Conifers are often infested with insects that can damage the health and appearance of the tree. Insects are especially problematic in conifers with commercial value, such as those being used for Christmas trees. Dwarf mistletoe is a parasitic flowering plant that can damage a variety of conifers by extracting important nutrients and water from the tree.
Spruce spider mites are common conifer pests, causing damage to the trees by feeding with piercing mouthparts. These mites are cool-season mites, and warm weather halts feeding and reproduction. However, many conifers are grown in areas where the temperatures never get high enough to slow mite reproduction and feeding. Trees grown in these areas may suffer serious damage year-round. Spruce spider mite feeding causes needles to develop yellow spots, which are typically found at the needle's base. Spider mites often place a web of silk on the needles and branches of host plants, which is a sign of heavy infestation. Scale insects are also damaging to a variety of conifers. Pine needle scale often infests spruce, pines and fir trees, and often cover the branches of the tree. Scale insects are white and in large numbers look like splattered paint or tufts of cotton on host trees. These pests feed on the bark and needles of host trees, removing the sap. Conifers infested with scale insects may become unsightly, suffer from reduced vigour, or experience premature needle drop and dieback.
Spruce spider mites are often controlled with brisk sprays of water several times each week. Spraying your tree with water removes spruce spider mites from the tree and reduces their population. Natural predatory insects also feed on spider mites and scale insects. Parasitic wasps and minute lady beetles often feed on damaging pests and can control light insect infestations. Heavy infestations of spruce spider mites and pine needle scale may be controlled with horticultural oils. These chemicals work by coating insects with oil, causing them to suffocate and die. Cover your entire tree thoroughly when applying horticultural oils, as these products must make contact with pests to be effective.
Conifers are susceptible to a parasitic flowering plant known as dwarf mistletoe. These parasitic plants invade the bark of host trees, producing root structures that grow just under the bark and into the wood of the tree. Dwarf mistletoes draw important nutrients from host trees, causing reduced vigour. Conifers infested with these parasitic plants often appear yellow and have reduced foliage that often takes on a witches' broom appearance. Looking closely at the tree, green or reddish-brown shoots are visible on the bark, which usually produce within three years of infection.
Parasitic Plant Control
Conifers infested with dwarf mistletoes may experience a gradual decline in health and vigour. Some conifers infested with dwarf mistletoes are more susceptible to damage from insects such as pine bark beetles and twig beetles, according to Colorado State University. Pruning infested branches and removing any trees with heavy infestations is the key to controlling the spread of dwarf mistletoe. Chemical sprays containing ethephon may control growing shoots, but is typically only used in conifers of high value.