Evergreen trees often seem impervious to insect pests, with their sharp needles and resinous sap. However, the spruce budworm is a serious pest of many conifers, such as pine trees, hemlocks, tamarack, balsam fir trees, and spruces. Organic gardeners must control spruce budworms before irreparable damage is done to the conifers in the landscape.
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According to the U.S. Department of Agriculture Forest Service, the initial stages of spruce budworm infestation are difficult to detect, as the young caterpillars are miniscule and seek refuge in the buds on which they feed. Although the bulk of the initial damage occurs at the crown of the tree, making identification of individual pests difficult, the appearance of web clusters at branch tips is a telltale sign.
On smaller conifers, where the gardener can examine damaged branch tips closely, he may notice the ® inch dark brown larvae covered with bristly hairs. The adult moths have a brown and gray pattern, which allows them to blend in with the bark of their host trees.
Spruce budworm caterpillar feed on the tender tips of conifers like pine trees and fir trees early in the growing season, when these trees are producing new susceptible growth. Not content to nibble one needle at a time, these destructive worms often tie together several needles with a sticky web-like substance, and then feast on the entire bundle as if it were a burrito.
The damage caused by the spruce budworm is more than cosmetic. A heavy spruce budworm infestation can defoliate an entire tree, causing death after one heavy feeding season.
If a gardener notices the beginnings of defoliation, webbing, or brown tips on her conifer branches, she must prune and discard those branch portions as soon as possible. This can result in the inadvertent topping off of the tree, a pruning faux pas that can result in a growth pattern that resembles a witch’s broom, with weak sprouts emanating from chopped stubby branches. Inexperienced gardeners or those dealing with larger trees should consult an arborist before administering a cure that is worse than the disease, but training a new central leader from a lower shoot may be an option.
A preventative budworm control method organic gardeners can implement is the use of pheromone traps to capture moths. This is only effective early in the season, before the moths lay their egg masses.
Like any species of caterpillar, spruce budworms are susceptible to the biological control agent BT, or Bacillus thuringiensis. This organic pesticide paralyzes the digestive tract of the budworm, so it’s only effective when the worms are actively feeding. Spinosad is a second organic spray option with OMRI approval. This fermentation product of bacteria kills caterpillars within two days of ingestion, after which it harmlessly breaks down into inert substances.
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