spring, check for last year’s bags filled with eggs. In late spring, look for tiny, moving bags on new growth.
Choose Your Insecticide: Select a product labeled for bagworm control on your specific tree type. Common effective ingredients include Spinosad, Bacillus thuringiensis (Bt), Cyfluthrin, or Permethrin. Bt is an excellent organic option for young larvae.
Time it Right: Plan to spray on a calm, dry day with no rain forecast for at least 24 hours. Early morning or late evening is best to avoid harming beneficial insects like bees.
Mix Thoroughly: Follow the label instructions exactly. Under-dosing is ineffective, and over-dosing can harm your plants.
Spray Completely: This is the most important step. You must thoroughly drench the entire tree, focusing on getting the spray inside the foliage where the bagworms feed. Hit every branch from top to bottom, as the larvae often start feeding at the top. The goal is to coat the plant material they will eat.
Repeat if Necessary: Check the product label for the recommended waiting period. A second application 7-14 days later is often advised to catch any late-hatching larvae.

Manual Removal: A Critical Companion Task

Spraying should always be combined with manual removal for the best, long-term control. This is especially vital in fall and winter.

When: Anytime from late fall through early spring, before the eggs hatch. The bags are easy to see on bare deciduous trees.
How: Use hand pruners or scissors to snip each bag off the branch. Be sure to cut the silk band that attaches it.
Disposal: Do not just throw them on the ground. Place the collected bags in a bucket of soapy water to drown the insects, or seal them in a plastic bag and dispose of them in the trash. This prevents the eggs from hatching elsewhere.

Preventing Future Bagworm Problems

Consistent vigilance is your best defense. A few simple habits can prevent a major outbreak.

Annual Inspection: Make it a routine to check your susceptible trees every spring (May) and again in the fall (October).
Encourage Natural Predators: Birds, such as chickadees and nuthatches, eat bagworm eggs and larvae. Providing bird feeders and houses can help attract these natural helpers to your garden.
Maintain Tree Health: A stressed tree is more suseptible to pest damage. Water your evergreens deeply during dry periods, especially before winter.
Isolate New Plants: Before planting a new evergreen, inspect it carefully for any signs of bagworms. You don’t want to introduce them yourself.

Frequently Asked Questions (FAQ)

What month is best for bagworm spray?

The best month is typically June, when the larvae are actively hatching and feeding. In warmer climates, this may begin in late May; in cooler areas, it may extend into early July.

Can you spray too late for bagworms?

Yes. Spraying in late August or fall is useless because the caterpillars have stopped feeding and entered the pupal stage. At that point, manual removal is your only current-year option.

What is the best product to spray for bagworms?

For young larvae, natural Bacillus thuringiensis (Bt) or Spinosad are highly effective and low-impact. For older larvae, a synthetic insecticide like cyfluthrin may be needed. Always read the label for proper application on your specific tree species.

Will bagworms come back after spraying?

A proper spray application in spring will kill the active larvae. However, new infestations can blow in from untreated nearby trees on silk threads. That’s why combining spray with annual manual removal of bags is the best strategy for lasting control.

Is it too late to treat bagworms in July?

It is not ideal, but it is not necessarily to late. Larvae in July are larger and harder to kill. You will need a thorough application of an effective insecticide, and you should still follow up with manual bag removal in the winter.

Can a tree recover from bagworms?

Evergreens that are completely defoliated often cannot recover because they cannot produce new needles from old wood. However, if the tree retains some living foliage, especially at the tips, and the infestation is stopped, it may slowly recover over several years. Deciduous trees are more resilient and can often releaf if treated in time.

By knowing exactly when do you spray for bagworms and combining that with physical removal, you can protect your trees effectively. Remember, the effort you put in during late spring saves you from much bigger problems down the road. Start monitoring your trees now, and you’ll be ready to act at the first sign of those tiny, moving bags.