Arborviate bagworms

Asked August 29, 2015, 3:18 PM EDT

Have a row of 20 10'-12' Arborviate. Just ONE in this row has contracted bagworms this season. Why just one?
Picked off as many as I could see. Been told that there is nothing else I can due but wait until next May or June to spray this tree. Is that correct advice and if so do I have to spray all 20 of them? Do you have a preferred spray that I can use? And finally, could you suggest literature re care and feeding of Arborviate?
Thank you very much in advance for your help.
Jerry Hughes

New Castle County Delaware

1 Response

Insects are often distributed randomly throughout a landscape, and other times are more clumped in their distribution. For whatever reason, the one tree managed to be in the direct drift of 'ballooning' neonates earlier this year and other trees either were colonized, were populated by very few (sometimes they are very difficult to see at any size), or were not in the path of any breezes that carried the neonates to the one tree. I do not know which tree in the row has the bagworms; therefore it is difficult to say why it had more bagworms than the others.

One of the best management plans is to physically remove the bagworms, as you have already done. Granted, you can only remove those found or those you can reach; thus some escape to live another day. Around mid-August (sometimes earlier) bagworms cease feeding and begin to form a pupa (similar to the Monarch's chrysalis). Any insecticide sprayed onto the tree to kill bagworms needs to be consumed by the caterpillar; thus, sprays from mid-August through the following May provide little to no control. The bagworm eggs hatch around the beginning of June, and neonates (really small caterpillars) throw silk into the breeze and are blown (called ballooning) to nearby suitable host plants (food) to begin to feed. This may be to another section of the plant they ate last year, or nearby plants. The caterpillar then ties small pieces of the plant they are eating around themselves for protection. Management during June may be obtained by spraying products containing Bacillus thuringiensis 'kurstaki' or spinosad. There are a variety of these products commonly available at garden centers or large box stores throughout the mid-Atlantic. Management during July (if necessary) may be obtained by the previously mentioned products if the caterpillars are still small, or products containing a pyrethroid (active ingredient ends in "-thrin"). Pyrethroids are broad spectrum and frequently reduce populations of beneficial insects such as lady beetles or green lacewings, as well as the targeted pest. These products are also commonly available throughout the mid-Atlantic. Any time a pesticide is applied, follow the instructions on the label because it is the law regarding its use.

I frequently use Dirr's Handbook of Woody Landscape plants for many of my plants I am interested in growing in the landscape, but there are probably other sources available for maintaining arborvitae. I would rely more on university, cooperative extension, or USDA forest service sites as more reliable, and use other websites with a little bit of caution.

hope this helps