Bagworms: Spray in Mid June or Do Not Bother

Bagworms are the most commonly mistreated pest in Kansas landscapes, and the reason is almost always timing.

Why timing decides everything

Bagworms spend most of their life inside a bag they build out of silk and bits of the plant they are eating. That bag is armor. An insecticide sprayed on the outside of a sealed bag does not reach the insect inside it.

There is a window, right after the eggs hatch, when the larvae are tiny, feeding openly, and building bags too small and thin to protect them. That window is when treatment works.

In northeast Kansas that is roughly mid June, give or take a week depending on the spring. Some years late May, some years early July.

Spray in August and you are spending money on a plant that is already stripped, killing nothing.

How to know the window has opened

Look for new bags about a quarter inch long, standing upright and moving. If the bags on your juniper are an inch or more and hanging still, you have missed it for this year.

The old bags from last season stay on the plant indefinitely. Do not use those as your signal. Look for small, new, moving ones.

What they attack

Eastern redcedar and junipers most of all, which matters here because redcedar is in every windbreak and juniper is in half the foundation plantings in Topeka and Lawrence.

Spruce, especially blue spruce.

Arborvitae, which they can kill outright.

They will also feed on deciduous trees including honeylocust, maple, and sycamore. A deciduous tree usually survives defoliation because it releafs. A conifer often does not, because most evergreens cannot regenerate needles from bare wood.

That is the part people underestimate. A bagworm infestation on a maple is unsightly. The same infestation on an arborvitae or a spruce is fatal.

If you missed the window

Hand picking works and is genuinely effective on a small plant. Every bag you pull off in winter is 500 to 1,000 eggs that will not hatch in June. Cut them off, do not just pull, since the silk band can girdle a twig. Bag them and put them in the trash rather than dropping them on the ground.

For a hedge or a windbreak, hand picking is not practical, and the answer is to mark next June on the calendar.

The one year plan

Winter. Hand pick bags off small plants and specimen trees.

Mid June. Treat, when the larvae are newly hatched and small.

Late summer. Assess the damage and identify which evergreens are stressed enough to need attention.

If you have a windbreak or a row of arborvitae you care about, the June treatment is worth putting on a recurring reminder. It is a fifteen minute job in the right week and an unfixable problem in the wrong one.