The spruce budworm (Choristoneura fumiferana) is a native moth and one of the most serious and damaging spruce and fir trees defoliator in North America. It primarily attacks spruces ( white, black Colorado) and balsam fir trees in Alberta. It is a wasteful feeder on current year needles. White spruce is naturally established on 9 million acres of privately own forest ( woodlots) as well as being planted in shelterbelts and windbreaks. Spruce provides not just economic values but also environmental, aesthetics and social values to Alberta landowners. Large scale infestation of spruce budworm usually occurs in 20 -year cycles, but local outbreaks may occur in a shorter period. Alberta Agriculture and Forestry perform annual aerial spruce budworm survey and you may find a aerial survey map on a current outbreak on this web site. This pest doesn’t kill spruce trees immediately, but consecutive spruce budworm infestation can kill and create significant damage to spruce and fir trees.
It takes a little bit effort to identify a spruce budworm. The young yellowish larvae with a dark brown head are relatively small up to 15 mm. Mature larvae ( caterpillars) are purple/reddish to brown colour with small white spots on their backs and a shiny black head. They grow between 18 mm to 25 mm long. Adult moths are grayish brown with a wingspan of 20 mm.
Symptoms and damages
This insect can significantly reduce growth and kill top branches within 3 years of severe infestation. It usually takes 5 consecutive years for trees to be dead after heavy infestation. The first symptoms of larvae feeding (mining) needles and buds are showing in early May feeding on last year’s growth. Later in May and June, larvae are mining on new developed needles and complete feeding in June. After a few years of infestation, trees are weakened and susceptible to other insects and diseases. There are several symptoms to recognize that elm trees are under the attack of this insect:
- The reddish/brown colour needles on top of trees are the first symptoms.
- Dry and partly consumed needles on the twigs
- As infestation quickly progresses, lower branches get infected.
- During heavy infestation, larvae can consume cones as well.
- The trees can have a scorched look, with green needles remaining in the interior.
Management and Control
As spruce budworm is a native insect, many birds, parasitizing insects (wasps and flies) , bacteria and viruses are controlling the budworm population. Weather is another nature way to control budworm populations. During the late cold/freezing spring as well as high wind events can reduce the number of eggs or larvae on trees.
Beside natural control of this insect, under epidemic outbreak you may consider using biological or chemical insecticides to control large budworm population. For using insecticides timing is everything and if you miss this window, you are wasting your time and money and mostly killing non-target insects.There are several management options for the control of the spruce budworm:
- On small trees you maybe can pick up larvae and destroy it
- Use Bacillus thuringiensis Kurstaki (B.t.k) that is readily available in local stores. This natural biological insecticide is best to use in the early stages of larvae development. Timing is crucial to spray – usually when bud caps come off and new needle growth begins.
- Use insecticidal soap with pressure gun.
- On small ornamental trees, vigorously shaking the tree may force larvae to fall off.
- Using powerful water jet will cause the larvae to drop to the ground.
- Insecticides registered for budworm control include: dimethoate, malathion and trichlorfon but it requires registered pesticide licence to obtain some of these products.
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