Insect Pests of the Washington State Douglas Fir

Identifying Pest Presence

• Douglas fir pests can show themselves in a variety of ways. A tree might display yellow or otherwise discolored foliage. The needles may have a botched appearance or be stunted. Boring dust from the beetle's entry may be found in bark crevices or at the base of the tree; it will be reddish or yellowish. Some pests chew holes in the bark; these holes then become covered with white, crystallized resin. Some pests feed on twigs, causing die back and deformed limbs. You also may see the pests or their galls on the trees.

Types of Pests

• A number of pests can plague Douglas fir trees. Black pineleaf scale infestations occur on the tree's needles. The cooley spruce gall adelgid forms characteristic white galls on needles. Generally, neither of these pests will kill a Douglas fir, but severe infestations of Douglas fir beetle, a voracious tunneler, can kill host trees. Another insect, Douglas fir needle midge, can lead to severe needle loss, particularly damaging in Douglas firs grown for Christmas trees. Douglas fir seed chalcid feeds on the developing seed, destroying it.

Prevention

• Prompt removal of fallen trees will help prevent infestation, since many pests are drawn to dead plant material. Well-maintained, healthy trees stand a better chance against pests of all sorts. A naturally occurring beetle repellent, methylcyclohexenone can be applied to fallen logs or to live trees to prevent attacks by Douglas fir beetles. Methylcyclohexenone is contained in a slow-release container that is stapled to dead or dying trees. The number of containers used per tree and frequency of application depend on the level of beetle infestation. Gardeners should also visually inspect landscape specimens regularly to be aware of potential invaders, which can be removed by hand.

Control

• Should your trees suffer a pest infestation, you can apply any of a number of topical agents designed to kill the bugs. Broad-spectrum chemical insecticides such as acephate can be applied at a rate of 10.5 ounces per 2 gallons of water two weeks before bud burst to rescue heavily invested stands. Insecticidal soap and horticultural oil are less harmful to your garden and to beneficial insects. Spray in early May and again in late July to early August. You can also prune out infested areas of infected trees to prevent migration. Biological controls in the form of other insects that eat the pests are also useful.