I have developed a problem with bagworms on my arborvitaes and Leyland cypress trees. Is there any pesticide or fertilizer that can be used to cure and prevent this problem?
The Gardener’s Answer
Both Leyland cypress (Cupressocyparis leylandii) and Arborvitae are susceptible to bagworms especially if their growing environment is not ideal. Bagworm infestations can cause severe defoliation and may be devastating to these evergreens if not controlled.
The most effective method of control is handpicking and then having a good stomp, or placing in a sealed bag and discarding them. Once the bags made of dead plant material are formed, spraying is ineffective. The bags themselves are very tightly woven and difficult to tear open.
It is helpful to understand their life cycle in terms of control. There is only one generation per year, but there are several hundred eggs within each bag. Have you noticed any bagworm caterpillars present and feeding on the foliage? If so, spraying may still be an option, but as they mature they will enter the pupa stage safe in their individual bags.
They feed for about six weeks before retreating to the bags during the pupa stage. The adult males will emerge this fall in search of females that remain in their bags. The female grubs leave their home after they have laid their eggs in the fall. They do not survive the winter, but the eggs that over-wintered in the bag start to emerge in the spring. Usually this happens in May, but it all depends on Mother Nature so keep an eye out for them next spring. While they are actively feeding is the only time a spray would be effective. We are at the tail end of this right now, but if you see the caterpillars go ahead and spray.
Otherwise, remove all bags that you can reach and when spring arrives be ready to take action. You will want to choose a product that is labeled for control of caterpillars. Any product that contains Bt (Bacillus Thuringiensis) as an active ingredient is a good option.