Biochemical ecology of the forest tent caterpillar: responses to dietary protein and phenolic glycosides

Summary Interactions between quaking aspen (Populus tremuloides) and the forest tent caterpillar (Malacosoma disstria) are likely to be influenced by leaf protein and phenolic glycoside levels, and insect detoxication activity. We investigated the direct and interactive effects of dietary protein and phenolic glycosides on larval performance and midgut enzyme activity of forest tent caterpillars. We conducted bioassays with six artificial diets, using both first and fourth stadium larvae. Four of the diets comprised a 2×2 factorial design-two levels of protein, each with and without phenolic glycosides. Additionally, we assayed high protein diets containing S,S,S-tributylphosphorotrithioate (DEF, an esterase inhibitor) and DEF plus phenolic glycosides. Enzyme solutions were prepared from midguts of sixth instars and assayed for -glucosidase, esterase and glutathione transferase activities. First instar mortality and development times were higher for larvae on diets low in protein or containing phenolic glycosides. Effects of phenolic glycosides were especially pronounced at low protein levels and when administered with DEF. Fourth instar development times were prolonged, and growth rates reduced, in response to consumption of low protein diets. Effects of phenolic glycosides on growth were less pronounced, although the effect for larvae on the low protein diet was nearly significant. Activity of each of the enzyme systems was reduced in larvae reared on low protein diets, and esterase activity was induced in larvae fed phenolic glycosides. Our results suggest that larval performance may be strongly affected by levels of protein and phenolic glycosides commonly occurring in aspen foliage, and that these factors may play a role in differential defoliation of aspen by forest tent caterpillars.     

Intraspecific variation in aspen phytochemistry: effects on performance of gypsy moths and forest tent caterpillars

Individual quaking aspen trees vary greatly in foliar chemistry and susceptibility to defoliation by gypsy moths and forest tent caterpillars. To relate performance of these insects to differences in foliar chemistry, we reared larvac from egg hatch to pupation on leaves from different aspen trees and analyzed leaf samples for water, nitrogen, total nonstructural carbohydrates, phenolic glycosides, and condensed tannins. Larval performance varied markedly among trees. Pupal weights of both species were strongly and inversely related to phenolic glycoside concentrations. In addition, gypsy moth performance was positively related to condensed tannin concentrations, whereas forest tent caterpillar pupal weights were positively associated with leaf nitrogen concentrations. A subsequent study with larvae fed aspen leaves supplemented with the phenolic glycoside tremulacin confirmed that the compound reduces larval performance. Larvae exhibited increased stadium durations and decreased relative growth rates and food conversion efficiencies as dietary levels of tremulacin increased. Differences in performance were more pronounced for gypsy moths than for forest tent caterpillars. These results suggest that intraspecific variation in defensive chemistry may strongly mediate interactions between aspen, gypsy moths and forest tent caterpillars in the Great Lakes region, and may account for differential defoliation of aspen by these two insect species.     

Growth strategies differentiate the spatial patterns of 11 dipterocarp species coexisting in a Malaysian tropical rain forest

We examined relationships between mortality rate, relative growth rate (RGR), and spatial patterns of three growth stages (small, medium, and large trees) for 11 dipterocarp species in the Pasoh 50-ha plot. Mortality rates for these species tended to be positively correlated with RGRs, although the correlation was significant only at the small-tree stage. Seven species with high growth and mortality rates exhibited peaks in spatial aggregation at small distances (<100 m) in small trees, but this aggregation disappeared in medium and large trees. In contrast, the other four species with low growth and mortality rates aggregated at large distances (>200 m) throughout the three growth stages in all but one species. Negative associations between different growth stages were observed only for the high-mortality species, suggesting density-dependent mortality. The high-mortality species showed habitat associations with topography, soil type, and the forest regeneration phase after gap formation, whereas the three low-mortality species only had associations with the forest regeneration phase. A randomization procedure revealed that these habitat associations explained little of their spatial aggregation. Our results suggest that the growth strategy has a large effect on the structuring of the spatial distribution of tree species through mortality processes.     

Polyphenol oxidase overexpression in transgenic<Emphasis Type="Italic"> Populus</Emphasis> enhances resistance to herbivory by forest tent caterpillar (<Emphasis Type="Italic">Malacosoma disstria</Emphasis>)

In order to functionally analyze the predicted defensive role of leaf polyphenol oxidase (PPO; EC 1.10.3.1) in Populus, transgenic hybrid aspen (Populus tremula × P. alba) plants overexpressing a hybrid poplar (Populus trichocarpa × P. deltoides) PtdPPO1 gene were constructed. Regenerated transgenic plants showed high PPO enzyme activity, PtdPPO1 mRNA levels and PPO protein accumulation. In leaf disk bioassays, forest tent caterpillar (Malacosoma disstria) larvae feeding on PPO-overexpressing transgenics experienced significantly higher mortality and reduced average weight gain compared to larvae feeding on control leaves. However, this effect was observed only when older egg masses were used and the resulting larvae showed reduced growth and vigor. In choice tests, no effect of PPO overexpression was detected. Although PPO in poplar leaves is latent and requires activation with detergents or trypsin for full enzymatic activity, in caterpillar frass the enzyme was extracted in the fully activated form. This activation correlated with partial proteolytic cleavage, suggesting that PPO latency and activation during digestion could be an adaptive and defense-related feature of poplar PPO.     

Foliar quality influences tree-herbivore-parasitoid interactions: effects of elevated CO<Subscript>2</Subscript>, O<Subscript>3</Subscript>, and plant genotype

This study examined the effects of carbon dioxide (CO₂)-, ozone (O₃)-, and genotype-mediated changes in quaking aspen (Populus tremuloides) chemistry on performance of the forest tent caterpillar (Malacosoma disstria) and its dipteran parasitoid (Compsilura concinnata) at the Aspen Free-Air CO₂ Enrichment (FACE) site. Parasitized and non-parasitized forest tent caterpillars were reared on two aspen genotypes under elevated levels of CO₂ and O₃, alone and in combination. Foliage was collected for determination of the chemical composition of leaves fed upon by forest tent caterpillars during the period of endoparasitoid larval development. Elevated CO₂ decreased nitrogen levels but had no effect on concentrations of carbon-based compounds. In contrast, elevated O₃ decreased nitrogen and phenolic glycoside levels, but increased concentrations of starch and condensed tannins. Foliar chemistry also differed between aspen genotypes. CO₂, O₃, genotype, and their interactions altered forest tent caterpillar performance, and differentially so between sexes. In general, enriched CO₂ had little effect on forest tent caterpillar performance under ambient O₃, but reduced performance (for insects on one aspen genotype) under elevated O₃. Conversely, elevated O₃ improved forest tent caterpillar performance under ambient, but not elevated, CO₂. Parasitoid larval survivorship decreased under elevated O₃, depending upon levels of CO₂ and aspen genotype. Additionally, larval performance and masses of mature female parasitoids differed between aspen genotypes. These results suggest that host-parasitoid interactions in forest systems may be altered by atmospheric conditions anticipated for the future, and that the degree of change may be influenced by plant genotype.     

Nuclear polyhedrosis virus as a biological control agent for Malacosoma americanum (Lepidoptera: Lasiocampidae)

In addition to damaging trees, the eastern tent caterpillar is implicated in early fetal loss and late‐term abortion in horses. In a field study, we evaluated the potential biological control of the caterpillar using eastern tent caterpillar nuclear polyhedrosis virus (ETNPV), a naturally occurring virus that is nearly species‐specific. Egg masses were hatched and second instar larvae were fed virus‐inoculated foliage to propagate the virus in vivo. Then, a viral pesticide was formulated at concentrations of 10⁴, 10⁶ and 10⁸ polyhedral inclusion bodies per ml. The pesticide was applied to foliage on which second, third and fourth instar caterpillars were feeding. When the majority of surviving larvae reached the sixth instar, colonies were collected and the surviving caterpillars counted. Mean numbers of surviving caterpillars per treatment were compared via 95% bootstrap confidence intervals. The data indicate second instar caterpillars were highly susceptible to the virus, but only at the highest concentration tested. Third instar caterpillars were also somewhat susceptible to high virus concentrations, while fourth instar caterpillars were fairly resistant. Our data provide the strongest evidence to date that ETNPV can be propagated, harvested and refined for formulation as a biological control agent for eastern tent caterpillar. Its use on this insect may be merited in circumstances where landowners and managers need to protect trees and horses.     

Fine root chemistry and decomposition in model communities of north-temperate tree species show little response to elevated atmospheric CO<Subscript>2</Subscript> and varying soil resource availability

Rising atmospheric [CO₂] has the potential to alter soil carbon (C) cycling by increasing the content of recalcitrant constituents in plant litter, thereby decreasing rates of decomposition. Because fine root turnover constitutes a large fraction of annual NPP, changes in fine root decomposition are especially important. These responses will likely be affected by soil resource availability and the life history characteristics of the dominant tree species. We evaluated the effects of elevated atmospheric [CO₂] and soil resource availability on the production and chemistry, mycorrhizal colonization, and decomposition of fine roots in an early- and late-successional tree species that are economically and ecologically important in north temperate forests. Open-top chambers were used to expose young trembling aspen (Populus tremuloides) and sugar maple (Acer saccharum) trees to ambient (36 Pa) and elevated (56 Pa) atmospheric CO₂. Soil resource availability was composed of two treatments that bracketed the range found in the Upper Lake States, USA. After 2.5 years of growth, sugar maple had greater fine root standing crop due to relatively greater allocation to fine roots (30% of total root biomass) relative to aspen (7% total root biomass). Relative to the low soil resources treatment, aspen fine root biomass increased 76% with increased soil resource availability, but only under elevated [CO₂]. Sugar maple fine root biomass increased 26% with increased soil resource availability (relative to the low soil resources treatment), and showed little response to elevated [CO₂]. Concentrations of N and soluble phenolics, and C/N ratio in roots were similar for the two species, but aspen had slightly higher lignin and lower condensed tannins contents compared to sugar maple. As predicted by source-sink models of carbon allocation, pooled constituents (C/N ratio, soluble phenolics) increased in response to increased relative carbon availability (elevated [CO₂]/low soil resource availability), however, biosynthetically distinct compounds (lignin, starch, condensed tannins) did not always respond as predicted. We found that mycorrhizal colonization of fine roots was not strongly affected by atmospheric [CO₂] or soil resource availability, as indicated by root ergosterol contents. Overall, absolute changes in root chemical composition in response to increases in C and soil resource availability were small and had no effect on soil fungal biomass or specific rates of fine root decomposition. We conclude that root contributions to soil carbon cycling will mainly be influenced by fine root production and turnover responses to rising atmospheric [CO₂], rather than changes in substrate chemistry.     

Use of pheromone mimic to cause the disintegration and collapse of colonies of tent caterpillars (Malacosoma spp.)

The disruption of the trail-based communication system of eastern tent caterpillars (ETC) ( Malacosoma americanum ) and forest tent caterpillars (FTC) ( M. disstria ) by spraying host trees with a trail pheromone mimic caused the disintegration and collapse of young colonies. A 1-ppm formulation of 5β-cholestan-3-one or a control formulation was sprayed on trees either before or after eclosion of the caterpillars in the spring. For pre-eclosion trials, an average of 92% of the FTC colonies on control trees, but only 17% of colonies on treated trees consisted of viable, cohesive aggregates when survival was assessed 5–23 days post-spray. The remaining colonies had either completely disappeared or were reduced to small, disintegrated fragments. For the ETC, 100% of colonies on control trees and an average of 12% of colonies on treated trees consisted of cohesive aggregates inhabiting tents when survival was assessed 3–41 days post-spray. The remaining colonies were completely destroyed, reduced to scattered individuals, or to small, disintegrated, shelter-less fragments. For post-eclosion trials with the FTC in which colonies were sprayed during their first or second larval stadium, an average of 63% of individuals in control groups but only 7% in treatment groups were still on the sprayed trees 5–18 days post-spray. For the ETC, colonies were largely unaffected when trees were not sprayed until after the caterpillars had eclosed and established tents. For all trials, the loss of individuals on treated trees was largely attributable to the inability of dispersed and isolated first or second instar caterpillars to maintain secure purchase, causing them to fall from trees and perish. To our knowledge, this is the first study to investigate the potential of manipulating populations of pest species by interfering with a trail-based, chemical communication system.     

Variability in thermal responses among Furia gastropachae isolates from different geographic origins

The effect of temperature ranging from 5-30 degrees C on in vitro vegetative growth and conidial germination of isolates of the entomophthoralean fungus Furia gastropachae was investigated. Eleven isolates were used for growth studies; two from Maryland, six from New York, and three from Ontario. A subset of four isolates, one each from Maryland and New York and two from Ontario, were used in conidial germination experiments. Growth and germination were significantly associated with temperature for all isolates, occurring throughout the range 5-30 degrees C, though both processes were inhibited to varying degrees at upper and lower extremes. Temperature optima for growth ranged from 20 to 27 degrees C, and for germination from 20 to 25 degrees C. Although significant variability was observed among isolates in growth at temperatures above 13 degrees C, temperature optima were not significantly different among isolates, and variability did not appear to relate to the geoclimatic origins of the isolates. In contrast, germination responses to temperature did appear to be related to geographic origin. Furia gastropachae isolates from New York and Maryland germinated more slowly at 10 degrees C than did Ontario isolates, although the percentage of conidia ultimately germinating at each temperature was the same for all isolates. The New York and Maryland isolates performed much better at 30 degrees C, with significantly greater overall germination and secondary conidial discharge, than the Ontario isolates. Compared with other isolates at 30 degrees C, Ontario isolates were the least active, often failing to successfully discharge any secondary conidia.     

Foraging behavior and growth of isolated larvae of a social caterpillar, Malacosoma americanum

In laboratory experiments, isolated eastern tent caterpillars, Malacosoma americanum (Fabricius) (Lepidoptera: Lasiocampidae), grew more slowly than their grouped siblings even though factors previously reported to give grouped caterpillars an advantage were eliminated from our experimental design. Analysis of time-lapse videorecordings of daily foraging bouts showed that, despite their slower growth, isolated individuals fed significantly more often than their grouped siblings. This finding is consistent with previous observations showing that the rate at which tent caterpillars assimilate food is largely independent of foraging frequency and suggests that increased metabolic costs associated with superfluous activity may cause isolated caterpillars to grow slower. More rapid growth of grouped caterpillars also appears attributable to the significantly longer periods of time solitary caterpillars spent inactive during episodes of molting. Our study shows that the distinctive temporal pattern of foraging characteristic of intact colonies of eastern tent caterpillars is a emergent property of the group.     

贡嘎山东坡亚高山针叶林的林窗动态研究

本文选择贡嗄山东坡亚高山针叶林的代表类型——麦吊杉群落和冷杉群落,从林窗的大小分布、林窗制造者（ＧＭ）的类型与特点、林窗更新的趋势与作用,以及林窗中优势种群的分布格局变化几方面,对亚高山针叶林的林窗动态进行了研究。结果表明：在麦吊杉和冷杉林中,林窗面积所占比例分别为３１．７％和３９．２％;但从林窗的平均大小来看,分别为１５８．５ｍ２和１２５．４４ｍ２,麦吊杉林的林窗要大于冷杉林。通过对４１个林窗的统计,共有ＧＭ９４个,平均每个林窗有２．３个。而林窗的形成主要是由于ＧＭ的根拔、枯朽或折倒作用。林窗的形成促进了优势种群的更新。各立木级分布格局的研究结果表明,林窗中麦吊杉和冷杉在发育过程中分布格局的动态变化是由集群分布过渡到随机分布。     

大青沟森林植物群落主要木本植物种群分布格局及动态的研究

本文运用方差／均值比率法及聚集强度的某些指数研究了大青沟森林植物群落主要木本植物种群的分布格局及其动态。主要木本植物种群的分布格局大多为集群型,只有部分群落中大果榆、色木槭及家榆种群为随机分布。格局动态的分析表明,水曲柳、黄菠萝、春榆、大果榆种群幼苗—幼树—小树—中树—大树分布格局由集群分布逐渐变为随机分布,而蒙古栎由于较强的萌生性,大树阶段仍为集群分布。分析表明影响种群分布格局的因子大致可归纳为两类：一是物种本身的生物学特性,二是由环境条件引起的种内种间竞争;方差／均值比率法是一种较好的分析种群分布格局的方法。     

Spatial variability in the nutrient composition of Populus tremuloides: clone-to-clone differences and implications for cervids

Summary Populus tremuloides, a clonal angiosperm, is a major food item for many temperate-region herbivores. Clonal variation in chemical composition has potentially important implications for vertebrate herbivores because a single clone may be areally large and hence, may comprise a large part of available forage. The objectives of this study were: (1) to examine spatial variability in nutrient concentrations and quality within and among 24 clones of trembling aspen, and (2) to evaluate these differences with respect to cervid nutritional requirements; notably Cervus elaphus and Odocoileus hemionus. Between-clone concentrations of all chemical components were significantly different. In terms of cervid nutrition, most clones provided adequate concentrations of Ca, Mg, Mn, and Zn. Crude protein was limiting in some clones and P in most. Additionally, P metabolism may be impaired by the high Ca:P ratios typical of some clones. Of the structural constituents, neutral detergent fiber showed the greatest intraclonal variability. Dry-matter digestibility varied sufficiently to conjecture that cervid energy balances may be affected. Overall, although clone-to-clone differences were not so substantial to conclude that selective browsing would be nutritionally cost-effective, it is proposed that cervids stand to benefit by fortuitously browsing on nutritionally superior clones owing to the multiplier effect.     

A spatial simulation model for vegetation dynamics

A fine-resolution, spatially explicit, stochastic model was developed to simulate the dynamics of species cover abundance and pattern in a single vegetation layer wherein neighbouring individuals are assumed to compete for growing space. Each species in the model is characterized by a small number of morphological and life-history parameters, which enter into equations that stand for a minimal set of vegetation processes. The model performed well in reproducing post-fire successional trends among the three codominant dwarf shrubs in a Dutch heathland community as recorded in an annually mapped permanent quadrat. Program inputs, outputs and an example of sensitivity analysis are illustrated. With suitable changes, the model could potentially describe any plant community in which the vertical structure is simple and community dynamics are determined by spatial interactions among neighbouring plants.Jacques de Smidt provided data, information and stimulus for this project. The model was developed during ICP's visit to Utrecht, arranged by Marinus Werger and financed by The Netherlands Science Research Council (ZWO). We also thank David Glenn-Lewin and Ernst Lippe for discussion and cooperation.