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Chapter 3


Insect infestations: Forest fragmentation from human activity exacerbates insect pest outbreaks.

Loss of ecological complexity: Reduced habitat for insect predators due to roads and other management activities is predicted to increase the severity of pest outbreaks.

Insect infestations: Forest fragmentation from human activity exacerbates insect pest outbreaks.

Key Finding: Forest fragmentation due to cleared forest increased the duration of tent caterpillar outbreaks.

Key Finding: Forest edges were predicted to be source populations for tent caterpillars.

Source: Roland, J. 1993. Large-scale forest fragmentation increases the duration of tent caterpillar outbreak. Oecologia 93: 25-30.

The author examines historical data on the spatial extent of tent caterpillar (Malacosoma disstria) outbreaks from 1950 through 1984 in Ontario, Canada, and compares these to township forest resource inventory maps. The degree of forest fragmentation was based on the percentage of cleared areas, forested areas, and the extent of edges. He found that the duration of tent caterpillar outbreaks was higher with increasing forest fragmentation. Townships with continuous forest had outbreaks lasting one to two years, while townships with 2-2.5 km of edge per km2 had outbreaks lasting four to six years.

The author did not investigate the mechanisms for this pattern but, based on research reported for other lepidopteran species, speculates that edges may have acted as source populations for caterpillar larvae. This could be either because more eggs were laid along edges (sunnier and warmer) than within the forest interior or because of more rapid development of larvae at the forest edge. He also suggests that forest fragmentation may limit the dispersal of parasitoids and pathogens that are natural enemies of tent caterpillars.

Key Finding: Mortality of tent caterpillars in the forest understory due to a natural virus (NPV) decreased as forest cover decreased and edge habitat increased.

Source: Rothman, L. D. and J. Roland. 1998. Forest fragmentation and colony performance of forest tent caterpillar. Ecography 21: 383-391.

The tent caterpillar (Malacosoma disstria) is reported to occur through most of the United States and southern Canada. A nuclear polyhedrosis virus (NPV) is the dominant natural enemy of the caterpillar. The authors introduced colonies of tent caterpillar larvae to two sites of trembling aspen/balsam poplar in Alberta, Canada. They measured larvae survival, net reproductive rates, and the relationship to forest cover. Their models showed forest cover to be the best predictor of tent caterpillar performance. As forest cover decreased, caterpillar colony performance improved, with the greatest effect during larval and prepupal/pupal stages. This relationship was due to increasing NPV-caused mortality with increasing forest cover. The authors report other studies where NPV became inactive after 10 hours of exposure to direct sunlight, confirming their hypothesis that increased area of edge habitat contributed to a greater caterpillar outbreak.

Key Finding: Abrupt edges along mature jack pine stands increased the levels of defoliation by jack pine budworm in Michigan.

Source: Kouki, J., D. G. McCullough and L. D. Marshall. 1997. Effect of forest stand and edge characteristics on the vulnerability of jack pine stands to jack pine budworm (Choristoneura pinus pinus) damage. Canadian Journal of Forest Research 27: 1765-1772.

The authors surveyed 104 managed jack pine stands in Hiawatha National Forest, Michigan, during a jack pine budworm (Choristoneura pinus pinus) outbreak. Defoliation by the budworm, its relationship with local stand factors, and characteristics of adjacent stands were evaluated. Pine stands ranged from 8 to 72 years old. Older stands had higher levels of defoliation than younger stands and, of these, mature stands with younger/shorter stands adjacent to them had the highest levels of defoliation.

The authors hypothesize that these higher rates of defoliation may have been due to higher light levels along edges. They discuss other studies that report pollen cone abundance (the food source for budworm larvae) to be higher than expected along roads and other forest edges where light availability was greater. This increased pollen cone production along edges may result in higher jack pine budworm populations.

Key Finding: Trees at forest edges created by roads had 2.4 times more gypsy moth egg masses than trees in the forest interior.

Source: Bellinger, R. G., F. W. Ravlin and M. L. McManus. 1989. Forest edge effects and their influence on gypsy moth (Lepidoptera: Lymantriidae) egg mass distribution. Environmental Entomology 18: 840-843.

The authors compared numbers of gypsy moth (Lymantria dispar) egg masses on forest edge trees versus interior trees in Virginia. A total of 160 trees (primarily oak species) were sampled in early May, along both sides of rural roads and fire roads. Trees designated as interior trees were 40.2 m from the edge. Edge and interior trees were of similar size and species. Edge trees had 2.4 times more gypsy moth egg masses than interior trees, and the edge side of edge trees had about 3.2 times more egg masses than the edge side of interior trees. Authors note that the edge effect still existed 40 m into the forest.

Loss of ecological complexity: Reduced habitat for insect predators due to roads and other management activities is predicted to increase the severity of pest outbreaks.

Key Finding: A diversity of predators is important for preventing pest outbreaks.

Key Finding: Old-growth and roadless areas, with their greater diversity of composition, structure, and predators, are predicted to be less vulnerable to pest outbreaks than forests simplified through management.

Source: Schowalter, T. D. and J. E. Means. 1989. Pests link site productivity to the landscape. pp. 248-250 in Maintaining the Long-Term Productivity of Pacific Northwest Forest Ecosystems. D. A. Perry, R. Meurisse, B. Thomas, R. Miller, J. Boyle, J. Means, C.R. Perry, R. F. Powers, eds. Timber Press, Portland, Oregon.

The authors discuss landscape patterns and their influence on pests. Three components are important in terms of their impact on pests - intersection by roads or other corridors, patch size, and diversity of stand age classes. They state that pest success increases with forest simplification as the diversity of habitats decreases, resulting in declines of important pest predators, such as spiders and birds. Similarly, reduced stand size and age-class diversity, planting of monocultures, and intersection by roads increases pests' likelihood of finding suitable hosts. They maintain that old-growth forests should be less vulnerable to pest outbreaks than the simplified forests created through management.

Key Finding: Species diversity and functional diversity of arthropods were much higher in old-growth stands than in regenerating logged stands.

Source: Schowalter, T. D. 1989. Canopy arthropod community structure and herbivory in old-growth and regenerating forests in western Oregon. Canadian Journal of Forest Research 19: 318-322.

Arthropod communities were compared in old-growth forest and regenerating forest by establishing study plots in six old-growth stands and six 7- to 11-year-old stands created through logging. All sites were located in the central western Cascades of Oregon, in the H. J. Andrews Experimental Forest. Old-growth stands were dominated by Douglas-fir, western hemlock, and western red cedar, while young stands were dominated by Douglas-fir. Foliage at upper, middle and lower crown levels was sampled in old-growth trees by climbing and clipping sample branches. Canopies of young Douglas-fir were sampled randomly from the ground. The sampling period extended from mid-June to early October.

Arthropod species diversity and functional diversity were much higher in canopies of old-growth trees compared to young trees. Aphid biomass was significantly higher in young stands compared to old-growth stands. The ratio of defoliating insects to sap-sucking herbivores was found to be different between the stands, with higher ratios in old-growth. The ratio of predator species to herbivore species was higher in old-growth than young stands, and predator species richness was much greater in the old-growth.

Key Finding: Old-growth forests, which have a greater diversity of insect predators, are predicted to help control pest populations.

Source: Franklin, J. F., D. A. Perry, T. D. Schowalter, M. E. Harmon, A. McKee and T. A. Spies. 1989. Importance of ecological diversity in maintaining long-term site productivity. pp. 82-97 in Maintaining the Long-Term Productivity of Pacific Northwest Forest Ecosystems. D. A. Perry, R. Meurisse, B. Thomas, R. Miller, J. Boyle, J. Means, C.R. Perry, R. F. Powers, eds. Timber Press, Portland, Oregon.

The authors discuss the importance of ecosystem resilience - the "ability to absorb stress or change without significant loss of function." Forest management has resulted in increased simplification of forests - structurally, genetically, on the landscape scale, and in terms of successional stages.

Stresses on forests such as pollutants, global climate change, and pests and pathogens are reviewed. The authors report studies indicating that disease and pest problems may be worse in managed stands than in natural stands, and that thinning practices contribute to diseases such as root rot. The authors also review findings that suggest that old-growth forests have a greater diversity of insect predators that may help limit pest populations. They state that damage by herbivorous insects could increase as the area of old-growth forests increasingly diminishes.

Key Finding: Ant and bird predation reduced adult western spruce budworm densities by approximately 10- to 15-fold at low budworm densities, and by approximately twofold at high budworm densities.

Source: Campbell, R. W., T. R. Torgersen and N. Srivastava. 1983. A suggested role for predaceous birds and ants in the population dynamics of the western spruce budworm. Forest Science 29: 779-790.

Ants, birds, or both were excluded from individual trees using sticky barriers for ants and whole-tree exclosures for birds in order to examine these predators' impacts on western spruce budworm (Choristoneura occidentalis) density. Study sites were in central Idaho (interior Douglas-fir/grand fir/ponderosa pine) and northcentral Washington (Douglas-fir/ponderosa pine). Budworm densities were sampled from the fourth instar to adult stage. At the lowest budworm density, trees protected from both birds and ants had 10 to 15 times as many adult moths as control trees. At higher densities of budworms (25 insects per m2 of foliage), protected trees had two times as many adult moths. Birds alone or ants alone also greatly dampened survival rates of spruce budworm.

Key Finding: Thatching ants play an important role in suppressing insect defoliator populations.

Source: McIver, J. D., T. R. Torgersen and N. J. Cimon. 1997. A supercolony of the thatch ant Formica obscuripes Forel (Hymenoptera: Formicidae) from the Blue Mountains of Oregon. Northwest Science 71: 18-29.

The authors review studies on thatching ants (Formica-rufa group). Their ability to reduce populations of defoliators during outbreaks was believed to be due to their high metabolic demands and ability to switch to foods that become more abundant. The authors also observe that stands with a large population of thatching ants experienced less defoliation from spruce budworm than stands lacking these ants, although experimental evidence had yet to be collected. Reasons for the decline in thatching ant populations in the Blue Mountains are unknown.

The authors describe a rare supercolony of western thatching ants (Formica obscuripes) from the Blue Mountains. Total nestbound population was estimated to be 56 million individuals. The authors calculated that this population would require at least 470 kg dry weight of food annually, which is about 11 times the total dry weight of western spruce budworm that might occur during an outbreak.

Key Finding: Ants, important predators of the western spruce budworm, require sufficient down wood in a range of sizes and decomposition stages.

Source: Torgersen, T. R. and E. L. Bull. 1995. Down logs as habitat for forest-dwelling ants ( the primary prey of pileated woodpeckers in northeastern Oregon. Northwest Science 69: 294-303.

Down logs, important habitat for forest-dwelling ants, were examined for their key characteristics and the species of ants using them. The 12 study sites were in uneven-aged mixed-conifer stands of the Blue Mountains, northeastern Oregon. Ants were of interest because many species are predators of the western spruce budworm (Choristoneura occidentalis), and 12 of the 13 ant species are reported to colonize standing or down dead wood. Ants are also an important food source for pileated woodpeckers.

All logs * 15 cm in diameter at the larger end and * 2 m long were sampled and classified according to five decomposition classes. In total, 1,385 logs were sectioned. Six logs per plot were sectioned to collect ant specimens. Ants were grouped into Camponotus species, Formica species, and other species.

Logs in the largest diameter class (51-120 cm), although only accounting for 8.3% of the logs, represented 37.6% of the down log volume. Nearly half of the total logs were in the smallest class (15-22 cm) and accounted for 11.9% of the volume.

Of the sectioned logs, 61.8% had ant groups in them, comprised of 13 species. Lasius alienus was the most common, followed by Formica neorufibarbis and Camponotus modoc. Camponotus ants occurred significantly more frequently in logs of the largest diameter class. Formica and other ants occurred in logs of all size classes but significantly favored logs * 7m long over 1-3 m long logs. Formica ants occurred significantly less commonly in the most decomposed logs compared to the other decomposition classes; Camponotus species also tended to avoid these logs.

The authors report that three Camponotus and three Formica species identified in this study are known to be predators of western spruce budworm. At least one of these species was found in 31.8% of the sectioned logs. The authors conclude that these ants use a range of logs of different sizes, species, and stages of decomposition. They caution that leaving insufficient amounts and kinds of down wood, because of logging or firewood cutting, could affect pileated woodpecker populations and the control of western spruce budworm.

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