Caterpillars & Norway Spruces
Stately Norway spruces (Picea abies) have spread from their native Northern European habitats to landscapes around the globe. Drooping branchlets of dark green needles line the 80- to 100-foot trees' narrow, strong branches. Mature Norway spruces have pyramidal forms with 25- to 40 foot spreads. Quick growth and tolerance for most soils make them popular windbreaks. These vigorous conifers, however, often suffer infestations of three caterpillar species. (Reference 1, Introduction; p.3, Pests, pars.3-5) Does this Spark an idea?
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Spruce Budworm
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Large spruce budworm populations (Choristoneura fumiferana) can decimate Norway spruce stands. The gray-and-brown moths lay clutches of green eggs on needle undersurfaces. Tiny green or yellow caterpillars hatch and crawl along the branches before spinning protective winter webs. They emerge in spring to mine needles. When the tree's foliage buds open, the caterpillars devour massive quantities of new needles. After reaching 1 inch long and developing white-spotted, reddish-brown skins, the pests pupate in cream-and-brown cocoons. Dead needles and cocoons give infested spruces a noticeably reddish-brown color.
European Spruce Needleminer
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The gray-and-white European spruce needleminer (Epinotia nanana) moth deposits its eggs at the base of Norway spruce needles in midsummer. Newly hatched, brown-headed tan caterpillars burrow into the needles' bases, eating their way to the opposite ends. They frequent overwinter in tubular silk connectors between needles. Alternatively, the pests drop to the ground and create cocoons among fallen needles. They pupate until spring. These caterpillars seldom cause significant damage.
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Bagworms
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Bagworm (Thyridopteryx ephemeraeformis) caterpillars present a serious threat to commercial Christmas tree growers. The wingless female bag moth lays eggs inside a brown bag stitched together with silk and bits of leaf debris. The eggs overwinter, hatching into black-backed, amber-bellied caterpillars. The larvae spin partial bags around their hind legs. They feed on needles until late summer. After attaching to twigs, they extend the leaf bags to cover themselves. The finished bags look remarkably like natural spruce cones. Adult male moths emerge to mate with the still-encased females. The leaf bags render trees worthless as potential Christmas trees.
Budworm Management
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Managing large budworm infestations -- those covering Norway spruce stands of 100 or more acres -- requires an aerial spray of diazanon insecticide. Ground spraying with carbaryl-based insecticide provides chemical control for individual trees. Spray with bacterial-based Bacillus thuringiensis (Bt) for a non-chemical treatment. Apply the carbaryl or Bt just after the new needles emerge.
Needleminer Management
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European needleminer populations rarely damage Norway spruces enough to merit control measures. Treat unusually heavy infestations by pruning back infested branches in early spring. Saturating the needles with a June spray of spinosad, chlorpyrifos, permethrin or bifenthrin insecticide provides chemical control.
Bagworm Management
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Hand removal of the leaf bags between fall and early spring prevents the enclosed bagworm eggs from hatching. Spray bags on branches too high for hand access with insecticide or Bt. Apply while the newly hatched caterpillars are leaving the bags. Appropriate insecticides include acephate, bifenthrin, spinosad and permethrin.
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References
- University of Florida IFAS Extension; Picea Abies Norway Spruce; Edward F. Gilman, et al.; October 1994
- Maine Department of Conservation; Spruce Budworm; May 2000
- Netherlands Biodiversity Information Facility; Arthropods of Economic Importance; Epinotia Nanana
- Penn State University; Woody Ornamental Integrated Pest Management; Bagworm Fact Sheet
- U. S. Forest Service; Christmas Tree Pest Manual, Second Edition; Deborah McCullough, et al.; 1998
- Penn State University; Woody Ornamental Insect, Mite and Disease Management; Arborvitae; December 2008
