What does elm spanworm damage look like?
All damage is caused by the larval stage of the elm spanworm. As soon as egg hatch occurs, larvae begin to feed on the underside of leaves, causing a shot hole effect. As larvae mature, they eat all leaf material between the major veins. Larvae are capable of completely defoliating shade trees and large areas of mixed hardwood forest during outbreaks.
How can I control elm spanworms?
Trees should be monitored from mid-May through early June for signs of elm spanworm larvae. If necessary, a registered insecticide should be applied when larvae are small. Where possible, prune small twigs that are infested with masses of eggs. Two egg parasitoids, Telenomus droozi and Ooencyrtus entomophagus are known to keep this pest at low population levels. These small natural predators can destroy more than 80 percent of eggs during an outbreak.
Bacillus thuringiensis var. kurstaki (Btk) can also be used to control Elm Spanworm. Btk is a selective biological insecticide which controls lepidopterous larvae (caterpillars). Btk crystals release a toxic protein when dissolved in the alkaline digestive system of the insect. The caterpillar stops feeding soon after, and dies within five days. Other insects, mammals, birds and fish are not affected by Btk. Btk is most effective when larvae are in their first instars. Btk has to be applied when larvae are actively feeding, and applied so that all foliage is thoroughly covered. Rain washes Btk off the leaves and sunlight breaks it down within a short period of time, approximately one to two days. All Pest Control products purchased and used must be registered with Health Canada and contain a Pest Control Product (P.C.P. or PCP) Number on their label.