Balsalm wooly adelgid (Adelges piceae) is a small, wingless insect that infests and kills true firs. It is invasive to North America. The insect was likely introduced to the United States around 1900 on imported nursery stock. It was first reported infesting true firs in Maine in 1908.
Balsalm wooly adelgid appears as a white, wooly spot on true firs. The insect damages vulnerable trees by feeding in bark fissures. As the insect feeds, it releases toxins contained within its saliva. The toxins disrupt the tree’s vascular system, inhibiting the distribution of nutrients and water throughout the crown. This often culminates in tree mortality.
Distribution & Habitat
Balsalm wooly adelgid is indigenous to Europe, where it is widespread. Since its introduction to North America, it has become prevalent across the United States.
Balsalm wooly adelgid infests all true firs. Balsalm fir, Fraser fir, and subalpine fir are the insect’s preferred hosts.
Balsalm wooly adelgid passes through three stages during its life cycle: an egg stage, a nymph stage, and an adult stage. The eggs are amber-colored, oblong-shaped, and flattened. They are fringed with a waxy coating. The nymphs are tiny, measuring around 0.35 mm in diameter. The adults have a dark purple to black coloration. They are wingless, and often covered in a white, wooly material. When mature, the adults may measure up to 1 mm long.
Balsalm wooly adelgid produces two to three generations per year, depending on the geographic location. The immature nymphs, also referred to as crawlers, overwinter, and resume activity the following spring. By May, the nymphs begin to feed. They progress through two additional instars before morphing into adults by June. As the nymphs mature, they gradually increase in size. During this period, the nymphs enshroud themselves in a wooly covering. The wooly coverings conceal the nymphs from predators. The adults are all female. Once fully grown, the adults each lay up to 250 eggs beneath the wooly shrouds they have crafted. The adults lay eggs on host trees for at least six weeks.
By July, the first generation of nymphs hatch, and settle on the bark of host trees. The nymphs become dormant for several weeks prior to feeding. Following this period, the nymphs feed until late summer, when they develop into adults. These adults lay eggs from late August to late October. The nymphs that hatch from these eggs overwinter, and emerge the following spring.
Occasionally, the nymphs may be dispersed by air currents, birds, or other vectors to nearby trees. Susceptible trees may be readily infested. The nymphs may also be inadvertantly transported to new locations on seedlings that have been purchased from nurseries or garden centers.
Symptoms of Infestation
The most visible symptom of infestation is the white wooly material the adults produce. The waxy covering is conspicuous on the bark or branches in summer and fall. Infested trees will often form a layer of hard wood over the damaged bark. While this layer can help to repel the nymphs, it may also disrupt the tree’s vascular system. Over time, the lack of sufficient water and nutrients to the crown will cause plant growth to become distorted. Eventually, the tree may expire. Swellings may develop on the outer branch nodes and terminal buds. If the swellings partially enclose the buds, new growth will be stunted. When the buds are completely enclosed by the swellings, no new shoots or needles will be formed.
- When planting, select trees that exhibit an increased resistance to balsalm wooly adelgid. Grand fir is one of the least vulnerable fir trees.
- Monitor potential hosts for signs of infestation. The insects are generally easiest to detect in summer and fall.
- Dense populations can severely damage the bark on infested trees. Many trees will form a protective layer over the damaged bark. After a few years, the nymphs will be unable to penetrate the thick layer, causing them to rapidly expire.
- Abonormally frigid temperatures in fall can decimate nymph and adult populations.
- In Europe, several natural predators and parasites help to limit adelgid populations. The most effective are three beetles, and three flies.
- Cull severely infested trees in winter to prevent the insects from spreading.
- Insecticides registered for use on balsalm wooly adelgid can be employed to provide some control. Applications must be performed in spring, before bud break occurs. To ensure that applications are effective, thoroughly drench the bark and branches.
Photo courtesy of Gilles San Martin CC-by-2.0