Eastern spruce budworm

Status

Eastern spruce budworm populations can experience significant oscillations. During outbreaks, populations grow to extremely high densities, compared to the relatively low levels that occur in between. These outbreaks are highly destructive and can cause both economic and ecological damage. As a result, methods of control are utilized. Several theories exist regarding these cyclical outbreaks: association with balsam fir maturation, catastrophe theory, dispersal from an epicenter, and oscillations synchronized by entrainment.

The first recorded outbreak of the spruce budworm in the United States occurred in Maine in about 1807. Another outbreak followed in 1878. Since 1909 there have been waves of budworm outbreaks throughout the eastern United States and Canada. The states most often affected are Maine, New Hampshire, New York, Michigan, Minnesota, and Wisconsin. These outbreaks have resulted in the loss of millions of cords of spruce and fir. In 20th-century eastern Canada, the major outbreaks occurred in periods circa 1910–20, c. 1940–50, and c. 1970–80. These outbreaks impacted, respectively, 10, 25, and 57 million hectares of forest. Longer-term tree-ring studies suggest that spruce budworm outbreaks have been recurring approximately every three decades since the 16th century. Paleoecological studies suggest the spruce budworm has been breaking out in eastern North America for thousands of years.

The spruce budworm disperse locally after hatching and practice more long-range migration as adults to avoid harsh weather conditions. While moving through the life cycle stages of egg, caterpillar, pupa, and adult, this species risks attack from predators including birds and spiders as well as numerous parasitoids. During mating, females release sex pheromones to attract males. After a mate has been successfully attracted, the males deliver a nuptial gift to the female during copulation.

Habitat

The range of the eastern spruce budworm is the largest of all budworms and coincides with the range of its hosts: fir and spruce trees in eastern North America, primarily in Canada. It includes the Boreal Forest as well as the Great Lakes-St. Lawrence, Northern, and Acadian forest regions. This range extends westward to Alaska. The spruce budworm is commonly found in boreal and sub-boreal forest regions, specifically those that consist of spruce-fir forests.

Food

Food shortages can occur in budworm populations if the budworms kill a significant amount of trees in the stand, such as during outbreaks. When food becomes depleted, the larvae feed on old foliage, which will result in slowed development and reduced fecundity in the female moths. However, food shortages generally do not lead to larval mortality.

Predators

Eastern spruce budworm egg masses are immobile and less vulnerable to predation than other stages of the cycle. Small larvae are exposed to predators during dispersal but become safer while bud or needle mining or overwintering in hibernacula. Large larvae actively escape predation by dropping from the tree foliage, which can also attract the attention of other predators. Predators outside of host plants on the forest floor include ants, spiders, beetles, and small mammals. Pupae are vulnerable because the budworm are generally immobile in this state. The adult moths can fly away from predators but are then exposed to various birds, dragonflies, and robber flies. Spruce budworm male and female moths experience increased vulnerability during copulation.

Insectivorous birds are a common and major predator of the spruce budworm, mainly preying on the larvae and pupae. Examples of species include sparrows, thrushes, and overstory warblers. Bird distribution will change to reflect budworm density in the forests. A 1989 study found that the highest populations of spruce budworm larvae and pupae occur in June and July. This coincides with the period when some species of birds require maximum food to feed their young. As a result, predation by birds helps control the growth of budworm predation. Other major predators include various invertebrates, primarily spiders.



In spruce-fir forests throughout Maine, the late-instar spruce budworm larvae is preyed on by eumenid wasps such as the ''A. adiabatus'', and ''E. leucomelas''. This predation is important in controlling populations of the spruce budworm in forest stands.

Migration

Long distance migration of the spruce budworm does occur as they can disperse across distances ranging from 20 to 450 km. In northern Minnesota, spruce budworm moths emigrate to the east lakeshore of Lake Superior in Ontario, Canada, because of seasonal changes. Dispersal is influenced heavily by temperature as low temperatures can slow down both take off and arrival. Other factors include dispersal direction, precipitation, altitude, and wind flow. It is likely that mass exoduses of the ''C. fumiferana'' over long distances are a result of food shortage in the local area.