Host Plants

Conifers include evergreen trees or shrubs of the class Coniferinae, such as the pine, fir, spruce, and other cone-bearing trees and shrubs, as well as yews of the class Taxaceae.

Regions impacted

Northern hemisphere.

Description

• Relatives of aphids, these insect pests can damage the growth of conifers and yews by sucking out the juices, much as aphids do. They can be found near the bark at the base of the needles and damage host plants by inserting their piercing mouthparts into the base of the plant needles and extracting fluids. This process can significantly weaken the tree’s overall health.
• The adelgid is tiny – 1.5-mm– and its color varies from dark reddish-brown to dark purplish-black; they may or may not have wings. The nymphs are tiny versions of the adults. As it matures, the insect produces a wool-like filament, which acts as a protective coating against enemies. These white masses, about half the size of a cotton swab, are recognizable on the conifer at the base of hemlock needles or attached to twigs.

Life Cycle

• Overwintering adult females lay eggs during the warmer seasons. The eggs are approximately 0.25 mm in length and are brownish-orange. Depending on the climate, eggs begin to hatch in April and are usually completed by late June. The nymphs mature in late September and spend their winters on trees.
• Several species, including the hemlock woolly adelgid, have a more complex life cycle involving two different tree host species and asexual and sexual life stages. It produces two generations a year, an overwintering generation and a spring generation, which overlap in spring. The overwintering generation has two forms: a wingless form that remains on the hemlock and a winged form that flies in search of another suitable host upon which to mate.

Damage and Detection

Adelgids feed on the sap of conifers and yews with piercing/sucking mouthparts. The result can include stunted growth and buds and needles that dry out, turn gray-green, and eventually drop. When present in large numbers, damage caused by adelgids may retard or kill trees, although vigorous plants can usually tolerate moderate adelgid populations. Infested trees are also more susceptible to attack by borers and other pests. Infestations are easily recognizable because of the preponderance of waxy white filaments that appear at the base of the hemlock. Other signs of an infestation include stunted growth, the death of trees, or any dieback.

Prevention and Control

The damage caused by adelgids is often minor and can usually be treated.

• Keep trees healthy by providing good site selection and care.
• Inspect conifers frequently to get ahead of infestations. Tall trees are virtually unreachable and untreatable, so vigilance when the tree is younger is important.
• To manage established populations, direct a forceful stream of water at cottony masses.
• Clip and dispose of damaged twigs and foliage before insects emerge.
• Prevent recurring problems by applying horticultural oil in the spring, summer, and fall.
• Avoid excessive fertilization and quick-release formulations that produce jolts of new growth preferred by adelgids.
• Adelgids are protected by waxy secretions and are difficult to control with insecticides. It is only possible to treat adelgids on small trees that can be sprayed thoroughly. Infestations on tall trees will have to be tolerated.