Choristoneura fumiferana
Choristoneura fumiferana, the eastern spruce budworm, is a species of moth of the family Tortricidae native to the eastern United States and Canada. The caterpillars feed on the needles of spruce and fir trees. Eastern spruce budworm populations can experience significant oscillations, with large outbreaks sometimes resulting in wide scale tree mortality. The first recorded outbreaks of the spruce budworm in the United States occurred in about 1807, and since 1909 there have been waves of budworm outbreaks throughout the eastern United States and Canada. In Canada, the major outbreaks occurred in periods circa 1910–20, c. 1940–50, and c. 1970–80, each of which impacted millions of hectares of forest. Longer-term tree-ring studies suggest that spruce budworm outbreaks have been recurring approximately every three decades since the 16th century, and paleoecological studies suggest the spruce budworm has been breaking out in eastern North America for thousands of years.
Budworm outbreaks can have significant economic impacts on the forestry industry. As a result, the eastern spruce budworm is considered one of the most destructive forest pests in North America, and various methods of control are utilized. However, the species is also ecologically important, and several bird species are specialised on feeding on budworms during the breeding season. Several theories exist to explain the cyclical budworm outbreaks.
Taxonomy
originally named the eastern spruce budworm, C. fumiferana, in 1865, which was recognized as a Nearctic representative of the genus Choristoneura. At this time, the name applied to populations in a variety of geographic regions and biotopes. The C. pinus, a distinct form of the Choristoneura, was later established as a separate species. However, a large group of this genus in the western part of North America remained taxonomically undefined as the "western complex" until T.N. Freeman established several new species in 1967. Field collections of late instar larvae of Choristoneura populations were taken from a range of localities in a wide arc, from the Atlantic seaboard along the edge of the Laurentian Shield to the Mackenzie River area near the Arctic Ocean. From these collections, only points east of the Rocky Mountain foothills yielded C. fumiferana. The two-year-cycle budworm C. biennis occurs only in the subalpine forest region, with alpine fir and Alberta spruce as hosts. Budworm populations from Rocky Mountain regions south of the area of introgressive hybridization of spruce differ from C. biennis. Other budworms are of little or no consequence with respect to spruces.Range and 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 resources
Host plant preferences
The main hosts of the eastern spruce budworm in eastern North America are balsam fir, white spruce, and black spruce, but the larvae feed almost exclusively on current-year needles of balsam fir and white spruce. In massive outbreaks, populations of the insect can become so high that the larvae will feed on old foliage after the current-year foliage has been depleted.Traditionally, the eastern spruce budworm prefers balsam fir over white spruce. However, one study showed contradicting evidence. In this study, Bichon sampled spruce budworm populations on branches from the upper mid-crowns of dominant or co-dominant balsam fir and white spruce. This was done at 20 randomly selected points in the Black Sturgeon Lake area near Thunder Bay, Ontario. The number of late-instar larvae captured in water traps was recorded throughout the dispersal period of the late instar larvae. The data indicated that white spruce canopies contained 2 to 3 times more spruce budworm than balsam fir canopies. A similar pattern was found in the understory. Water traps under white spruce trees captured more than 3 times as many larvae as did those under balsam fir trees for most of the dispersal period.
Impact on hosts
Balsam fir is the most susceptible host to outbreaks of the spruce budworm. Annual defoliation of current-year growth for 5 to 8 years will kill the host tree. Defoliation by the spruce budworm is most clearly reflected in the fir's radial growth. The population of mature balsam fir in a forest is greatly reduced by a combination of factors: its shorter lifespan and a great vulnerability to lethal budworm attacks. The dietary preference for balsam fir over white spruce has the potential to alter the structure and composition of spruce-fir forests. Similarly, the next-generation stand of trees are influenced by the late instar larvae that disperse to the understory of the forest and feed on the regeneration of plants. During a 1957 budworm outbreak in Quebec, balsam fir mortality was greater than 75% in stands in which no mortality was reported among the smaller component of white spruce.However, while balsam fir is the preferred host, severe outbreaks have occurred among white spruce stands in the Prairie Provinces and Northwest Territories containing little or no balsam fir. The white spruce is less susceptible to budworm attack but can experience extreme defoliation during severe outbreaks. Young white spruce and black spruce trees that had been transplanted to cleared areas became infested with dozens of late-stage larvae during severe outbreaks in north-central Ontario. Mortality among white spruce also occurred in northwestern Ontario and the Algoma District of Ontario, as well as in certain areas of New Brunswick.
Significant damage is also caused to subalpine fir. Red spruce, in its limited distribution, and tamarack are also attacked.
Food shortage
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.Microbiome
The midgut microbiota of the spruce budworm larvae is primarily composed of Pseudomonadota, mainly from the genus Pseudonomas. Two specific species from this genus are P. fluorescens and P. paea. Enterococcus and Staphylococcus bacteria were found in lower abundance as well.Diet plays a role in influencing the gut microbiome. One study found possible negative consequences of ingesting balsam fir as it may release juices that adversely affect the midgut microbiota.
Reproduction and life cycle
Mating
Pheromones and mate choice
As with other species in the genus Choristoneura, spruce budworm females produce sex pheromones to attract males as potential mates and enhance their base level of sexual activity. C. fumiferana females emit aldehydes, using a 95:5 mix of E- and Z11-tetradecenals while some other species of Choristoneura emit acetates and alcohols. The pheromone is made with palmitate using β-oxidation and Δ11-desaturation and stored as unsaturated E/''Z11-14Ac.Male size is an important factor in reproductive success, but male C. fumiferana also emit a pheromone that helps attract females. This pheromone becomes available through larval and adult feeding. While attempting to copulate, males will exhibit their abdominal hair pencils to the female. These hair pencils release a volatile pheromone and play a role in attracting females. The chemical composition of the volatile pheromone is currently unknown. Some studies suggest that these pheromones are indicators of male mate quality and prevent other males from approaching the female.
Females exhibit selective mate choice as they show more parental investment. Pheromones allow females to recognize and assess males as mates. Male C. fumiferana'' prefer to mate with virgin females.