Insect Herbivore Populations Research Articles

Subsidized predators, landscapes of fear and disarticulated carnivore communities

The subsidization of predators occurs when humans directly or indirectly alter resource availability in such a way as to increase the density of a predator population above levels that would occur without the additional resources. Subsidized predators can drastically impact prey populations because subsidies insulate the predator populations from the effects of declines in prey populations (Sinclair et al., 1998). Such scenarios become problematic for conservation practitioners when the enhanced predator population influences species of conservation concern. Examples of this are widespread: subsidized ravens influence tortoise ecology (Boarman, 2003); subsidized predatory beetles influence insect herbivore populations (Rand & Louda, 2006); subsidized mountain lions influence bighorn sheep demographics (Rominger et al., 2004); and in the example presented by Shapira, Sultan & Shanas (2008), subsidized red foxes Vulpes vulpes influence gerbil ecology. Shapira et al.’s fox-gerbil example is particularly interesting because the influence of foxes is best seen not in altered densities of gerbils per se, but in the behavior of these taxa. In areas near farms, the availability of crops and favorable habitat apparently subsidize foxes. As a result, in areas where foxes are putatively more abundant, gerbils are not only less abundant but are also less active, and show decreased foraging efficiency. This suggests that foxes in this agricultural region have altered gerbil biology not by direct predation alone, but also by creating a landscape of fear (Brown, Laundre & Gurung, 1999; Laundre, Hernandez & Altendorf, 2001), from which the gerbils have somehow rendered a microhabitat-level map of the perceived risk of predation. It is harder, however, to address the effect of actual predation on gerbils as Shapira et al. do not present information on the diet of foxes. Indeed, the differential activity rates and behaviors of gerbils at sites near and far from foxes are potentially confounded by several concerns. Foxes are not uniformly distributed but are found near farms. While this is, of course, a primary point of Shapira et al.’s paper, it nonetheless means that many of the factors making the farmed areas so different from the more natural surrounding areas are also presumably correlated with fox distributions and may also influence gerbil biology. In addition, which of the two gerbil species, or the ratio of the two species, that is being examined at each study site is not clear. This is important, because if the two gerbil species differ in their foraging and behavior, then observed differences in gerbil ecology and behavior that are attributed to foxes may in fact be a function of which gerbil species is visiting the seed tray. Finally, the foraging behavior of the predator itself may differ across study sites. Shapira et al. did not study the ecology of the foxes, but we know that predator behavior is not static. Just as prey alter their behavior to address perceived risks of predation, predator behavior also changes as a function of prey density and behavior (Lima, 2002; Quinn & Cresswell, 2004). Nonetheless, differential intensity of fox use of the landscape does appear to be the most parsimonious explanation for the observed patterns of gerbil ecology, as these patterns match our understanding of how subsidized predators and their prey should interact. In a world in which anthropogenic landscape change is a seemingly ubiquitous concern for conservationists, can we predict a prioriwhere and which predators are likely to become problematic in such situations, and can we make recommendations as we attempt to identify solutions? Although a full examination of these two questions is beyond the scope of this commentary, we know enough about the ecology of predators, of prey and of their intraand interguild interactions to make some generalizations that may carry predictive power and applied relevance. For instance, we know that larger carnivores may suppress populations of other guild members in the same way in which predators influence prey: through predation and by creating landscapes of fear (Linnell & Strand, 2000; Creel, Spong & Creel, 2001). At the Israeli–Jordanian study sites, the generalist and mid-sized red fox apparently outcompetes

Reproduction and dispersal of wing-clipped predatory stinkbugs, Podisus nigrispinus in cotton fields

Release of arthropod predators holds promise for suppressing herbivorous insect populations in commercial crops below economic threshold levels, but dispersal from release sites remains a practical issue that can limit their impact. This study examined mating behavior, survival, reproduction, and dispersion of membranous wing-clipped and wing-intact adults of the predatory stinkbug Podisus nigrispinus (Dallas) (Hemiptera: Pentatomidae), a generalist predator of cotton pests in South America. After laboratory studies demonstrated that wing-clipping did not alter mating behavior or reproductive output, field studies were conducted in experimental plots. In each experiment, 50 mated females (5–10 days old) were released at a central point. After 24, 48, 72, and 96 h, dispersion was measured via drop-cloth samples at distances up to 16 m from the release point. After 96 h, plants on each survey and release point were collected and, in the laboratory, they were inspected for egg masses. During the first 24 to 48 h after release, predators from both groups disappeared, but recovery of the wing-clipped predators was significantly greater (38.6% for wing-clipped and 17.3% for wing-intact predators) in all three releases. Importantly, oviposition rates on the release site were about three times greater for wing-clipped females (0.07 vs. 0.02 egg masses per plant), indicating that limiting flight induces females to stay and lay their eggs, hence, allowing local establishment of a new generation of predators.

HOST PLANT GENOTYPE INFLUENCES SURVIVAL OF HYBRIDS BETWEEN EUROSTA SOLIDAGINIS HOST RACES

Extrinsic, host-associated environmental factors may influence postmating isolation between herbivorous insect populations and represent a fundamentally ecological cause of speciation. We investigated this issue in experiments on hybrids between the host races of Eurosta solidaginis, a fly that induces galls on the goldenrods Solidago altissima and S. gigantea. To do so, we measured the performance of parental host races and their hybrids on five genotypes of S. gigantea and nine genotypes of S. altissima to test hypotheses about how variation in plant genotype affects performance (i.e., fitness) and potentially influences gene flow between these host races. We found that rates of gall induction and of survival to adult emergence by hybrid larvae were significantly lower than those of both parental host races on both host species, adding support to the hypothesis that there is partial postmating isolation between the host races. Hybrid flies significantly varied in their performance across plant genotypes of both host species. A significant interaction between the effects of plant genotype and mating treatment (parental vs. hybrid crosses) on larval performance indicated that the relative suitability of particular plant genotypes differed between the parental host races and their hybrids. These patterns illustrate a poor correspondence between optimal parental and hybrid environments, consistent with the hypothesis that these host races are partially isolated due to extrinsic (ecological) factors. Based on these findings, we discuss the possibility that plant genotypes in which hybrid performance is high can facilitate hybridization and gene flow between partially reproductively isolated populations of herbivorous insects, thus affecting the dynamics of ecological speciation.

Can intra‐specific genetic variation in arbuscular mycorrhizal fungi (Glomus etunicatum) affect a mesophyll‐feeding herbivore (Tupiocoris notatusDistant)?

Abstract1. Arbuscular mycorrhizal fungal (AMF) infection can have negative, positive or neutral effects on insect herbivore populations, but patterns are difficult to predict.2. Intra‐specific genetic variation in nutrient uptake ability between fungal isolates may also have indirect effects on insect herbivores due to changes in plant quality. In preliminary studies mirid (Tupiocoris notatus) populations were significantly reduced on tobacco (Nicotiana rustica) colonised by AMF but it was unknown if same‐species fungal isolates differed in their effect.3. An experiment was performed as a first test of the effect of intra‐specific genetic variation in the mycorrhizal fungusGlomus etunicatumon mirid nymphal population structure, dynamics, and growth rate.4. Mirid nymphal populations were lower on mycorrhizal fungal‐infected plants. Population size, however, did not differ between the mycorrhizal isolates. While no statistical difference in population between isolates was found, one isolate consistently had 1.7–2.4 times lower mirid populations compared with the controls, indicating that the magnitude of effect is different between mycorrhizal isolates.5. The significantly negative effect of AMF on mirid populations likely resulted from AMF‐induced changes in plant quality (e.g. increased defence). This study lends further support to recent demonstrations that below‐ground symbionts significantly influence above‐ground processes. In addition, mycorrhizal fungi can affect insect population structure, which may have consequences for future herbivory.

Interactions Across Spatial Scales among Forest Dieback, Fire, and Erosion in Northern New Mexico Landscapes

Ecosystem patterns and disturbance processes at one spatial scale often interact with processes at another scale, and the result of such cross-scale interactions can be nonlinear dynamics with thresholds. Examples of cross-scale pattern-process relationships and interactions among forest dieback, fire, and erosion are illustrated from northern New Mexico (USA) landscapes, where long-term studies have recently documented all of these disturbance processes. For example, environmental stress, operating on individual trees, can cause tree death that is amplified by insect mortality agents to propagate to patch and then landscape or even regional-scale forest dieback. Severe drought and unusual warmth in the southwestern USA since the late 1990s apparently exceeded species-specific physiological thresholds for multiple tree species, resulting in substantial vegetation mortality across millions of hectares of woodlands and forests in recent years. Predictions of forest dieback across spatial scales are constrained by uncertainties associated with: limited knowledge of species-specific physiological thresholds; individual and site-specific variation in these mortality thresholds; and positive feedback loops between rapidly-responding insect herbivore populations and their stressed plant hosts, sometimes resulting in nonlinear “pest” outbreak dynamics. Fire behavior also exhibits nonlinearities across spatial scales, illustrated by changes in historic fire regimes where patch-scale grazing disturbance led to regional-scale collapse of surface fire activity and subsequent recent increases in the scale of extreme fire events in New Mexico. Vegetation dieback interacts with fire activity by modifying fuel amounts and configurations at multiple spatial scales. Runoff and erosion processes are also subject to scale-dependent threshold behaviors, exemplified by ecohydrological work in semiarid New Mexico watersheds showing how declines in ground surface cover lead to non-linear increases in bare patch connectivity and thereby accelerated runoff and erosion at hillslope and watershed scales. Vegetation dieback, grazing, and fire can change land surface properties and cross-scale hydrologic connectivities, directly altering ecohydrological patterns of runoff and erosion. The interactions among disturbance processes across spatial scales can be key drivers in ecosystem dynamics, as illustrated by these studies of recent landscape changes in northern New Mexico. To better anticipate and mitigate accelerating human impacts to the planetary ecosystem at all spatial scales, improvements are needed in our conceptual and quantitative understanding of cross-scale interactions among disturbance processes.

Top‐down effects on population dynamics of Eriocrania miners (Lepidoptera) under pollution impact: does an enemy‐free space exist?

In areas disturbed by pollution, populations of herbivorous insects may reach high densities. This study was conducted to test one of the hypotheses attempting to explain this phenomenon – that pollution creates an enemy‐free space for herbivores. We monitored the population densities of Eriocrania leaf‐mining moths on mountain birch, Betula pubescens subsp. czerepanovii, in the vicinity of the nickel–copper smelter in Monchegorsk (Kola Peninsula, NW Russia) over twelve years (1994–2005) and assessed larval mortality from parasitoids, ants and birds. The mean density (mines/birch leaf area) of Eriocrania populations in severely disturbed habitats (industrial barrens) was about 2.7 times higher, and peak densities 2–4 times higher, than in pristine forests. Temporal population variability (measured as the coefficient of variation of log‐transformed densities) increased with an increase in pollution load. The proportion of infested trees was not affected by pollution, but mine distribution among trees was more clumped in the polluted sites. Eriocrania populations in disturbed sites fluctuated independently of each other, whereas populations in forest sites fluctuated in synchrony. Larval mortality caused by parasitoids was lower in disturbed sites only during those years when populations of Eriocrania reached high densities; mortality from ants and birds did not differ between disturbed and undisturbed habitats in either high or low density years. In undisturbed forests the rate of population change correlated negatively with previous‐year parasitism, suggesting that parasitoids are the key demographic factor in Eriocrania population dynamics. In the habitats heavily disturbed by pollution no such correlation was found, which means that negative feedback with parasitoids is disrupted: parasitoids fail to follow host population growth, thus creating an enemy‐free space for Eriocrania leafminers.

Do Top-Down Or Bottom-Up Forces Determine Stephanitis Pyrioides Abundance In Urban Landscapes?

This study examined the influence of habitat structural complexity on the collective effects of top-down and bottom-up forces on herbivore abundance in urban landscapes. The persistence and varying complexity of urban landscapes set them apart from ephemeral agroecosystems and natural habitats where the majority of studies have been conducted. Using surveys and manipulative experiments. We explicitly tested the effect of natural enemies (enemies hypothesis), host plant quality, and herbivore movement on the abundance of the specialist insect herbivore, Stephanitis pyrioides, in landscapes of varying structural complexity. This herbivore was extremely abundant in simple landscapes and rare in complex ones. Natural enemies were the major force influencing abundance of S. pyrioides across habitat types. Generalist predators, particularly the spider Anyphaena celer, were more abundant in complex landscapes. Predator abundance was related to greater abundance of alternative prey in those landscapes. Stephanitis pyrioides survival was lower in complex habitats when exposed to endemic natural enemy populations. Laboratory feeding trials confirmed the more abundant predators consumed S. pyrioides. Host plant quality was not a strong force influencing patterns of S. pyrioides abundance. When predators were excluded, adult S. pyrioides survival was greater on azaleas grown in complex habitats, in opposition to the observed pattern of abundance. Similarly, complexity did not affect S. pyrioides immigration and emigration rates. The complexity of urban landscapes affects the strength of top-down forces on herbivorous insect populations by influencing alternative prey and generalist predator abundance. It is possible that habitats can be manipulated to promote the suppressive effects of generalist predators.

Climatic unpredictability and parasitism of caterpillars: Implications of global warming

Insect outbreaks are expected to increase in frequency and intensity with projected changes in global climate through direct effects of climate change on insect populations and through disruption of community interactions. Although there is much concern about mean changes in global climate, the impact of climatic variability itself on species interactions has been little explored. Here, we compare caterpillar-parasitoid interactions across a broad gradient of climatic variability and find that the combined data in 15 geographically dispersed databases show a decrease in levels of parasitism as climatic variability increases. The dominant contribution to this pattern by relatively specialized parasitoid wasps suggests that climatic variability impairs the ability of parasitoids to track host populations. Given the important role of parasitoids in regulating insect herbivore populations in natural and managed systems, we predict an increase in the frequency and intensity of herbivore outbreaks through a disruption of enemy-herbivore dynamics as climates become more variable.

Variation in plant quality and the population dynamics of herbivores: there is nothing average about aphids

In the attempt to use results from small-scale studies to make large-scale predictions, it is critical that we take into account the greater spatial heterogeneity encountered at larger spatial scales. An important component of this heterogeneity is variation in plant quality, which can have a profound influence on herbivore population dynamics. This influence is particularly relevant when we consider that the strength of density dependence can vary among host plants and that the strength of density dependence determines the difference between exponential and density- dependent growth. Here, we present some simple models and analyses designed to examine the impact of variable plant quality on the dynamics of insect herbivore populations, and specifically the consequences of variation in the strength of density dependence among host plants. We show that average values of herbivore population growth parameters, calculated from plants that vary in quality, do not predict overall population growth. Furthermore, we illustrate that the quality of a few individual plants within a larger plant population can dominate herbivore population growth. Our results demonstrate that ignoring spatial heterogeneity that exists in herbivore population growth on plants that differ in quality can lead to a misunderstanding of the mechanisms that underlie population dynamics.

Bell-miner-associated dieback at the tree crown scale: a multi-trophic process

Summary This paper examines some of the factors and processes which interact within eucalypt stands affected by bell-miner-associated dieback. A key symptom of this form of dieback is sustained foliar damage from herbivorous insects, in particular psyllids. Repeated cycles of defoliation/refoliation result in branch death and crown contraction. Weakened trees become more susceptible to secondary stressors such as wood borers, soil fungal pathogens and abnormal levels of soil moisture. Psyllid outbreaks can occur through a reduction in the efficacy of their natural enemy complex, and the provision of sufficient favourable foliage to sustain high insect populations. The young foliage preferred for feeding and oviposition by most eucalypt insect herbivores can occur when soil moisture and nutrients and canopy irradiation are non-limiting. Bell miner colonies require high insect densities in their food source and they reinforce this situation through their feeding behaviour and territorial defence. The study suggests the effect of bell miners on the populations of herbivorous insects, and hence the amount of insect-damaged foliage, is density dependent. Bell miners also require dense understorey, about 2–5 m high, for nesting sites, and the presence of surface water. Any management practice that reduces bell miner density will in turn reduce the density of insect herbivores and hence foliar damage in the eucalypt tree crowns. Then, if the trees have not become too debilitated, crown recovery is likely to occur.

Aphid response to vegetation diversity and insecticide applications

A field experiment was conducted to determine the effects of vegetation diversity and pesticide disturbance on insect herbivore populations in a broccoli agroecosystem. Varying concentrations of the selective insecticide imidacloprid were applied to patches of broccoli surrounded by bare ground or weedy vegetation during one growing season in western Washington (USA). Aphids responded to an interaction between vegetation diversity and pesticide concentration, and their response varied as a function of time after pesticide disturbance. These results suggest that simplistic, linear predictions of the effects of chemical disturbance and natural enemies on insect herbivore populations will not be forthcoming.

Temporal effects on jasmonate induction of anti-herbivore defense in Physalis angulata: seasonal and ontogenetic gradients

Patterns of defensive enzyme (polyphenol oxidase (PPO)) induction were examined in Physalis angulata L. (cutleaf ground cherry) across seasonal and ontogenetic time gradients, in response to foliar jasmonic acid (JA) application. These studies showed strongest JA induction of PPO activity in young plants and during the month of July (mid-summer). In older plants, JA treatment was correlated with a decrease in PPO activity, and no significant effects of JA treatment were observed beyond September. Developmental patterns of induction corresponded with published studies of defense induction in other species, indicating active defense of young tissues and potential shifts in defensive strategy as plants transition from growth to reproduction. Peak induction during summer corresponds with periods of high solar irradiance and herbivorous insect populations, both of which are stresses against which PPO has been demonstrated effective.

Effects of a simple plant morphological mutation on the arthropod community and the impacts of predators on a principal insect herbivore.

Plant features that enhance predator effectiveness can be considered extrinsic-resistance factors because they result in reduced insect herbivory. In this paper we test the hypothesis that reduced epicuticular wax (EW) in Pisum sativum L. is an extrinsic-resistance factor contributing to field resistance to Acyrthosiphon pisum (Harris). We monitored pea aphid populations in the field on reduced EW and normal EW near isolines of peas for two seasons and confirmed that aphid populations are lower on reduced EW peas than on normal EW peas. We also monitored predators within the canopies of the two pea lines to discover community level patterns in response to differences in EW. We found that while predator numbers were similar between the two lines, there were more syrphids on the normal EW peas, and a trend towards more coccinellids on reduced EW peas. We tested the impact of predators on pea aphids on the two EW lines by monitoring their population levels in cages that excluded predators, and in cages that allowed predators to enter. We found that pea aphid populations were similar on the two EW lines when predators were excluded. When predators were allowed access to the plants, pea aphid populations were reduced more on reduced EW peas than on normal EW peas. We also examined the intrinsic resistance to aphids in reduced EW peas with laboratory dual-choice tests comparing aphid response to reduced EW and normal EW peas, and found that walking, apterous aphids displayed no preference for one pea line over the other. Bioassays to measure growth and fecundity of the pea aphid on the two EW types in the greenhouse and in the field showed that intrinsic rate of increase, and other life table parameters, were not different for aphids on the two lines. Together these results support the hypothesis that reduced EW in peas is a predator-dependent extrinsic resistance factor. Genetically reducing EW bloom in peas and other waxy crop plants might improve the effectiveness of arthropod natural enemies of insect pests. More generally, the results show that a subtle change in plant morphology can substantially influence the impact of predators on insect herbivore populations. The benefit of extrinsic resistance to herbivory conferred by reduced EW may balance any benefits of a prominent EW bloom, thereby sustaining EW polymorphisms in some natural plant populations.

Effects of endemic densities of canopy herbivores on nutrient dynamics along a gradient in elevation in the southern Appalachians

In southern Appalachian forests, outbreaks of insect herbivores have been shown repeatedly to increase the availability of nutrients in soil and the export of nitrate in forest streams. The mechanisms underlying herbivore-induced changes in nutrient dynamics include inputs of insect frass (feces) and modification of precipitation as it passes through the forest canopy (throughfall). Here, we consider the effects of endemic (non-outbreak) populations of insect herbivores on soil processes in the southern Appalachians. We measured inputs of frass and throughfall at three elevations at the Coweeta Hydrologic Laboratory, North Carolina. We also measured soil nutrient availability and soil respiration. Inputs of total frass, frass nitrogen and frass carbon exhibited early- and late-season peaks, with those peaks occurring earlier at low elevation where leaf flush begins first. The C:N ratio of frass generally increased over time at all elevations, presumably reflecting seasonal declines in foliar nitrogen. Nitrate in throughfall generally increased over time, whereas throughfall phosphate declined and throughfall ammonium remained relatively constant. Relationships among frass deposition and throughfall nutrients varied with elevation. At low elevation, frass nitrogen was strongly correlated with throughfall nitrate, but this relationship was absent at mid and high elevation. The relationships between frass deposition and throughfall ammonium were inconsistent among elevations. The availabilities of nitrate and ammonium in soil were both related to frass deposition. For example, frass deposition in May explained about 62 % of the variance in soil nitrate availability. Soil respiration exhibited summer maxima at all elevations and was related primarily to soil temperature. There was also a weak positive relationship between the C:N ratio of frass and soil respiration. Overall, we suggest that endemic densities of canopy herbivores can influence forest soil processes, but that the relationships exhibit pronounced spatial and temporal variability.

Community structure, survival and mortality factors in arctic populations of Eupontania leaf gallers

We studied survival, mortality factors, and community structure of nine species of leaf-galling sawflies, Eupontania spp., living on ten willow species (Salix spp.) at six sites on the Russian arctic tundra. The sawfly species represented two different gall types: the viminalis-type, which forms pea-shaped galls on the underside of leaf blades, and the vesicator-type, which forms bean-shaped galls on both sides of the leaf blade. Gall communities in the northernmost site had only one parasitoid species, but up to six parasitoids were found at the southernmost site. Inquiline parasitoids were encountered only in the two southern sites. Survival of the larvae varied between 20.0 and 82.8% among galler species at different sites. Parasitoids were the most important mortality factor for the sawflies. They caused mortality of 7.8-65.4%, depending on galler species and site, and it was highest in the northernmost site. Plant-specific mortality varied from 1.7 to 28.4% by galler species and it tended to decrease towards the north. Mortality from parasitoids was greater in the vesicator-type gallers than in the viminalis-type gallers. The total mortality caused by parasitoids in the arctic communities does not appear to differ from that in the diverse southern communities of Eupontania in Middle Europe, Scandinavia and North America, despite the assemblage having only a few members in the Arctic. The largest difference between the southern and the northern communities was the lack of inquiline parasitoids in the north. Our data do not support the hypothesis that abiotic, rather than biotic, factors would be more important in determining the abundance of populations of herbivorous insects in the harsh arctic environment.

Indirect effect of aquatic insect emergence on a terrestrial insect population through by birds predation

AbstractA manipulative field experiment was performed to determine the effect of birds, subsidized by aquatic insect emergence, on the insect herbivores in a riparian deciduous forest. Insectivorous birds were observed more frequently in the riparian forest than in upland forest away from the stream, utilizing both herbivorous insects feeding on the riparian vegetation and aquatic insects emerging from the stream as their prey. Field experiments revealed that the insect herbivore population in the riparian forest was more depressed by bird predation than that in the upland forest. This suggests that allochthonous prey input to the in situ prey population was responsible for a modification in the interaction between birds and herbivorous insects, resulting in a heterogeneous food web structure in the forest.

Selection on herbivore life-history traits by the first and third trophic levels: the devil and the deep blue sea revisited.

The timing of life-history events in insects can have important consequences for both survival and reproduction. For insect herbivores with complex life histories, selection is predicted to favor those combinations of traits that increase the size at metamorphosis while minimizing the risk of mortality from natural enemies. Studies quantifying selection on life-history traits in natural insect herbivore populations, however, have been rare. The purpose of this study was to measure phenotypic selection imposed by elements of the first and third trophic levels on variation in two life-history traits, the timing of egg hatch and pupal mass, in a population of oak-feeding caterpillars, Psilocorsis quercicella (Lepidoptera: Oecophoridae). Larvae were collected from the field throughout each of two generations per year for three years and reared to determine the effects of the date of egg hatch on both the risk of attack from parasitoids and the pupal mass of the survivors. The direction and strength of phenotypic selection attributed to aspects of the first and third trophic levels, as well as their combined effects, on the date of egg hatch was measured for each of the six generations. Heritabilities of and genetic correlations between pupal mass and the date of adult emergence from diapause (the life-history trait expected to have the largest influence on the timing of egg hatch, and thus larval development) were estimated from laboratory matings. In four of the six generations examined, significant directional selection attributed to the first trophic level was detected, always favoring early-hatching cohorts predicted to experience higher leaf quality than late-hatching cohorts. Directional phenotypic selection by the third trophic level was detected in only one of three years, and in that year the direction of selection was in opposite directions during the two successive generations. The combined effect of selection by both trophic levels indicated that the third trophic level acted to either reduce or enhance the more predictable pattern of selection attributed to the first trophic level. In addition, I found evidence of truncation selection acting to increase the mean and decrease the variance of pupal mass during the pupa-adult transition in the laboratory. Pupal mass and diapause duration were found to vary significantly among full-sibling families; upper bounds for heritability estimates were 0.57 and 0.30, respectively. Furthermore, these two traits were found to be positively genetically correlated (families with larger pupae had longer diapause durations), resulting in a fitness trade-off, because larger pupae enjoy higher survival through metamorphosis and female fecundity but emerge later, when average leaf quality for offspring is generally poorer.

How foraging tactics determine host-plant use by a polyphagous caterpillar.

The use of multiple host-plant species by populations of insect herbivores can result from a variety of possible ecological and behavioral mechanisms. An understanding of the foraging mechanisms determining polyphagy in relation to local ecological conditions is therefore essential to understanding the evolutionary ecology of polyphagy. Here, we evaluate patterns of host-plant use by the polyphagous caterpillar Grammia geneura (Lepidoptera: Arctiidae) in relation to host-plant availability and foraging tactics of individuals. Field surveys of caterpillar feeding and plant abundance carried out across several sites, seasons, and years showed that: (1) G. geneura consistently preferred forbs to grasses and woody plants, (2) forb-feeding was opportunistic, supporting the idea that caterpillars sample locally available host-plants, and (3) there were consistent patterns of host-plant use that were not explained by host-plant availability (electivity). An independent set of 7-h observations of 11 caterpillars showed that electivity for a subset of caterpillar-host associations could be explained by variation in the probability of initiating feeding and the average duration of feeding bouts on different hosts but not by variation in the probability of encountering different hosts, thus providing a behavioral basis for the observed variation in host-plant use. The use of detailed foraging tactics by larvae to explain host-plant use at the population level is a novel contribution of this study.

Effects of elevated atmospheric carbon dioxide on insect–plant interactions

Effects of elevated atmospheric carbon dioxide on insect–plant interactions

Relative roles of top‐down and bottom‐up forces in terrestrial tritrophic plant–insect herbivore–natural enemy systems

Whether resources (bottom‐up forces), natural enemies (top‐down forces), or both, determine the abundance of insect herbivore populations in plant–insect herbivore–natural enemy systems remains a major issue in population ecology. Unlike recent surveys of the tritrophic literature we do not seek to quantify whether top‐down or bottom‐up forces predominate in any given set of experimental systems. Acknowledging the dearth of empirical synthesis we employ two contrasting literature surveys to determine whether the plant–insect herbivore–natural enemy literature is currently adequate to form a conceptual synthesis of the relative roles of top‐down and bottom‐up forces. The emergence of a synthesis of the relative roles of top‐down and bottom‐up forces in plant–insect herbivore–natural enemy systems appears to have been largely prevented by (1) the absence of appropriate empirical data; (2) failure to appreciate the merits of existing data; (3) a continued desire to emphasise either top‐down or bottom‐up forces to the exclusion of the other; and (4) confusion regarding which processes regulate and which influence the abundance of insect herbivores.