This is the time of year when bagworms munch on landscape plants and evidence of their destruction will soon appear. Bagworms can cause irreversible damage unless they are treated at the right time. Think of this as your annual warning.
The feeding of bagworms has a negative impact on conifers. Their favorites are junipers, spruces and arborvitae. Due to the wide range of host materials, be sure to take a quick look at any landscape plants.
At first they are difficult to spot, but once too large to control, they are some of the easiest insects to identify. The young, worm-like insects eat the foliage, which causes the area to initially brown, followed by the plant’s death under severe feeding. Understanding the bagworm life cycle helps you control its damaging effects.
Bagworms form a silky sac mixed with parts of plants, up to 3 inches in length. Bagworms spend most of their lives attached to a branch or stalk, eating and never leaving the comfort of the bag. Only the males leave the sacs to mate with the female, then they die and the female lays eggs for hatching the following year.
Bagworms overwinter in the egg stage in bags attached to the plant. The eggs hatch from late May to mid-June. Newborns are tiny at first, starting about the size of a sharp pencil point.
They grow quickly, spinning a larger sac until mid to late summer. At this point, the bagworms mate, the eggs are laid, and the whole process is ready to start again for the next year.
Populations are growing rapidly to damaging levels, as each egg sac produces more than 1,000 hungry tiny worms. They are best controlled during hatching in mid-June. Just about any insecticide will control sackworms when they are small, and the silky sack is not very formed.
The larger the bag, the less effective the control. At the end of the summer, chemical applications are unnecessary. At this point, it is recommended to sort and destroy the bags by hand. It’s a slow task that most people would rather not do.
The small developing bagworms treated now are easy to kill. Most insecticides are effective against bagworms this time of year. Products to be applied include Spinosad, Acephate, Cyfluthrin or Permethrin. A biological product called Bacillus thuringiensis is effective. Complete coverage of the plant is vital for full control. Make sure to soak the developing bags with the spray.
Now is a great time to carefully inspect your evergreen plants for bagworms. Extension receives numerous calls each year from people trying to identify this pest. They say something like “they appeared overnight”. While this is what it seems, they are indeed already present in your landscape.
Keep in mind that they can do a lot of damage in a short time, so identifying bagworms early on to get them under control before it’s too late and you end up with an ugly plant is essential. .
Dennis Patton is a Horticultural Research and Extension Officer at Kansas State University. Have a question for him or for other academic extension experts? Email them at firstname.lastname@example.org.