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University of Illinois Extension

Bagworms – control them now!

July 8, 2014

Landscape and garden problems seem to pop up overnight when actually many of them have been developing for a while – you just need to know when and what to look for.  One pest that goes unnoticed until their damage shows up is bagworms.

Bagworms catch our attention after they have stripped branches of needles and foliage creating their protective bag.  At this stage they are a mature caterpillar and difficult to control.  The best time to achieve optimum control of this pest in the Quad Cities area is mid-July when they are young and actively feeding.  Take the time to really look at your evergreens – especially at the top of arborvitae, spruce and Juniper and locate any brown or bare spots where foliage used to be.

Bagworm is a solitary, tent-building caterpillar.  They feed on plant material using it to build a protective bag around itself, thus the common name of bagworm.  People are amazed to see these bags way up high in their trees –how do they get there?  Eggs overwinter within the bag.  The larvae hatch out and immediately spin a conical tent around them and cover it with bits of foliage, bark, or anything else that is handy.  They climb to the top of the tree, appearing as moving, 1/8-inch long, conical hats.  They then spin out and dangle at the end of a two to three foot strand of silk that they attach near the top of the tree.  Winds detach these strands and carry the associated larvae up to several miles.  This process is called ballooning and is the main way these guys get around.  The larvae hatch out over a span of several days and balloon repeatedly for a couple of weeks before settling down to feed in earnest.  They have to try to feed on wherever the wind blows them so we find them on many kinds of trees and shrubs.  Not only evergreens such as arborvitae, spruce, Eastern white pine, Eastern red cedar, and other junipers are damaged, but they also feed on deciduous hosts including crabapples, oaks, maples, and hackberry.

If uncontrolled the larvae continue to eat and grow through the summer, increasing the size of their tents (bags) as they get larger.  Because of this process of continual growing, an actively feeding bagworm will have green foliage at the top of the bag that has only recently been clipped off and applied to the outside of the silk tent.  This is a sign that the larvae is still feeding and therefore able to connect with pesticides.  Once the bag is completely brown the optimum window of time for control is past.

As long as the bags are still moving insecticide sprays can be effective.  Once they stop feeding your only management option is to pick them off and destroy the bags.  If you see browned out or bare areas and have really good eyesight (or get out a pair of binoculars) look for the actively feeding moving bags.  Insecticide options for active bagworms include Btk, Bacillus thuringiensis kurstaki, (sold as Dipel, Thuricide, Safer Caterpillar Killer and others), spinosad (sold as Conserve and some Ferti-lome products), and cyfluthrin (sold as some Bayer products).  Read and follow all label directions.  Because caterpillars often start feeding at the top of the tree, sprays need to reach the top to provide complete control.

A week or so after applying insecticide sprays go out and give the plants another good look-over.  Remember any caterpillars still feeding after spraying show green leaves at the top of the bags and are still moving.

Source: Martha A. Smith, Extension Educator, Horticulture, smithma@illinois.edu

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