Example
of a spruce budworm larvae. Click on photo to make it larger.The spruce budworm (Choristoneura fumiferana) is the most significant conifer defoliator found in the boreal ecosystems of North America. Outbreaks can result in the loss of billions of board feet of timber in both the eastern and western U.S. (Johnson and Lyon 1991). Extremely extensive and damaging outbreaks have occurred in the Maritime provinces of Canada, Quebec, Ontario, Maine, and in the Great Lakes States (Coulson and Witter 1984). There have been periodic spruce budworm outbreaks for centuries, therefore, there have been several studies to determine those trees most susceptible and vulnerable to severe damage and the most effective control measures to reduce the destruction caused by the spruce budworm.
The spruce budworm has a 12 month lifecycle. From late June to late July, moths emerge from the pupae and mate. Female moths ascend 150-300 m above the canopy to encounter strong wind currents which allow them to migrate distances over 50 miles (80 km) and/or until they come upon a frontal system (Coulson and Witter 1984; Johnson and Lyon 1991). They usually make several of these evening flights, and therefore can disperse eggs extremely long distances.
In July, the female deposits her eggs in clusters on needles, usually in the upper half of the crown. A female normally deposits around 170 eggs in 5-10 egg clusters with 5-50 eggs each (Coulson and Witter 1984). The eggs hatch at the end of 10-14 days (Coulson and Witter 1984). During this first instar, there is very little, if any, defoliation. The larvae are photopositive and therefore migrate to the distal portion of the crown unless they encounter high temperatures, in which case they settle earlier (Coulson and Witter 1984). This migration can also be interrupted if turbulent winds cause the larvae to fall from the trees via silken threads (Coulson and Witter 1984). Once the larvae settle in the distal ends of the branches of either host or nonhost trees, they spin a hibernaculum (cocoonlike shelter) in bark fissures, staminate flower bracts, or lichen mats on branches to overwinter. Before the onset of winter, the larvae molts into the second instar, which overwinters in the hibernaculum until late-April to mid-May (Coulson and Witter 1984).
Example
of a spruce budworm larvae. Click on photo to make it larger.
When the second instar emerges, it begins to move towards feeding sites. The larvae initially feed on old needles, unopened buds, or staminate flowers of the host tree. When the buds begin to open and the shoots begin to elongate, the larvae can enter the expanding buds and feed on the new foliage. Often, the larvae destroy the bud before shoot elongation is initiated (Coulson and Witter 1984).
Spruce budworm larvae go through 6 instars. During the first instars, the half-grown larvae usually build and feed inside a web-encased shelter. Older larvae may prefer new foliage, but often have to 'backfeed' (consume old foliage) in a dense population. They can reach a size of 20-30 mm and are recognized by their brownish-black prothoracic shield, head, and thoraic legs. The rest of their body is brown, black, and green with light areas on each side. The fourth and fifth instars cause the majority of damage seen in forest stands (Coulson and Witter 1984). During this period, the larvae form a nest by tying the tips of shoots together and feed heavily upon severed needles they have attached to the twigs with silk (Johnson and Lyon 1991). The attached needles eventually turn brown and cause the infected tree to appear to have needleless branch tips and scorched foliage (Johnson and Lyon 1991). After the sixth instar, the larvae spin a pupae during mid-June to early-July. The adult moth usually emerges 10-14 days later to complete the 12-month life cycle (Coulson and Witter 1984).
Throughout its range, the spruce budworm primarily infests balsam fir (Abies balsamea) and species of spruce including white (Picea glauca), red (Picea rubens), and black (Picea mariana). It will also feed upon associated conifers such as larch (Larix spp.) and hemlock (Tsuga spp.) when its population reaches high densities.
All ages and sizes of trees are susceptible to defoliation by the spruce budworm, but stands with the following conditions are the most vulnerable:
As stated above, there have been periodic spruce budworm outbreaks for centuries throughout the boreal forests of North America. The intensity and duration of an outbreak usually dictates the severity of the damage; the spruce budworm can be responsible for growth loss, top-kill, tree mortality, stand mortality, and changes in stand composition (Coulson and Witter 1984). Studies in North America have shown a 30-90% decrease in radial growth of trees in spruce-fir stands that were heavily defoliated for 2-6 years, and one study done in Minnesota reported that 78% of the balsam fir had at least 183 cm to top kill after 3 years of defoliation of new growth (Coulson and Witter 1984). In general, heavy defoliation during an outbreak lasting for 3 or more years will lead to the mortality of seventy-100% trees within mature fir stands, and 30-70% within immature stands (Coulson and Witter 1984).
Example
of the damage to spruce forests caused by the spruce budworm. Click on
photo to make it larger.
Because of the high tree mortality associated with heavy outbreaks, the spruce budworm can be responsible for the destruction of billions of board feet of merchantable timber in the spruce-fir forests of eastern North America. As of 1984, the estimated loss associated with the current outbreak was 283 million m3. This could severely impact the economy of areas that depend on the spruce-fir forests for the pulp and paper industry and recreation. ~(Coulson and Witter 1984).
Extensive and often very destructive infestations have not led to the discontinuation of the spruce-fir forests. In fact, the spruce budworm is an important intermediate disturbance agent within the spruce-fir ecosystem (Coulson and Witter 1984). The major disturbance agent within most of these regions is fire, however, with the decrease in fire frequency, spruce budworm infestations have become very influential on stand dynamics (Kneeshaw and Bergeron 1996). Budworm outbreaks within a stand create gaps that release shade tolerant species such as the balsam fir (Kneeshaw and Bergeron 1996). However, the larger balsam fir trees within the understory also can be infected and killed by a severe outbreak (Kneeshaw and Bergeron 1996). The elimination of both the subcanopy and canopy trees in fir-dominated forest stands can cause larger gaps and possibly lead to an increase in hardwood recruitment (Kneeshaw and Bergeron 1996). This changes the forest from a conifer-dominated to a mixed stand. However, the spruce budworm outbreaks with fir-dominated forests also can contribute to the perpetuation of the fir within an area, and consequently could be considered as part of a self-regulating system within the balsam fir forests (Morin 1994). Thus, outbreaks by spruce budworm, in the absence of fire, can instigate significant changes in or help maintain the current forest composition.
The early larval stages are the most susceptible to mortality by natural agents. The majority of losses (60-80%) are correlated with the weather and/or the characteristics of the stand and occur during the dispersal of the newly hatched larva and second instar after emerging from the hibernaculum. A large density of open crowns and/or nonhost species within a stand significantly contributes to larval death. Also, a cooler, wetter climate during dispersal can increase mortality (Coulson and Witter 1984).
Predators, parasites, and disease can also help control spruce budworm numbers, especially when the population is in decline (Coulson and Witter 1984). Several of species of songbirds will prey upon spruce budworm (Montgomery et al. 1984) Wasps, such as Meteorus trachynotus and Phaeogenes hariolus, are the most prevalent parasites, however, flies such as Lypha setifacies, will also prey upon the spruce budworm (Johnson and Lyon 1984).
There are chemical and microbial insecticides that can be used to control spruce budworm outbreaks. Managers usually try to limit insecticide use to those areas with the greatest amount of damage and spray during the third and fourth instar (from late-May to mid-June). However, no pesticide consistently protects a stand from an outbreak, therefore, managers have been integrating the use of pesticides with other forest management practices (Coulson and Witter 1984).
First, forest managers must evaluate the susceptibility of their stand (see 'Hosts' to review which stands are more susceptible) and the possibility of a spruce budworm outbreak. If the stands are highly vulnerable and outbreak occurs, most of the spruce and fir trees are expected to die within 5-10 years. There are several options that can be implemented to reduce the vulnerability of the stand, and thereby prevent high rates of mortality. The following are some possible management strategies that can be used individually or in combination to help reduce damage by the spruce budworm ~(Montgomery et al. 1984).
1. Harvest any spruce and/or fir trees within the stand that are over 50 years old and/or have heart rot and/or.
2. Implement a series of strip- or patch-cuts within a mature spruce-fir stand. This creates gaps, and allows some shade-intolerant species to become established. It also retards successful dispersal of larvae by enlarging the distances between host trees. Likewise, it creates edge and meadow areas that can be used by wildlife, including those songbird species that prey on the budworm and/or.
3. Another way to limit host trees is to harvest individual spruce and/or fir trees and replace them with non-host trees. Shelterwood cuts, seed-tree, selection cutting, and clearcutting are possible ways to do this.