Bagworms Hatching Date

Asked May 9, 2016, 8:21 AM EDT

I have an infestation of bagworms in my 25 foot tall arborvitaes. When will the bagworms hatch and how should I best treat the trees?

New Castle County Delaware

1 Response

The eggs in the bags should begin hatching around the end of this month. Sometime before then you should physically remove all the bags you can reach. The male to female ratio is usually about 1 to 1; thus it averages out to be every other bag picked off will be remove eggs from your tree. After the bags are removed, cut up the bags, burn them (if possible), or place them in a plastic bag and dispose of them in the garbage. This will remove a large portion of potential insects from your trees. Yes, you will not be able to get them all because your trees are so tall, but everything you remove makes management of the population this summer much easier.

Around the middle of June, your trees can be sprayed with any of the products locally available that have Bacillus thuringiensis (B.t.) or spinosad. These products work well against small caterpillar pests. You will need to get thorough coverage of the foliage because spraying bags does not work. You may need to make another application around the beginning of July. If you forget to make applications in June and find bagworms on your tree after mid-July there are many other products out there available for controlling bagworms such as carbaryl or any of the pyrethroids (active ingredients end in -thrin). These are broad spectrum products that will have impacts on beneficial species of insects. Regardless of what you apply, thorough coverage of the foliage is required because the insect is protected inside their bags. They obtain their toxic dose from eating treated foliage. Bagworms are typically finished feeding on trees by mid-August; thus treatments at this time and through the following May do not work.