The western spruce budworm, Choristoneura occidentalis, is a very destructive forest defoliator. Nurseries in or near infested areas can be severely infested by larvae that have overwintered on mature trees. Another budworm, Choristoneura rosaceana, has also been an incidental problem in nurseries in British Columbia. Larvae found in early spring, web the top needles of seedlings together.
Western spruce budworm affects container and bareroot Douglas-fir (Figure 109) and spruce seedlings. In spring, larvae bore into needles until buds begin to swell, and then they mine expanding buds. Later, they spin loose webs around new foliage and feed within the webbing, often chewing needles off at their bases. They will feed on old foliage once new growth has been destroyed. They are voracious feeders, causing severe defoliation.
Adult moths (Figure 111) are mottled orange brown and have a wingspan up to 28 mm. Eggs are light green and laid in shinglelike masses on the underside of needles in mid-July. The resulting larvae overwinter in small silken cocoons which are hard to locate. The following spring, larvae up to 32 mm long (Figure 112), with brownish head and a body having prominent ivory-colored spots, emerge and spin long silken threads that aid in their dispersal. They can continue invading nurseries up to their last instars in early July when they pupate (Figure 113). As well, newly planted seedlings in reforestation sites can be attacked in the spring by larvae, acquired in the nursery, which have survived cold storage on stock lifted the preceding winter.
Because of the small size of nursery seedlings and the voracious feeding behaviour of budworm larvae, it is important that nursery workers carefully monitor seedlings in the spring and control larvae that blow onto the stock after they have overwintered in mature trees in and around the nursery site.
Adult moths pose a further threat because they can oviposit on nursery stock, greenhouse structures, or nursery equipment such as styroblocks. The resulting larvae will accompany lifted stock to planting sites or invade newly sown seedlings at the nursery the following year. Pheromone or light traps can be used to monitor adult populations. Significant infestations may require use of a foliar spray to prevent oviposition, applied at the beginning of moth emergence and repeated at 2-week intervals as long as adults are present. This spray works partly as a repellant to moths and, through contact activity, kills moths that alight on foliage to lay eggs.
Furniss, R.L. and V.M. Carolin. 1980. Western forest insects. U.S. Dep. Agric. For. Serv., Misc. Publ. No. 1339, Washington, D.C.
Unger, L.S. 1986. Spruce budworms in British Columbia. Can. For., Serv. Pac. For. Cent., For. Pest Leafl. 31. Victoria, B.C. 4 p.
|
Principal, locally grown hosts |
Host age and season when damage appears |
|
Nursery type and location |
|
|
|
|---|---|---|---|---|---|---|
|
Bareroot |
Container |
|||||
|
Age |
Season |
Coastal |
Interior |
Coastal |
Interior |
|
|
Douglas-fir and spruces |
1+0 |
Spring through early summer |
No |
Yes |
No |
Yes |
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Figure 109. Bareroot Douglas-fir with western spruce budworm feeding damage.
Figure 110. Life history of western spruce budworm (one generation per year).
Figure 111. Adult western spruce budworm (courtesy of Forest Insect and Disease Survey, P.F.C., Victoria, B.C.).
Figure 112. Western spruce budworm larva. Note webbing on right (courtesy of Forest Insect and Disease Survey, P.F.C., Victoria, B.C.).
Figure 113. Western spruce budworm pupa (courtesy of Forest Insect and Disease Survey, P.F.C., Victoria, B.C.).