Peter Dalin

Adult beetle grazing induces willow trichome defence against subsequent larval feeding

Abstract. Induced increases in trichome density to grazing by herbivores have been suggested to increase plant resistance to future herbivore attack. In this paper we present results which show that plants of Salix cinerea L. respond to adult leaf beetle (Phratora vulgatissima L.) grazing by developing new leaves with an increased trichome density. The same plants are usually attacked again later in the season when the next generation of larvae hatches on the plants. The effect of the induced response was studied by comparing larval growth and feeding on newly developed leaves of plants exposed to different defoliation treatments. Larvae on plants previously exposed to adult grazing consumed less total leaf area and showed more dispersed feeding than larvae on plants protected from previous grazing. Larvae on plants exposed to mechanical defoliation responded intermediately. These results corresponded to the increased trichome density of defoliated plants. However, we found this larval response only in whole plant tests – when reared on single, excised leaves in petri dishes, larvae in all treatments behaved similarly. This discrepancy between the on-plant experiment and that in petri dishes highlights how experimental design may alter the conclusion of a study. We suggest that the induced response to adult grazing may act as a defence against subsequent larval feeding.

Evolution of critical day length for diapause induction enables range expansion of Diorhabda carinulata, a biological control agent against tamarisk (Tamarix spp.)

In classical weed biological control, small collections of arthropods are made from one or a few sites in the native range of the target plant and are introduced to suppress the plant where it has become invasive, often across a wide geographic range. Ecological mismatches in the new range are likely, and success using the biocontrol agent may depend on postrelease evolution of beneficial life history traits. In this study, we measure the evolution of critical day length for diapause induction (day length at which 50% of the population enters dormancy), in a beetle (Diorhabda carinulata) introduced into North America from China to control an exotic shrub, Tamarix spp. Beetle populations were sampled from four sites in North America 7 years after introduction, and critical day length was shown to have declined, forming a cline over a latitudinal gradient At one field site, decreased critical day length was correlated with 16 additional days of reproductive activity, resulting in a closer match between beetle life history and the phenology of Tamarix. These findings indicate an enhanced efficacy and an increasingly wider range for D. carinulata in Tamarix control.

Generalist natural enemies of a willow leaf beetle (Phratora vulgatissima): Abundance and feeding habits

The natural enemies attacking eggs (and young larvae) of the willow leaf beetle Phratora vulgatissima were identified in the field. Three heteropterans were common natural enemies. The mirid Orthotylus marginalis was the most abundant and had an intermediate consumption rate in the lab, whereas the mirid Closterotomus fulvomaculatus was the least abundant but had the highest consumption rate. The anthocorid Anthocoris nemorum was intermediate in abundance but had the lowest consumption rate. However, the experimental situation (in petridish or on shoot) affected the ranking of the predators and illustrates behavioral differences. The anthocorid was very mobile and could be characterized as a “run and eat” predator, whereas the mirids were less mobile and behaved to a “find and stay” principle. Possible consequences of interspecific variation in behavior, from a biological control perspective, are discussed.

Reduced population control of an insect pest in managed willow monocultures.

Background There is a general belief that insect outbreak risk is higher in plant monocultures than in natural and more diverse habitats, although empirical studies investigating this relationship are lacking. In this study, using density data collected over seven years at 40 study sites, we compare the temporal population variability of the leaf beetle Phratora vulgatissima between willow plantations and natural willow habitats. Methodology/Principal Findings The study was conducted in 1999–2005. The density of adult P. vulgatissima was estimated in the spring every year by a knock-down sampling technique. We used two measures of population variability, CV and PV, to compare temporal variations in leaf beetle density between plantation and natural habitat. Relationships between density and variability were also analyzed to discern potential underlying processes behind stability in the two systems. The results showed that the leaf beetle P. vulgatissima had a greater temporal population variability and outbreak risk in willow plantations than in natural willow habitats. We hypothesize that the greater population stability observed in the natural habitat was due to two separate processes operating at different levels of beetle density. First, stable low population equilibrium can be achieved by the relatively high density of generalist predators observed in natural stands. Second, stable equilibrium can also be imposed at higher beetle density due to competition, which occurs through depletion of resources (plant foliage) in the natural habitat. In willow plantations, competition is reduced mainly because plants grow close enough for beetle larvae to move to another plant when foliage is consumed. Conclusion/Significance To our knowledge, this is the first empirical study confirming that insect pest outbreak risk is higher in monocultures. The study suggests that comparative studies of insect population dynamics in different habitats may improve our ability to predict insect pest outbreaks and could facilitate the development of sustainable pest control in managed systems.

Leaf beetle grazing does not induce willow trichome defence in the coppicing willow Salix viminalis

Abstract  1 Willows are frequently attacked and defoliated by adult leaf beetles (Phratora vulgatissima L.) early in the season and the plants are then attacked again when new larvae emerge. The native willow Salix cinerea has previously been shown to respond to adult grazing by producing new leaves with an increased trichome density. Subsequent larval feeding was reduced on new leaves. This type of induced plant response may reduce insect damage and could potentially be utilized for plant protection in agricultural systems.

Diapause induction and termination in a commonly univoltine leaf beetle (Phratora vulgatissima)

Abstract  The leaf beetle Phratora vulgatissima (Linnaeus 1758) is commonly univoltine in south‐central Sweden but may sometimes initiate a partial second generation. The current study was set out to investigate under what abiotic conditions the beetles initiate a second generation. Using climate chamber experiments, the beetles were shown to have a facultative reproductive diapause induced by declining day‐length. The critical day‐length (CDL) for diapause induction was estimated to be 18 h and 10 min. In the field, first‐generation beetles developing to adulthood before August in 2009 became reproductively active and produced a second generation, but most individuals emerged later and were in reproductive diapause. P. vulgatissima overwinter as adults and diapause was shown to be maintained until mid‐winter in 2008/2009. The cumulative temperature requirement for oviposition after diapause termination was estimated to be 222 day‐degrees with a 5.5°C temperature threshold. Three different day‐degree models that were developed to predict the phenology of female oviposition in the spring were validated by comparing model results with field data on the timing of oviposition in previous years. The study suggests that P. vulgatissima may initiate a second generation in Sweden if development of the first generation is completed before August. Warmer spring and summer temperatures due to ongoing climate change may cause advanced insect phenology and faster completion of insect life‐cycles at northern latitudes, which will affect the proportion of insects that initiate a second generation.

Predator foraging strategy influences prey population dynamics: arthropods predating a gregarious leaf beetle

We examined whether behavioural variation within an enemy complex attacking the willow leaf beetle, Phratora vulgatissima, influences the population dynamics of this gregarious prey. The most common enemies are three species of heteropteran arthropods: the two mirids Orthotylus marginalis and Closterotomus fulvomaculatus and the anthocorid Anthocoris nemorum. When attacking egg clusters on plants in the laboratory, the two mirids consumed a greater proportion of eggs within egg clusters than the anthocorid. The anthocorid visited and ate eggs from more egg clusters than both the mirids. The two foraging strategies have been characterized as ‘find and stay’ for the mirids and ‘run and eat’ for the anthocorid. By using a stochastic exponential growth model we showed that model prey experienced different temporal dynamics when exposed to predators that differ in the probabilities of finding prey aggregations and of consuming prey within aggregations. Model prey exposed to the find and stay type of predator was less likely to become established and to increase in abundance than model prey exposed to the run and eat type. In a field study, we found a correspondence between high abundance of find and stay mirids and low densities of leaf beetles. The results suggest that, even when average predation rate is constant, the foraging strategy of the predator can have population level consequences for the prey. The consumption of prey in dense patches seems to be important in the control of gregarious prey, especially in the early phase of prey population establishment.

Geographic variation in photoperiodic diapause induction and diapause intensity in Sericinus montelus (Lepidoptera: Papilionidae)

Abstract  Due to the risk of extinction and ornamental value of the swallowtail butterfly, Sericinus montelus Gray (Lepidoptera: Papilionidae) in China, knowledge about local adaptations is important for the conservation and economical utilization of the species. In the present study, photoperiodic diapause induction and diapause intensity of S. montelus populations from Jiamusi (46°37′N), Beijing (40°15′N), Zibo (36°48′N), Fangxian (32°36′N), Wuhan (30°33′N) and Huaihua (27°33′N) were characterized at 25°C. Logistic regression analysis revealed a significant population × hours of light interaction, confirming that photoperiodic responses varied among populations. The critical photoperiod was positively correlated with latitude and increased toward the north at a rate of about 1 h for each 6.67 degrees of latitude. Survival analyses indicated that survival time of diapausing pupae before adult eclosion differed significantly among populations at 25°C and 16 : 8 L : D h. The mean duration of pupal diapause was also positively correlated with latitude. Our study reveals geographic variation in the critical photoperiod for diapause induction and in diapause intensity of S. montelus. These results provide useful information for our general understanding about seasonal adaptation in insects and may also be used to predict how geographic populations respond to climate warming.

Native Insects Colonizing Introduced Tree Species—Patterns and Potential Risks

Biological invasions are today considered to be among the greatest threat to biodiversity on earth (Gurevitch & Padilla 2004). The many examples of introduced plant species that have spread and appear to out-compete native plants illustrate the severity of this problem (Crawley 1997, Vitousek et al. 1997, Richardson & Higgins 1998). One commonly accepted mechanism behind plant invasions is that the plants experience decreased regulation by herbivorous insects and other enemies in the new environment. This is often referred to as the enemy-release hypothesis (Keane & Crawley 2002) or the escape-from-enemy hypothesis (Wolfe 2002). However, not all introduced plant populations have spread and become invasive (Grotkopp et al. 2002, Louda & Rand 2003). For example, some plants might be rapidly colonized by herbivorous insects (Strong 1974, Strong et al. 1977, Auerbach & Simberloff 1988), which may in turn limit the spread of the species (Maron & Vila 2001, Louda & Rand 2003). The control of exotic plant species by the native biota is often referred to as the biotic resistance hypothesis (Elton 1958, Maron & Vila 2001) which, similar to the enemy-release hypothesis, assumes that herbivorous insects can be participants in the regulation of plant populations. This suggests that the ability and the rate at which native insects can shift to feed on introduced plants might determine the plants’ success in the new environment. Plants usually establish in new environments through natural dispersal in response to environmental change. However, some plants establish in new environments by the influence of humans, either by accident or by purpose. For example, trees are moved between continents and planted in new environments to be used as new sources of economic development and to reduce the harvest of native forests. Ecologists are concerned that these plant introductions may affect forest biodiversity and ecosystem processes in the recipient communities (see e.g. Vitousek et al. 1997). Among foresters, there are also concerns that introduced tree species may be more subject to insect pests and diseases than native tree species (Zobel et al. 1987). Several examples from the literature show that native insect pests can reach outbreak levels on introduced tree species (Zobel et al. 1987, Watt &

Methods to identify the prey of invertebrate predators in terrestrial field studies

Abstract Predation is an interaction during which an organism kills and feeds on another organism. Past and current interest in studying predation in terrestrial habitats has yielded a number of methods to assess invertebrate predation events in terrestrial ecosystems. We provide a decision tree to select appropriate methods for individual studies. For each method, we then present a short introduction, key examples for applications, advantages and disadvantages, and an outlook to future refinements. Video and, to a lesser extent, live observations are recommended in studies that address behavioral aspects of predator–prey interactions or focus on per capita predation rates. Cage studies are only appropriate for small predator species, but often suffer from a bias via cage effects. The use of prey baits or analyses of prey remains are cheaper than other methods and have the potential to provide per capita predation estimates. These advantages often come at the cost of low taxonomic specificity. Molecular methods provide reliable estimates at a fine level of taxonomic resolution and are free of observer bias for predator species of any size. However, the current PCR‐based methods lack the ability to estimate predation rates for individual predators and are more expensive than other methods. Molecular and stable isotope analyses are best suited to address systems that include a range of predator and prey species. Our review of methods strongly suggests that while in many cases individual methods are sufficient to study specific questions, combinations of methods hold a high potential to provide more holistic insights into predation events. This review presents an overview of methods to researchers that are new to the field or to particular aspects of predation ecology and provides recommendations toward the subset of suitable methods to identify the prey of invertebrate predators in terrestrial field research.