The elm spanworm is a member of the Geometridae family (looper) of the order Lepidoptera (moths and butterflies). This insect was mainly responsible for the total defoliation of thousands of deciduous trees in St. John’s during the summer of 2002 and caused great concern as to the fate of those trees. The insects common name is misleading, for it is a general feeder on a wide range of deciduous trees, such as maples, lindens, chestnut, beech, oaks and elms. There are approximately 150 species of spanworms, inchworms or loopers which have been collected on the island of Newfoundland. It is important to bear this in mind when determining the appropriate course of action to take as many species look the same, but their life cycles are entirely different. Therefore, the first step is the correct identification of the insect because what works for one type does not necessarily work for all of them.
Description and Life Cycle
Larvae are slate gray to brownish black in colour with a rust- colored head and usually grow to about 50 mm long before they pupate. The adults are snow white moths. Over wintering eggs hatch in early spring when the tree buds break, usually in late May to early June depending on the weather. The larvae feed for four to six weeks and then pupate in net-like cocoons on the host tree. Six to 10 days later, the adult moths emerge, mate and deposit their egg clusters on the underside of twigs where they overwinter. There is only one generation per year.
Two important egg parasites appear in great numbers when moths are abundant. Other natural enemies are also important in keeping infestations in check. Chemical controls may be needed to protect high value trees, but is usually limited to smaller trees. It is practically impossible to reach insects in mature trees due to the height and congested crowns. Defoliating insects don’t normally cause any long term damage to the host tree, unless the tree is in poor health prior to infestation or suffers from repeated years of defoliation.
Unlike flower gardens, trees and shrubs are often neglected. They suffer from nutrient deficiency, insect infestations and diseases. Trees and shrubs are lasting investments for your property and the environment. They, therefore, deserve all the care we can give them. Think of preventive health care for trees as an investment in natural beauty that pays big dividends. Remember, that curing a problem once it develops is much more difficult, annoying, time consuming and costly than preventing one. The most critical step in promoting tree health is providing a tree with suitable space and environment to ensure healthy development. It's about getting the right tree for the right space. An effective tree maintenance program should include four major practices: inspection, mulching, fertilizing and pruning.
Dormant oil spray is quite effective against the eggs when sprayed at the correct time. Thoroughly wet the trunk and crown prior to bud break. At this time it is easier to get the spray solution into the tree because there is less congestion. However, once the leaves start to appear it is too late to spray because the dormant oil could damage the new leaves.
Spray with Bacillus Thuringiensis (BTk). BTk is a naturally occurring spore-forming bacterium that causes a fatal disease in spanworms. Commercial formulations of the bacterium, under several trade names, are available from garden stores and have proved effective in the control of spanworms. The best results are obtained if spray is applied when all eggs have hatched and larvae are still small.
Banding trees in the early spring with a registered systemic insecticide such as cygon will offer some protection to younger trees during the period of larval feeding. Systemic insecticide must be applied in early spring so it can travel throughout the tree prior to larvae feeding.
Insecticide Sprays - Most common insecticides, including insecticidal soap, are effective against young spanworms. Best results are obtained if the spray is applied after all the eggs have hatched and the larvae are still small. Please remember that most insecticides are non-selective, which means that they kill all insects, not just the 'bad ones'.
The recommended treatment is to spray the entire tree (from trunk to tip) with dormant oil prior to bud break. Dormant oil is a dometic product available at any of the local garden centers and can be applied by the homeowner.