Ewwwwww!!! What is that thing on my plant?

That funky new ornamentation on your shrubs that seemingly sprung up over night is not a hot new style of plant jewelry. Those oval shaped bags festooned all over your Arborvitae or Juniper are called Bag worms. These nasty little guys are easily visible once they have formed their spindle shaped bags. Large populations of bag worm larvae can strip plants of their needles and leaves. While deciduous shrubs and trees are able to produce new leaves within a 4-6 week period, evergreens are much slower to recuperate and the damage may be severe enough to cause eventual death. Plant damage is most noticeable in June, July and August. The bags are made up of silk and the vegetation the bag worm is feeding upon. On evergreens the bags can resemble a small pine cone, making it difficult to detect until the damage is done and the plant is partially defoliated. During this stage the bag is attached to the larvae which is dragged around as the nasty critter munches on your unsuspecting plant. The bag continues to grow as the larvae grow. When disturbed, the larvae will pull their head and upper body back into their bag and hold the top opening closed. Gross!! In early fall, when the bags are one to two inches long, the bags are suspended from branches and the larvae change into pupae before becoming adults. The adult male emerge as a moth and start their search for females. The female emits a scent to attract the male to her while remaining in her silken home. The male Bag worm inserts his abdomen into a hole at the bottom of the sack to mate, he will die shortly thereafter. The female lays up to a thousand eggs in her sack and then she will drop to the ground from the bottom of the sack to die. The eggs overwinter in their bag until May through June and emerge from the bottom of the bag. They spin a fine silk and either attach themselves to the same plant as their parents or are picked up on the wind and blown to the next plant over. There is one generation per year.

Bag worms are not that food specific but seems to prefer juniper, arborvitae, pine, spruce, and an array of deciduous plants. To control Bag worms use cultural and or chemical cultural control. Pick off by hand as many of the nasty bags as you can and destroy or burn them. If you choose not to go this route, (there could just be too many to pull off) then use a chemical spray. Sprays are most effective while larvae are in their newly emerged state or while feeding so apply May through August. Bacillus thuringiensis or Bt, Sevin, Neem Oil, Hi-Yield 38+ or Permethrin are good choices. There you go and good luck!!!

Ruth Whiter
Horticulturist