Forest Tent Caterpillar Populations Research Articles

Effects of forest spatial structure on insect outbreaks: insights from a host-parasitoid model.

Understanding how cycles of forest-defoliating insects are affected by forest destruction is of major importance for forest management. Achieving such an understanding with data alone is difficult, however, because population cycles are typically driven by species interactions that are highly nonlinear. We therefore constructed a mathematical model to investigate the effects of forest destruction on defoliator cycles, focusing on defoliator cycles driven by parasitoids. Our model shows that forest destruction can increase defoliator density when parasitoids disperse much farther than defoliators because the benefits of reduced defoliator mortality due to increased parasitoid dispersal mortality exceed the costs of increased defoliator dispersal mortality. This novel result can explain observations of increased outbreak duration with increasing forest fragmentation in forest tent caterpillar populations. Our model also shows that larger habitat patches can mitigate habitat loss, with clear implications for forest management. To better understand our results, we developed an approximate model that shows that defoliator spatial dynamics can be predicted from the proportion of dispersing animals that land in suitable habitat. This approximate model is practically useful because its parameters can be estimated from widely available data. Our model thus suggests that forest destruction may exacerbate defoliator outbreaks but that management practices could mitigate such effects.

Isolation and characterization of plaque-purified strains of Malacosoma disstria Nucleopolyhedrovirus

Seven plaque-purified genotypic variants or strains, derived from a previously described field isolate of the Malacosoma disstria Nucleopolyhedrovirus (MadiNPV) from Alberta populations of forest tent caterpillar, were characterized based on distinctive restriction endonuclease fragment patterns. Two strains, MadiNPV-pp3 and MadiNPV-pp11, were selected for further characterization, as they represented strains producing high and low budded virus (BV) titres, respectively, in the M. disstria cell line UA-Md203. Analysis of restriction endonuclease fragment profiles indicated the genomes differed significantly in size, 133.8 +/- 2.4 kb for MadiNPV-pp3 and 118.1 +/- 3.5 kb for MadiNPV-pp11. These strains were characterized based on their BV production in three different cell lines derived from M. disstria haemocytes. Compared with MadiNPV-pp11, MadiNPV-pp3 produced two- to three-fold more BVs in UA-Md203 and 210 other cell lines; however, BV production was only marginally higher for MadiNPV-pp3 in the UA-Md221 cell line. Similarly, the yield of polyhedral inclusion bodies was significantly higher for MadiNPV-pp3 in UA-Md203 and 210 cell lines than for MadiNPV-pp11 but not in the UA-Md221 cell line. This data, although derived from a limited number of cell lines, suggested MadiNPV-pp3 may have a broader tissue tropism than MadiNPV-pp11.

Variation in the Suitability of Host Tree Species for Geographically Discrete Populations of Forest Tent Caterpillar

The forest tent caterpillar, Malacosoma disstria Hubner (Lepidoptera: Lasiocampidae), is distributed throughout North American hardwood forests. Although considered polyphagous, regional populations tend to use only a few host species for oviposition, suggesting that M. disstria is more oligophagous than commonly thought. We tested this premise using larvae from Manitoba, Canada; Michigan; and Louisiana in a factorial, reciprocal transplant experiment. Pupal mass, development time, and survival were recorded for each population after rearing larvae in Louisiana on three primary hosts used by southern populations (water tupelo, Nyssa aquatica L.; sweetgum, Liquidambar styraciflua L.; and water oak, Quercus nigra L.) and in Michigan on three northern host trees (red oak, Quercus rubra L.; trembling aspen, Populus tremuloides Michaux; and sugar maple, Acer saccharum Marshall). Manitoba, Canada and Michigan populations had the highest pupal mass, best survival, and most rapid development on trembling aspen and red oak, both northern species. Louisiana larvae attained the highest pupal mass on water tupelo, a primary host across the Gulf States. Northern populations grew poorly on water tupelo, whereas Louisiana caterpillars had the smallest pupal mass and poorest survival on sugar maple. Both red and water oaks were acceptable hosts for all three populations. Our results indicate that M. disstria is actually a composite of regionally adapted populations rather than an extreme generalist. It is unclear, however, what mechanisms might reduce gene flow, allowing such specialization to evolve and persist. We suggest that varying phenology in adult flight times among populations feeding on different hosts could provide at least a partially isolating mechanism, allowing for the evolution of host adaptation.

Developing techniques for monitoring forest tent caterpillar populations using synthetic pheromones

AbstractTo effectively monitor forest tent caterpillar, Malacosoma disstria Hübner (Lepidoptera: Lasiocampidae), populations using sex pheromone baited traps, we field-tested pheromone dispenser (lure) type, lure age, and trap design using (Z,E)-5,7-dodecadienal:(Z,Z)-5,7-dodecadienal (100:1). Rubber septa lures, polyurethane lures, and two trap types [sticky-type pheromone traps (Wing Trap I) and bucket-type pheromone traps (Universal Moth trap)] were evaluated. Traps baited with polyurethane lures produced higher trap catches and lower zero-catch frequencies than did rubber septa traps. There was no detectable difference in trap catch among polyurethane lures aged 0–28 days. Wing traps reached a functional saturation point in outbreak M. disstria populations and caught fewer moths than Universal traps in nonoutbreak populations. A nonsaturating trap such as the Universal trap in conjunction with the polyurethane lure should be effective for monitoring M. disstria populations.

Macrogeographic clines in fecundity, reproductive allocation, and offspring size of the forest tent caterpillar Malacosoma disstria

Summary1. The fecundity of the forest tent caterpillar varies considerably across its geographic range. Field data indicate that populations in the southern United States (Gulf States) produce nearly twice as many eggs as females from Canada or the Lake States, with little or no difference in the size of adult females.2. In controlled rearing experiments, female forest tent caterpillar from the southern United States (Louisiana) had much larger clutch sizes than same sized females from northern populations in Michigan or Manitoba, Canada. Increased fecundity in Louisiana females was achieved through a significant reduction in egg size and a concomitant increase in the allocation of resources to egg production.3. Comparison of 10 forest tent caterpillar populations spanning a 27° latitudinal gradient, validated the results of detailed comparisons among the three populations above by confirming the strong negative correlation between latitude and clutch size.4. Neonate forest tent caterpillars from Manitoba were significantly larger than larvae from either Michigan or Louisiana. Michigan larvae were intermediate in size. It is postulated that large neonates are advantageous in thermally limiting environments. More than three times as many degree‐days are available to Louisiana neonates during the first 2 weeks after hatching. A consistently favourable climate during the vulnerable post‐hatching period may have allowed the evolution of larger clutches at the expense of neonate size in southern populations.

Forest edge effects on the biological performance of the forest tent caterpillar (Lepidoptera: Lasiocampidae) in sugar maple stands

Fragmentation of forests has considerably increased the amount of edge habitats. This edge effect could have a direct influence on the dynamics of forest insects by altering the nutritional quality of foliage. In this study, we examined from 1994 to 1997 the effects of foliage from edge trees and interior trees on biological performance of the forest tent caterpillar (Malacosoma disstria Hbn.). Each year, ten mature sugar maple trees (Acer saccharum Marsh.) were selected at the forest edge and in the forest interior. Leaves were collected on branches located at 3-6 m above ground at intervals of one or two days. These leaves were immediately brought back to the laboratory for rearing of insects in Petri dishes (8 larvae/Petri dish) and for chemical analyses. Total nitrogen levels were significantly higher in leaves from edge trees as were nonsignificantly higher levels of soluble sugars and total phenolics and lower water content. Generally, performance of the forest tent caterpillar, except for larval and pupal developmental time, varied between the forest edge and the forest interior habitats. Pupal weight, egg production and pupal survivorship were greater for larvae reared on leaves from edge trees. Stepwise multiple regression analyses revealed that total nitrogen, soluble sugars and water content were closely associated with the forest tent caterpillar’s performance. These results suggest that forest edge effects, in terms of leaf nutritional quality, could have important implications for forest tent caterpillar populations in sugar maple stands.

Effects of Constitutive and Inducible Traits of Hybrid Poplars on Forest Tent Caterpillar Feeding and Population Ecology

Abstract Hybrid Populus spp. clones, differentially affected forest tent caterpillar, Malacosoma disstria, behavior, development, and population dynamics. Increased larval cohort size and larval silk generally enhanced development. Larval behavior affected developmental success. Under choice conditions, larvae selected more suitable clones, and their growth was enhanced when they moved from less to more suitable hosts. Herbivory by the forest tent caterpillar caused significant reductions in the quality of foliage for subsequent larval development. Polar and nonpolar foliar extracts varied between constitutive and previously damaged plants, and elicited significant larval behavioral responses. The extent to which prior feeding induced foliar changes that affected larval feeding and development varied among clones. The ability of forest tent caterpillar to exploit clones includes genetic and environmental components affecting foliar quality, and includes positive and negative density dependent feedback. In whole-tree field cages, forest tent caterpillar populations increased 9.7x during a 2 yr period on a highly suitable clone, and declined 2.7x on a poor quality clone. The relationships between clonal suitability and forest tent caterpillar population growth were strongly influenced by bud phenology and exudates, in addition to foliar traits. For. Sci. 43(2):252-267.

Responses of natural enemies to experimentally increased populations of the forest tent caterpillar, Malacosoma disstria

1. The responses of predators and parasitoids to increased forest tent caterpillar populations were studied by introducing eggs to two trembling aspen, Populus tremuloides Michx., forests where natural populations were at very low density.2. Of five parasitoid species recovered, only the braconid Aleiodes malacosomatos (Mason) and the tachinid Patelloa pachypyga (Aldrich & Webber) exhibited spatially density‐dependent responses, in dry upland forest and mesic lowland forest, respectively. Forest type restrictions on the abundance of these species and the weak density‐dependent response by A. malacosomatos suggests that parasitoids may not be capable of regulating low‐density tent caterpillar populations.3. Predation of final‐instar larvae and pupae by birds, in particular the northern oriole, Icterus galbula (L.), virtually eliminated the experimental populations. Avian predation was widespread and dominated mortality at all densities.4. Implications for the initiation of outbreaks of cyclic defoliators are discussed.

LARVAL AND PUPAL PARASITISM OF THE FOREST TENT CATERPILLAR, MALACOSOMA DISSTRIA HÜBNER (LEPIDOPTERA: LASIOCAMPIDAE), IN ALBERTA, CANADA

AbstractLarval and pupal parasitoids were studied from collections made in endemic, outbreak, and post-outbreak populations of forest tent caterpillar (FTC), Malacosoma disstria, at 21 sites in Alberta, Canada, from 1989 to 1994. The parasitoid complex included 18 identified species. Aleiodes malacosomatos (Mason) (Hymenoptera: Braconidae) was common in low density populations and was the only species to attack FTC larvae prior to the fourth instar. Leschenaultia exul (Townsend) (Diptera: Tachinidae) was the most abundant larval parasitoid in endemic populations and after the 2nd year of outbreak. Although prevalent early in outbreaks, Patelloa pachypyga (Aldrich and Webber) (Diptera: Tachinidae) declined in older outbreaks possibly because it completes larval development later than other tachinids, leaving it vulnerable to competition from aggressive pupal parasitoids. I hypothesize that early initiation of oviposition allows L. exul to be more successful than P. pachypyga in low density populations because caterpillars have dispersed before the latter species initiates oviposition. I recorded lower pupal parasitism by Arachnidomyia aldrichi (Parker) (Diptera: Sarcophagidae) in Alberta than has been recorded in other regions, although it was still the most abundant parasitoid reared in this study. Parasitism by A. aldrichi was significantly higher in cocoons collected from undergrowth vegetation than from the forest canopy. Other pupal parasitoids contributed relatively little to FTC mortality.

Large-scale forest fragmentation increases the duration of tent caterpillar outbreak.

I examined historical data (1950-1984) on the duration of outbreaks of the forest tent caterpillar (Malacosoma disstria) in northern Ontario, Canada. Outbreak duration was compared to host tree species dominance and forest structure over large areas of boreal forest partially cleared for agriculture. Abundance of the principal host tree species Populus tremuloides had no consistent effect on duration of outbreak within forest districts, and was negatively correlated with duration of outbreaks among the eight forest districts examined. The amount of forest edge per km2 was the best, and most consistent, predictor of the duration of tent caterpillar outbreaks both within individual forest districts and among forest districts. Because forest tent caterpillar populations are driven largely by the impact of parasitoids and pathogens, results here suggest that large-scale increase in forest fragmentation affects the interaction between these natural enemies and forest tent caterpillar. Increased clearing and fragmentation of boreal forests, by agriculture and forestry, may be exacerbating outbreaks of this forest defoliator.

Population Fluctuations and Causes of Mortality for the Forest Tent Caterpillar, Malacosoma disstria (Lepidoptera: Lasiocampidae), on Three Different Sites in Southern Louisiana

Population fluctuations of forest tent caterpillar, Malacosoma disstria Hubner, were investigated at two permanently flooded plots and one plot that was dry during summers of 1980–83 in southern Louisiana. Tupelo gum, Nyssa aquatica L., and swamp tupelo, Nyssa sylvatica var. biflora (Walt.) Sarg., made up 63% of the dry plot forest stand and 93.8 and 87% of the two wet plots. Egg parasitism ranged from 8.8 to 29.7%, and egg infertility varied from 1.2 to 6.7%. An analysis of egg mortality factors demonstrated no density-dependent relationships or significant plot differences. Pupal parasitism by Sarcophaga houghi Aldrich varied from 18 to 78% during 1981–82. Lower rates of parasitism were found on wet plots due, presumably, to drowning of parasite puparia. Populations of forest tent caterpillar at the dry plot were reduced by apparent density-dependent parasitism by S. houghi . We found that a generation survival rate of 0.1% resulted in population decline and that survival ≥0.7% resulted in a population increase the following season. Starvation may have been a major factor in population fluctuation at wet plots.

Artificial Initiation of Virus Epizootics in Forest Tent Caterpillar Populations

AbstractNuclear-polyhedrosis virus, introduced into virus-free populations of the forest tent caterpillar, Malacosoma disstria (Hübner), in 1963, was carried over into the 1964 host generation and was spread into the surrounding areas from the points of introduction. The extent of spread in 1964 appeared to be influenced by the size of the area treated in 1963. In 1964 the epizootics developed slowly during the early instars and reached the highest levels late in the last instar. Many infected larvae crawled to the tops of the trees and died while eggs were being laid, thus, some egg masses were contaminated. This is one means by which virus may be spread from one generation to the next, but egg-transmission by adults appears to be the main method of spread over wide areas. Adults of the dipterous parasite, Sarcophaga aldrichi Park, may be important dispersal agents of disease in localized areas because they feed on virus-diseased larvae and are present from the time larvae enter the third instar until after adults have oviposited.

Outbreaks of the Forest Tent Caterpillar, Malacosoma disstria Hbn., a Periodic Defoliator of Broad-leaved Trees in Ontario

Severe defoliation of aspen stands by the forest tent caterpillar, Malacosoma disstria Hbn., became apparent in 1948 in three widely separated regions in Ontario, and by 1949 it was clear that another general outbreak of this defoliator had begun. In 1950 the Forest Insect Survey (cf. McGugan, 1958) initiated an intensive program of observation, collecting, and rearing to record the progress of the outbreak and study the parasite complex of the insect. This paper compares the outbreak in the early ‘fifties’ with earlier ones, and attempts to describe the pattern of build-up and decline of forest tent caterpillar populations within extensive forested areas.