Spruce trees can grow very tall and make, providing a dramatic effect to your landscaping. However, infestations of certain insects can quickly defoliate and kill your trees. Two of the most common types of infestations are sawflies and spruce needle miners. Inspect your trees regularly for signs of these pests. Catching the problem early is essential to a simple solution. Driving these bugs away before they become overwhelming can preclude the application of expensive and dangerous pesticides. Identify the insect you are dealing with and take care of it immediately.
Spruce Needle Miners
Adult spruce needle miners resemble brown and white moths. They emerge in May and June to mate. The female lay groups of six or seven eggs on spruce needles in an arrangement similar to roofing shingles. The larvae look like pale white mealworms. They tunnel into the needles shortly after hatching. As the larva grows, it cuts off needles and spins them into a nest using silk thread. By April, the larva spins itself into a cocoon and changes into its adult form in time for the next mating season. One generation of miners occurs every year. An infestation of spruce needle miners can defoliate the entire crown of small ornamental trees. In larger trees, they dwell in the inner reaches of the lower branches. A visual inspection of an infected tree quickly reveals unsightly clumps of dead needles held together with silk. To prevent an infestation, wash off the tree with a garden hose once per season. Remove unhatched eggs, silk larva nests, and cocoons. Collect these materials and burn them.
An adult saw fly looks like a wasp or bee with a broad connection between its abdomen and thorax. Sawflies have dark bodies with bright orange, red, or yellow markings. The female has a saw-shaped ovipositor which she uses to cut open spruce needles and lay eggs inside. The larva of the sawfly resembles a caterpillar but has more prolegs on its abdomen. Adults mate and lay eggs in September and October. The larvae hatch inside the needle and begin eating it from within. They emerge from the original needle and begin feeding on others by May or June. The sawfly larva only eats the previous season’s needles. The larvae move from branch to branch and tree to tree as they strip away all of last year’s needles. They will not eat new needles but will strip the bark from saplings. If left unchecked, they generally will not kill the spruce trees, but the appearance of an infected tree is very unpleasant. Try to prevent infestations by washing the larva and flies from the tree before their numbers become overwhelming.
If it is too late to contain the infestation by washing or combing the tree branches, consider using pesticide to control the swarm. Apply pesticides according to the instructions and safety warnings on the label or hire a licensed exterminator.