Dylan Parry

Repeated insect defoliation effects on growth, nitrogen acquisition, carbohydrates, and root demography of poplars

Large-scale outbreaks of defoliating insects are common in temperate forests. The effects of defoliation on tree physiology are expected to cascade through the entire forest ecosystem, altering carbon, nitrogen, and water fluxes, and subsequently affecting nitrogen cycling and plant-herbivore interactions. If these post-defoliation changes are largely driven by N deficiency, tree root system responses to defoliation should be central to regulating the long-term effects of defoliation; N fertilization should reverse the effects. We examined these phenomena in a 3-year large-scale replicated manipulative field experiment in a hybrid poplar plantation, where we regulated defoliation by gypsy moths as well as nitrogen availability. To our knowledge, this is the first manipulative field experiment at this scale to examine the effects of severe insect defoliation on whole-tree physiology. Defoliation decreased tree growth and increased the rate of top dieback in the stand. Defoliation led to transient declines in carbon allocation to starch in fine roots, trunk, and twigs in the year of heaviest defoliation. Root production and root mortality were unaffected by the heaviest defoliation, but nitrate and ammonium uptake were strongly depressed. N fertilization increased tree growth, but did not alter defoliation effects on starch accumulation or top dieback. Defoliation and fertilization treatments did not interact. In this system, defoliation effects on tree recovery of leaf nitrogen lost to herbivory were primarily driven by effects on nitrogen uptake, rather than effects on root production or mortality.

Climate change effects on native fauna of northeastern forests

We review the observed and potential effects of climate change on native fauna of forests in northeastern North America by focusing on mammals, birds, amphibians, and insects. Our assessment is placed in the context of recent regional-scale climate projections. Climate change, particularly in recent decades, has affected the distribution and abundance of numerous wildlife species. Warming temperatures, alterations to precipitation regimes, seasonality, and climatic extremes are projected to affect species directly or indirectly in each of the focal taxa. Greatest climate change will occur during winter, and the survival of winter-active species as well as the survival, distribution, and abundance of hibernating mammals, amphibians, resident birds, and diapausing insects may be altered. Even under low emissions scenarios, effects on native fauna may be profound, affecting iconic species, endangered species, and species that provide economically valuable services, such as pollination and regulation of insec...

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

1. 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.

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.

RESPONSES OF AN INSECT FOLIVORE AND ITS PARASITOIDS TO MULTIYEAR EXPERIMENTAL DEFOLIATION OF ASPEN

Foliage quality may decline in deciduous trees following defoliation, thus af- fecting the insect generation responsible for the herbivory (rapid induced resistance, RIR), or future generations (delayed induced resistance, DIR). During outbreaks, trees often suffer partial or complete defoliation for two or more successive years, yet most studies have examined induced resistance following only one season of defoliation, which may not reveal its full impact on herbivores. In a field experiment, 40 trees from each of two clones of trembling aspen, Populus tremuloides, were severely defoliated for one, three, and four years in succession by experimentally manipulating densities of an outbreak folivore, the forest tent caterpillar, Malacosoma disstria. Treatments were applied such that the individual and combined effects of RIR and DIR on the fitness of the forest tent caterpillar could be assessed independently. In field assays, defoliation treatments did not affect larval development time and only marginally affected survival. However, fecundity in both clones was significantly reduced by a single season of defoliation concurrent with the bioassay (effects of RIR), by three consecutive years of defoliation prior to the year of the bioassay (effects of DIR), and by four consecutive years of defoliation (combined effects of RIR and DIR). There were no differences among the three defoliation treatments, indicating that the effects of DIR and RIR combined were not greater than each acting alone. Reductions in fecundity were less than half those observed during natural outbreaks, suggesting that other factors also must contribute to declining fecundity during the collapse of outbreaks. Short-term laboratory bioassays indicated that defoliation effects observed in long-term field assays were not due to changes in relative growth rate (RGR) of second instars or final-instar males, which were unaffected, possibly because of increased relative con- sumption rates (RCR). Defoliation treatments decreased RGR of final-instar females in laboratory bioassays, despite elevated RCR. Both defoliation treatment and aspen clone influenced parasitism by tachinid flies that detect hosts through volatiles released from leaves damaged by caterpillars. Parasitism was highest on trees defoliated concurrently with the larval bioassay. However, there were no differences between trees defoliated prior to the bioassay and control trees, indicating that effects on parasitism were not due to defoliation-induced changes in host quality per se. Thus, there were no additive interactions between DIR and parasitism that would amplify delayed density-dependent effects on population dynamics. Spatial responses of these tach- inids to host density or to current defoliation rather than the defoliation history of the trees may enhance the stabilizing effect of RIR on population dynamics. Conversely, differences in parasitism among clones could contribute to spatial variation in tent caterpillar population density. Neither defoliation effect on host quality nor parasitism was sufficient to slow reproductive rates to levels observed in declining outbreaks in nature, suggesting that single- factor explanations for tent caterpillar population dynamics are unlikely.

IMPLICATING AN INTRODUCED GENERALIST PARASITOID IN THE INVASIVE BROWNTAIL MOTH'S ENIGMATIC DEMISE

Recent attention has focused on the harmful effects of introduced biological control agents on nontarget species. The parasitoid Compsilura concinnata is a notable example of such biological control gone wrong. Introduced in 1906 primarily for control of gypsy moth, Lymantria dispar, this tachinid fly now attacks more than 180 species of native Lepidoptera in North America. While it did not prevent outbreaks or spread of gypsy moth, we present reanalyzed historical data and experimental findings suggesting that parasitism by C. concinnata is the cause of the enigmatic near-extirpation of another of North Americas most successful invaders, the browntail moth (Euproctis chrysorrhoea). From a range of approximately 160,000 km2 a century ago, browntail moth (BTM) populations currently exist only in two spatially restricted coastal enclaves, where they have persisted for decades. We experimentally established BTM populations within this area and found that they were largely free of mortality caused by C. concinnata. Experimental populations of BTM at inland sites outside of the currently occupied coastal enclaves were decimated by C. concinnata, a result consistent with our reanalysis of historical data on C. concinnata parasitism of the browntail moth. The role of C. concinnata in the disappearance of browntail moth outside these enclaves has not been reported before. Despite the beneficial role played by C. concinnata in reversing the browntail moth invasion, we do not advocate introduction of generalist biological control agents. Our findings illustrate that the impact of such organisms can be both unpredictable and far-reaching.

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

Abstract The forest tent caterpillar, Malacosoma disstria Hübner (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.

Beyond Pandora's Box: quantitatively evaluating non-target effects of parasitoids in classical biological control

A seminal paper by Howarth (Proc Hawaii Entomol Soc 24:239–244, 1983) entitled “Classical biological control: Panacea or Pandora’s Box” ignited a sometimes acrimonious debate over the relative safety of introductions for classical biological control. Extolled for years as environmentally benign, the litany of negative non-target effects profiled by Howarth heightened awareness of this issue. Several factors have muddied this debate including the conflation of frequency of effects with their strength, grouping the effects of disparate biological control agents together, and the lack of quantitative data on either side of the argument. Here, I examine the potential for non-target effects among insect parasitoids, the most common group used for biological control of arthropods. In response to calls for better quantitative studies, I highlight three different techniques, quantitative food webs, life table analysis, and experimental populations, respectively, to quantitatively assess or reassess non-target effects in different systems. I also explore three methodological approaches employed to ascertain the strength of competitive interactions between native and introduced parasitoids, a potential non-target effect that has received little attention in the literature. These types of studies may greatly increase our understanding of the nature of non-target interactions with introduced parasitoids and bring more rigor to a debate often dominated by rhetoric.

Using hybrid and backcross larvae of Papilio canadensis and Papilio glaucus to detect induced phytochemical resistance in hybrid poplar trees experimentally defoliated by gypsy moths

Sub‐plots of hybrid poplars were experimentally defoliated using 10 million gypsy moth larvae. Half of the defoliated (and undefoliated control) plots were fertilized to see if this would ameliorate the predicted induction of carbon‐based phenolic defenses in the regrowth leaves. In order to bioassay the leaves of the four different treatments, we employed a continuum of genotypes (different hybrids and backcrosses of two different species of tiger swallowtail butterflies) with different abilities to detoxify these allelochemicals. Based on our previous studies with phytochemicals from the Salicaceae plant family, Papilio canadensis was likely to consume and process all Populus spp treatments, whereas P. glaucus predicted to either not consume or else quickly die on all Populus treatment leaves. Hybrid and backcross larvae of these two butterfly species are known to have intermediate levels of esterase detoxication enzymes and would therefore be likely to provide a continuum or at least varying degrees of sensitivity in bioassays for even the most subtle induction responses in the regrowth leaves. This presumption was supported in the feeding and growth studies conducted at different times post‐defoliation during the 1997 growing season in Michigan.

The Influence of Climate Change on Insect Invasions in Temperate Forest Ecosystems

Climate change could potentially become one of the most important influences on forest ecosystem function and diversity due to its profound effect on many biotic processes. Additionally, climate change could interact with other anthropogenically driven agents of forest alteration, such as non-native invasive species. Although their arrival is primarily facilitated by global trade and travel, climate and changes to climate have affected and will likely continue to affect rates of invasive species establishment, range expansion, and impact to native ecosystems. In this chapter, we attempt to synthesize broadly the interaction between climate change and non-native insect invasions in temperate forest ecosystems. We highlight four primary effects: changes in distributional ranges, outbreak frequency and intensity, seasonality and voltinism, and trophic interactions. A paucity of data for some processes necessitated the use of exemplar native species in native ranges, and their extrapolation to non-native species. Future studies should give greater attention to the complexity associated with these interacting forces of change in forest ecosystems.