Caterpillar Infestation on Globe Spruce

Globe spruce (Picea pungens "Glauca Globosa") enhances the sunny small-space garden in U.S. Department of Agriculture plant hardiness zones 2 through 7 with year-round color and architectural interest. Growing a scant 2 to 4 inches annually to an ultimate 3- to 5-foot height and 4-to 6-foot spread, this flat-topped, shimmering silver-blue conifer flourishes in acidic, well-drained soil and a cool-summer climate. Globe spruce benefits from supplemental water during drought, and may occasionally require treatment for needle-devouring caterpillars.

You’ll never see the female bagworm moths (Thyridopteryx ephemeraeformis) on your globe spruce. These unusual insects exist within black pupal sacs encased in rust-brown, spindle-shaped bags made of silk webbing and bits of plant debris. The winged, charcoal-black male moths survive just long enough to mate after emerging from their own protective bags. Before dying, the females produce up to 1,000 creamy-white, wax-covered eggs. In spring or early summer, the newly hatched brown larvae feed on globe spruce’s needles while building their own camouflage bags.

The tips of a globe spruce in the early stages of bagworm infestation fade to brown. Heavy feeding may strip the spruce of its needles. The young caterpillars sometimes hang from the branches on silky threads and move to nearby plants on wind currents. While the pests’ inability to migrate more than a few yards limits the damage they do, the original host may spread infestations for years.

Spruce budworms (Choristoneura fumiferana and Choristoneura occidentalis) periodically decimate wild spruces and occasionally attack cultivated ones. Their presence on your spruce won’t become noticeable for several months. In early fall, the yellow-green caterpillars hatch from clusters of pale-green eggs that 3/4-inch, gray- or brown mottled female moths deposit on the needle's undersides. They shelter beneath bud scales or in bark crevices, spinning cocoonlike structures and emerging in spring. During their first four molts, they feed on developing buds before weaving the new needles into protective nests. By midsummer, the mature, white-spotted brown caterpillars stop eating and pupate.

Budworm caterpillars consume huge numbers of new globe spruce needles. Those they don't eat, they sever at the bases and leave clinging to the shrub inside their webs. These needles fade to brown over the summer, giving the spruce a scorched look. The pupating caterpillars' rust-colored cocoons magnify this effect. In addition to ruining its appearance, the needle loss causes a major slowdown in photosynthesis and stunts your spruce's growth. A budworm infestation lasting three or more years may be fatal.

Managing bagworm infestations on compact globe spruces is as easy as pruning and crushing the egg-containing bags between fall and spring. To treat budworms, spray the plant thoroughly during the two weeks after bud break with a mixture of 1 to 2 teaspoons -- or the label’s specified amount -- of wettable Bacillus thuringiensis powder for each 1 gallon of soft water. This solution starves newly hatched caterpillars by impeding their digestion. Treat your spruce in early evening to protect the UV-sensitive Bt organisms.