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Quantifying tree defoliation by insects over large areas is a major challenge in forest management, but it is essential in ecosystem assessments of disturbance and resistance against herbivory. However, the trajectory from leaf-flush to insect defoliation to refoliation in broadleaf trees is highly variable. Its tracking requires high temporal- and spatial-resolution data, particularly in fragmented forests. In a unique replicated field experiment manipulating gypsy moth Lymantria dispar densities in mixed-oak forests, we examined the utility of publicly accessible satellite-borne radar (Sentinel-1) to track the fine-scale temporal trajectory of defoliation. The ratio of backscatter intensity between two polarizations from radar data of the growing season constituted a canopy development index (CDI) and a normalized CDI (NCDI), which were validated by optical (Sentinel-2) and terrestrial laser scanning (TLS) data as well by intensive caterpillar sampling from canopy fogging. The CDI and NCDI strongly correlated with optical and TLS data (Spearman's ρ = 0.79 and 0.84, respectively). The ΔNCDII\(_{Defoliation(A−C)}\) significantly explained caterpillar abundance (R\(^{2}\) = 0.52). The NCDI at critical timesteps and ΔNCDI related to defoliation and refoliation well discriminated between heavily and lightly defoliated forests. We demonstrate that the high spatial and temporal resolution and the cloud independence of Sentinel-1 radar potentially enable spatially unrestricted measurements of the highly dynamic canopy herbivory. This can help monitor insect pests, improve the prediction of outbreaks and facilitate the monitoring of forest disturbance, one of the high priority Essential Biodiversity Variables, in the near future.

The adaptive significance of adjusting behavioral activities to the right time of the day seems obvious. Laboratory studies implicated an important role of circadian clocks in behavioral timing and rhythmicity. Yet, recent studies on clock-mutant animals questioned this importance under more naturalistic settings, as various clock mutants showed nearly normal diel activity rhythms under seminatural zeitgeber conditions. We here report evidence that proper timing of eclosion, a vital behavior of the fruit fly Drosophila melanogaster, requires a functional molecular clock under quasi-natural conditions. In contrast to wild-type flies, period01 mutants with a defective molecular clock showed impaired rhythmicity and gating in a temperate environment even in the presence of a full complement of abiotic zeitgebers. Although period01 mutants still eclosed during a certain time window during the day, this time window was much broader and loosely defined, and rhythmicity was lower or lost as classified by various statistical measures. Moreover, peak eclosion time became more susceptible to variable day-to-day changes of light. In contrast, flies with impaired peptidergic interclock signaling ( Pdf01 and han5304 PDF receptor mutants) eclosed mostly rhythmically with normal gate sizes, similar to wild-type controls. Our results suggest that the presence of natural zeitgebers is not sufficient, and a functional molecular clock is required to induce stable temporal eclosion patterns in flies under temperate conditions with considerable day-to-day variation in light intensity and temperature. Temperate zeitgebers are, however, sufficient to functionally rescue a loss of PDF-mediated clock-internal and -output signaling

The adaptive significance of adjusting behavioural activities to the right time of the day is intuitive. Laboratory studies have implicated an important role of circadian clocks in behavioural timing and rhythmicity. Yet, recent studies on clock-mutant animals questioned this importance under more naturalistic settings, as various clock mutants showed nearly normal diel activity rhythms under semi-natural Zeitgeber conditions.We here report evidence that proper timing of eclosion, a vital behaviour of the fruit fly Drosophila melanogaster, requires a functional molecular clock even under quasi-natural conditions. In contrast to wildtype flies, period01 mutants with a defective molecular clock eclose mostly arrhythmically in a temperate environment even in the presence of a full complement of abiotic Zeitgebers. Moreover, period01 mutants eclose during a much larger portion of the day, and peak eclosion time becomes more susceptible to variable day-to-day changes of light and temperature. Under the same conditions, flies with impaired peptidergic inter-clock signalling (pdf01 and han5304 mutants) stayed largely rhythmic with normal gate sizes. Our results suggest that the presence of natural Zeitgebers can mitigate a loss of peptide-mediated phasing between central clock neuron groups, but cannot substitute for the lack of a functional molecular clock under natural temperate conditions.

The life cycle of many social insects living in seasonal environments is characterized by a period of (pure) investment into worker brood followed by a phase of pure allocation of resources to the production of sexuals. Researchers have invested some effort to find possible correlates (proximate triggers) of this transition but current empirical knowledge does not provide clear evidence. Utilizing an established theoretical model—modified to account for brood developmental time—we show that a strategy that switches to the production of sexuals at a particular (fixed) date may typically perform better than strategies linking the transition to the status of the colony in terms of attributes like worker or brood number, or the colony’s age. Our results provide rationale why the search for cues triggering the transition in investment has been inconclusive at best and suggests a number of experiments and observations that allow validating our arguments.

Background Many species are shifting their ranges in response to global climate change. Range expansions are known to have profound effects on the genetic composition of populations. The evolution of dispersal during range expansion increases invasion speed, provided that a species can adapt sufficiently fast to novel local conditions. Genetic diversity at the expanding range border is however depleted due to iterated founder effects. The surprising ability of colonizing species to adapt to novel conditions while being subjected to genetic bottlenecks is termed ‘the genetic paradox of invasive species’. Mutational processes have been argued to provide an explanation for this paradox. Mutation rates can evolve, under conditions that favor an increased rate of adaptation, by hitchhiking on beneficial mutations through induced linkage disequilibrium. Here we argue that spatial sorting, iterated founder events, and population structure benefit the build-up and maintenance of such linkage disequilibrium. We investigate if the evolution of mutation rates could play a role in explaining the ‘genetic paradox of invasive species’ for a sexually reproducing species colonizing a landscape of gradually changing conditions. Results We use an individual-based model to show the evolutionary increase of mutation rates in sexual populations during range expansion, in coevolution with the dispersal probability. The observed evolution of mutation rate is adaptive and clearly advances invasion speed both through its effect on the evolution of dispersal probability, and the evolution of local adaptation. This also occurs under a variable temperature gradient, and under the assumption of 90% lethal mutations. Conclusions In this study we show novel consequences of the particular genetic properties of populations under spatial disequilibrium, i.e. the coevolution of dispersal probability and mutation rate, even in a sexual species and under realistic spatial gradients, resulting in faster invasions. The evolution of mutation rates can therefore be added to the list of possible explanations for the ‘genetic paradox of invasive species’. We conclude that range expansions and the evolution of mutation rates are in a positive feedback loop, with possibly far-reaching ecological consequences concerning invasiveness and the adaptability of species to novel environmental conditions. Electronic supplementary material The online version of this article (doi:10.1186/s12862-017-0998-8) contains supplementary material, which is available to authorized users.

1. The link between multi-host use and host switching in host–parasite interactions is a continuing area of debate. Lycaenid butterflies in the genus Maculinea, for example, exploit societies of different Myrmica ant species across their ranges, but there is only rare evidence that they simultaneously utilise multiple hosts at a local site, even where alternative hosts are present. 2. We present a simple population-genetic model accounting for the proportion of two alternative hosts and the fitness of parasite genotypes on each host. In agreement with standard models, we conclude that simultaneous host use is possible whenever fitness of heterozygotes on alternative hosts is not too low. 3. We specifically focus on host-shifting dynamics when the frequency of hosts changes. We find that (i) host shifting may proceed so rapidly that multiple host use is unlikely to be observed, (ii) back and forth transition in host use can exhibit a hysteresis loop, (iii) the parasites' host use may not be proportional to local host frequencies and be restricted to the rarer host under some conditions, and (iv) that a substantial decline in parasite abundance may typically precede a shift in host use. 4. We conclude that focusing not just on possible equilibrium conditions but also considering the dynamics of host shifting in non-equilibrium situations may provide added insights into host–parasite systems.

For an increasing number of species dispersal decisions have been shown to depend on local population density. This implies that organisms perceive – with sufficient accuracy – information about population density. However, research on the fitness value of such information, the adequate use of imprecise information, and how information use affects overall dispersal has only just started. A critical issue in such research necessarily concerns the adequate rule linking an individual's population density estimate to its readiness to emigrate; a spectrum of such rules has been proposed in the past. We use three prevalent rules and one rule specifically derived for situations of incomplete information to demonstrate that the adequacy of each rule strongly depends on the accuracy of information about population density: Simple bang–bang (threshold) behavior performs best if information is highly inaccurate, whereas a saturating response function (dispersal propensity gradually increasing with population density) out-competes all other strategies when individuals can estimate density accurately. The decision rule as well as the precision of information significantly influence average emigration probabilities establishing in metapopulations. Our results show that use of inadequate rules in models of dispersal evolution may lead to false dispersal decisions. Based on this analysis we present a rule that allows approximating an adequate dispersal response at the population level even when individuals possess incomplete information about local density. We conclude that individuals should generally invest little into the acquisition of (accurate) information even if such acquisition is associated with only small costs.

Proper timing of activities is one of the principal challenges faced by most organisms. Organisms need to account for various aspects in decision making like avoiding inordinate risks, synchronizing with resource availability, or finding mates. We provide analytical and simulation models to investigate the influence of life expectancy, resource competition and unpredictable environmental conditions (environmental uncertainty) on the evolutionarily stable distribution of emergence times in organisms depending on seasonally available resources. We focus on the partitioning of total phenotypic variance in emergence times into 1) genetic variance in mean emergence times between lineages and 2) environmental trait variance that determines the intra-lineage variance in the timing of emergence. Both, life expectancy of organisms and intensity of competition severely influence the evolutionary response to environmental uncertainty. Our main findings can be summarized as follows: 1) in general diversifying bet hedging (environmental trait variance) is the adequate mechanism to reduce the risk arising from environmental uncertainty while conservative bet hedging, i.e. delaying emergence into ‘safe’ phases of the season is restricted to short lived organisms and to situations with vanishing competition. 2) Environmental trait variance increases with increasing environmental uncertainty whereas 3) significant genetic variance evolves only under severe resource competition; it is driven by selection for an ideal free distribution of emergence times. 4) The level of genetic variance evolving declines with increasing life expectancy of organisms. 5) With sufficiently short life expectancy evolutionary branching and coexistence of distinctly different emergence strategies occurs; the number of co-occurring strategies is determined by the level of environmental uncertainty. Our model provides cues for understanding how different ecological factors contribute and interact to shape the evolution of emergence strategies.

“Simultaneous progressive provisioners” feed their offspring gradually as they develop—and typically feed more than one offspring simultaneously (SIM) at a time. In contrast, “sequential mass provisioners” supply offspring one after another (SEQ). Utilizing individual-based simulations, Field (Nature 404:869–871, 2005) compared the lifetime reproductive success of these strategies in different scenarios. Accordingly, SEQ should evolve in the majority of cases—SIM only has an evolutionary benefit if offspring depend on their mothers’ protection until adulthood even past the provisioning period. However, this is only one potential explanation for the evolution of SIM. Here, we present an alternative mechanism for solitary individuals with overlapping generations. We propose an analytical model (comprising Field’s former approach) utilizing growth rate instead of lifetime reproductive success as a measure of fitness. Our model shows that multiplicative geometric effects on fitness would typically compensate for the demographic disadvantages of SIM (due to prolonged dependency) and consequently support the evolution of SIM over SEQ for a wide range of life history parameters. The optimal level of SIM (i.e., the optimal number of eggs laid simultaneously) is determined by offspring development time, survival rates, and foraging efficiency of the mother. Only extreme values of these demographic parameters would favor a transition to SEQ behavior. Our model provides a coherent explanation of selective favoring of SIM over SEQ that may also contribute to understanding why SIM is the dominant strategy among social insect species. Workers in social insects typically feed several offspring simultaneously while solitary species with parental care—apart from a few exceptions—provision brood cells one after another. The provisioning pattern might play a prominent role in the evolutionary pathway to higher social organization. Based on a novel theoretical approach, we show that geometric growth benefits increase selection pressure towards simultaneous progressive provisioning in species with generation overlap. Such geometric benefits may specifically emerge in seasonal eusocial species. This result alters former assessment of causal mechanisms and extends findings focusing on solitary insects. It adds a new and reasonable explanation for the dominance of simultaneous provisioning among social species.

Empirical studies of annual eusocial insects in agricultural landscapes report contrasting findings with regard to colony responses to mass-flowering of crops such as oilseed rape. In particular, total sexual production is often unaffected by such events, whereas worker number responds with a prominent increase. To resolve these conflicting observations, we model-using an established approach-the expected change in worker and sexual numbers in response to an increased worker productivity induced by mass-flowering events at different times of the season. We find that the predicted response pattern is mainly shaped by the degree to which individual worker productivity is reduced by an increasing number of workers in the colony. Different environmental conditions and colony characteristics result in different levels of interference of workers, for example, during foraging or nest construction. Reduction in individual productivity is low, when worker interference is negligible ("weak limitation") and high when an increasing number of workers substantially decreases per-capita efficiency ("strong limitation"). For weak limitation, any mass-flowering event that ends before the production of sexuals starts has a strong multiplicative impact on both worker and sexual numbers. The magnitude of the effect is quite independent of the precise timing of such an event. After the onset of sexual production, mass-flowering has a weaker effect, as the added resource supply is only linearly transferred into production of additional sexuals. For colonies under strong limitation, the predicted impact of mass-flowering events is generally weaker, especially on the production of sexuals, and the timing of mass-flowering events becomes more influential: Production of sexuals profits more from late than from early mass-flowering events. Consequently, early mass-flowering events are predicted to have a prominent effect on worker numbers but a negligible one on the output of sexuals. The model presented provides a mechanistic explanation of why increased worker abundances do not necessarily translate into increased production of sexuals. The model is also applicable to other eusocial insects such as paper wasps whenever brief pulses of massive resource availability shortly elevate resource intake rates above the "normal" levels.

International audience; Caterpillars of the butterfly Maculinea rebeli develop as parasites inside ant colonies. In intensively studied French populations, about 25% of caterpillars mature within 1 year (fast‐developing larvae [FDL]) and the others after 2 years (slow‐developing larvae [SDL]); all available evidence indicates that this ratio is under the control of egg‐laying females. We present an analytical model to predict the evolutionarily stable fraction of FDL (pESS). The model accounts for added winter mortality of SDL, general and kin competition among caterpillars, a competitive advantage of SDL over newly entering FDL (priority effect), and the avoidance of renewed infection of ant nests by butterflies in the coming season (segregation). We come to the following conclusions: (1) all factors listed above can promote the evolution of delayed development; (2) kin competition and segregation stabilize pESS near 0.5; and (3) a priority effect is the only mechanism potentially selecting for . However, given the empirical data, pESS is predicted to fall closer to 0.5 than to the 0.25 that has been observed. In this particular system, bet hedging cannot explain why more than 50% of larvae postpone growth. Presumably, other fitness benefits for SDL, for example, higher fertility or longevity, also contribute to the evolution of delayed development. The model presented here may be of general applicability for systems where maturing individuals compete in small subgroups.

Explaining the evolution and maintenance of cooperative breeding or eusociality remains a challenge. Surprisingly, fundamental ecological factors, specifically competition for limited resources and resource variance, are frequently ignored in models of animal sociality. We here develop a mathematical model that includes density-dependent population growth and quantify the influence of cooperative foraging on resource use efficiency. We derive optimal resource sharing strategies, ranging from egalitarian to cooperatively breeding and eusocial groups. We find that, while egalitarian resource sharing is a risk-reducing foraging strategy, eusociality yields additional benefits: like egalitarian strategies, eusocial groups can reduce their members’ starvation risk by reducing resource variance. Additionally, eusocial groups increase their reproductive output by increasing intra-group variance in resources allocated to reproduction. This allows reproduction even when resources are so scarce that solitary animals would not be able to reproduce. In a majority of environmental situations and life-histories, this twofold benefit of eusociality increased resource use efficiency and led to supersaturation, that is, to a strong increase in carrying capacity. Supersaturation provides indirect benefits to group members even for low intra-group relatedness and represents one potential explanation for the evolution and maintenance of eusociality and cooperative breeding.

Risk-sensitive foraging theory is central to behavioural ecology. It relates individual fitness to variance in foraging success (risk) and predicts which foraging strategy maximizes fitness under applicable constraints. Fitness usually comprises survival and reproduction. Yet, most models of risk-sensitive foraging have focused on only one of these two factors. Consequently, such models cannot account for the interaction between resource availability and population size, i.e. density dependence. Here, we incorporate both mortality and reproduction as functions of an individual’s risk-sensitive foraging strategy. In our model the individual strategy thus determines the mean availability of resources per capita and consequently the equilibrium population size. From a continuum of possible strategies we are able to pinpoint the exact risk-sensitive strategies that are favoured by natural selection in saturated habitats and demonstrate that, in addition to risk proneness and risk aversion, a number of optimal intermediate variances can be selected for. In contrast to predictions based on models that ignore the interaction between behaviour, population density and resource availability, our results show that high baseline mortality (e.g. predation risk) does not necessarily lead to risk proneness. In addition to this novel finding, our model confirms the crucial importance of resource-independent (baseline) mortality for optimal risk-sensitive behaviour.

Foundress queens of social Hymenoptera require considerable amounts of energy for survival, solitary nest founding, provisioning of the first brood, and egg production. Energy reserves in insects mostly consist of fat. We investigated how hibernation and the subsequent flight season, especially the solitary nest founding phase, influenced the abdominal fat content of gynes in the primitively eusocial sweat bee, Lasioglossum malachurum (Hymenoptera, Halictidae). In our study population, sexuals are produced in both the second and the third broods. Emerging gynes of the third brood had significantly more fat than those of the second brood, whereas there was no such difference in males. As expected, fat reserves in samples of female sexuals caught at emergence, after hibernation, during solitary nest founding, and at the end of the social phase of the nest cycle indicate a severe decrease of reserves that was highest during the 7 weeks of the solitary founding phase. Thus, the amount of fat reserves of foundress queens seems to be crucial, particularly for nest founding. However, investment of energy reserves in the solitary nest founding phase has probably to be balanced with the subsequent social phase in a way that maximizes the queen’s fitness. Possible consequences for the complexity and progress of the nest cycle are discussed.

Solitary bees in seasonal environments must align their life-cycles with favorable environmental conditions and resources; the timing of their emergence is highly fitness relevant. In several bee species, overwintering temperature influences both emergence date and body weight at emergence. High variability in emergence dates among specimens overwintering at the same temperatures suggests that the timing of emergence also depends on individual body conditions. However, possible causes for this variability, such as individual differences in body size or weight, have been rarely studied. In a climate chamber experiment using two spring-emerging mason bees (Osmia cornutaandO. bicornis), we investigated the relationship between temperature, emergence date, body weight, and body size, the last of which is not affected by overwintering temperature. Our study showed that body weight declined during hibernation more strongly in warm than in cold overwintering temperatures. Although bees emerged earlier in warm than in cold overwintering temperatures, at the time of emergence, bees in warm overwintering temperatures had lower body weights than bees in cold overwintering temperatures (exception of maleO. cornuta). Among specimens that experienced the same overwintering temperatures, small and light bees emerged later than their larger and heavier conspecifics. Using a simple mechanistic model we demonstrated that spring-emerging solitary bees use a strategic approach and emerge at a date that is most promising for their individual fitness expectations. Our results suggest that warmer overwintering temperatures reduce bee fitness by causing a decrease in body weight at emergence. We showed furthermore that in order to adjust their emergence dates, bees use not only temperature but also their individual body condition as triggers. This may explain differing responses to climate warming within and among bee populations and may have consequences for bee-plant interactions as well as for the persistence of bee populations under climate change.

Vegetation structure can profoundly influence patterns of abundance, distribution, and reproduction of herbivorous insects and their susceptibility to natural enemies. The three main structural traits of herbaceous vegetation are density, height, and connectivity. This study determined the herbivore response to each of these three parameters by analysing oviposition patterns in the field and studying the underlying mechanisms in laboratory bioassays. The generalist leaf beetle, Galeruca tanaceti L. (Coleoptera: Chrysomelidae), preferentially deposits its egg clutches on non-host plants such as grasses. Earlier studies revealed that oviposition within structurally complex vegetation reduces the risk of egg parasitism. Consequently, leaf beetle females should prefer patches with dense, tall, or connected vegetation for oviposition in order to increase their reproductive success. In the present study, we tested the following three hypotheses on the effect of stem density, height, and connectivity on oviposition: (1) Within habitats, the number of egg clutches in areas with high stem densities is disproportionately higher than in low-density areas. The number of egg clutches on (2) tall stems or (3) in vegetation with high connectivity is higher than expected for a random distribution. In the field, stem density and height were positively correlated with egg clutch presence. Moreover, a disproportionately high presence of egg clutches was determined in patches with high stem densities. Stem height had a positive influence on oviposition, also in a laboratory two-choice bioassay, whereas stem density and connectivity did not affect oviposition preferences in the laboratory. Therefore, stem height and, potentially, density, but not connectivity, seem to trigger oviposition site selection of the herbivore. This study made evident that certain, but not all traits of the vegetation structure can impose a strong influence on oviposition patterns of herbivorous insects. The results were finally compared with data on the movement patterns of the specialised egg parasitoid of the herbivore in comparable types of vegetation structure.

Understanding decisions about the allocation of resources into colony growth and reproduction in social insects is one of the challenging issues in sociobiology. In their seminal paper, Macevicz and Oster predicted that, for most annual insect colonies, a bang–bang strategy should be favoured by selection, i.e. a strategy characterised by an “ergonomic phase” with exponential colony growth followed by a “reproductive phase” with all resources invested into the production of sexuals. Yet, there is empirical evidence for the simultaneous investment into the production of workers and sexuals in annual colonies (graded control). We, therefore, re-analyse and extend the original model of Macevicz and Oster. Using basic calculus, we can show that sufficiently strong negative correlation between colony size and worker efficiency or increasing mortality of workers with increasing colony size will favour the evolution of graded allocation strategies. By similar reasoning, graded control is predicted for other factors limiting colony productivity (for example, if queens’ egg laying capacity is limited).

The classical model of colony dynamics developed by Macevicz and Oster predicts that optimal colony fitness in annual eusocial insects is achieved by a bang-bang strategy of reproduction: exclusive production of workers (ergonomic phase) followed by exclusive production of sexuals (reproductive phase). We propose an alternative model that assumes colony development in discrete broods and a limited overall investment potential of the queen. Based on the costs for producing eggs, workers, and sexuals and efficiency of individuals we predict the optimal number of workers and sexuals in the colony for each brood of the colony cycle that maximizes overall colony fitness. To link our model assumptions to the real world we chose model parameters according to field data of the halictid bee Lasioglossum malachurum. However, our model is representative of a large number of species with an annual life cycle and with discrete broods. Our model shows that the optimal partitioning of resources, i.e. the optimal workers/sexuals ratio depends on rearing cost for sexuals as well as productivity of workers but not on the queens’ total investment, egg cost, or rearing cost for workers. In complete accordance to Macevicz and Oster we predict a bang-bang reproduction strategy despite the differences in the basic assumptions. Potential deviations from this strategy and transitions from social to solitary breeding are discussed in the framework of our model.

Mass-flowering crops may affect long-term population dynamics, but effects on pollinators have never been studied across several years. We monitored wild bees in oilseed rape fields in 16 landscapes in Germany in two consecutive years. Effects on bee densities of landscape oilseed rape cover in the years of monitoring and in the previous years were evaluated with landscape data from three consecutive years. We fit empirical data to a mechanistic model to provide estimates for oilseed rape attractiveness and its effect on bee productivity in comparison to the rest of the landscape, and we evaluated consequences for pollinator densities in consecutive years. Our results show that high oilseed rape cover in the previous year enhances current densities of wild bees (except for bumble bees). Moreover, we show a strong attractiveness of and dilution on (i.e., decreasing bee densities with increasing landscape oilseed rape cover) oilseed rape for bees during flowering in the current year, modifying the effect of the previous year's oilseed rape cover in the case of wild bees (excluding Bombus). As long as other factors such as nesting sites or natural enemies do not limit bee reproduction, our findings suggest long-term positive effects of mass-flowering crops on bee populations, at least for non-Bombus generalists, which possibly help to maintain crop pollination services even when crop area increases. Similar effects are conceivable for other organisms providing ecosystem services in annual crops and should be considered in future studies.

Existing models explaining the evolution of sexual dimorphism in the timing of emergence (SDT) in Lepidoptera assume equal mortality rates for males and females. The limiting assumption of equal mortality rates has the consequence that these models are only able to explain the evolution of emergence of males before females, i.e. protandry—the more common temporal sequence of emergence in Lepidoptera. The models fail, however, in providing adaptive explanations for the evolution of protogyny, where females emerge before males, but protogyny is not rare in insects. The assumption of equal mortality rates seems too restrictive for many insects, such as butterflies. To investigate the influence of unequal mortality rates on the evolution of SDT, we present a generalised version of a previously published model where we relax this assumption. We find that longer life-expectancy of females compared to males can indeed favour the evolution of protogyny as a fitness enhancing strategy. Moreover, the encounter rate between females and males and the sex-ratio are two important factors that also influence the evolution of optimal SDT. If considered independently for females and males the predicted strategies can be shown to be evolutionarily stable (ESS). Under the assumption of equal mortality rates the difference between the females’ and males’ ESS remains typically very small. However, female and male ESS may be quite dissimilar if mortality rates are different. This creates the potential for an ‘evolutionary conflict’ between females and males. Bagworm moths (Lepidoptera: Psychidae) provide an exemplary case where life-history attributes are such that protogyny should indeed be the optimal emergence strategy from the males’ and females’ perspectives: (i) Female longevity is considerably larger than that of males, (ii) encounter rates between females and males are presumably low, and (iii) females mate only once. Protogyny is indeed the general mating strategy found in the bagworm family.

The physiology and behavior of ectothermic organisms is strongly influenced by temperature. For ground nesting species like the primitively eusocial halictid bee, Lasioglossum malachurum, soil temperature might influence the life cycle as well as the complexity of the social group since the number of broods that can be fitted into the flight season might increase with increasing temperature. Our study populationof L. malachurum at Wuerzburg exhibits a remarkable variability with respect to the number of broods and the pattern of sexual production. Broods are separated by activity pauses during which the larvae develop. In this study we investigate the influence of soil temperature on the pattern of nesting activity (duration of broods and pauses) and on the number of broods in L. malachurum. We observed a total of 1138 nests in 13 aggregations near Wuerzburg. As expected, soil temperature shortened the duration of the pauses, resulting in an overall shortening of the nesting cycle. This is most probably due to a physiological effect of soil temperature on the development of the larvae. With regard to the nesting strategies, we hypothesized that a shortening of the nesting cycle within the limited flight season should enhance the success of a strategy with more worker broods. In fact, patches with higher soil temperature showed more broods. However, this effect was rather weak, suggesting that other factors might have a stronger impact on the variability in nesting strategy within our study population of L. malachurum.

Increasing interest is directed on understanding how individuals utilize information to come to dispersal decisions. We assume individuals base emigration decisions on male and female density in their natal patches. We derive gender-specific functions for emigration probability of species with discrete generations and polygynous mating under the premise that dispersal strategies equalize fitness expectations of emigrants and philopatric individuals: migration decisions should then always depend on a critical threshold density of the own gender. Whether density of the opposite sex affects emigration depends on details of resource competition: (1) Without competition, females should never emigrate, while males should emigrate in response to local sex ratio. (2) Under extreme competition among females or offspring, females and males should respond to the local density of their own gender only. (3) If both sexes compete over resources, emigration responds to the density of both sexes, but the dependence differs quantitatively between females and males. (4) Male-biased dispersal is the general expectation for polygynous species, but the model allows specifying conditions under which more females than males might nonetheless emigrate. The model provides guidelines for implementing density-dependent dispersal in simulations and specifies principal patterns that should emerge in empirical data.

Numerous invertebrates inhabit social insect colonies, including the hoverfly genus Microdon, whose larvae typically live as brood predators. Formica lemani ant colonies apparently endure Microdon mutabilis infections over several years, despite losing a considerable fraction of young, and may even produce more gynes. We present a model for resource allocation within polygynous ant colonies, which assumes that whether an ant larva switches development into a worker or a gyne depends on the quantity of food received randomly from workers. Accordingly, Microdon predation promotes gyne development by increasing resource availability for surviving broods. Several model predictions are supported by empirical data. (i) Uninfected colonies seldom produce gynes. (ii) Infected colonies experience a short-lived peak in gyne production leading to a bimodal distribution in gyne production. (iii) Low brood : worker ratio is the critical mechanism controlling gyne production. (iv) Brood : worker ratio reduction must be substantial for increased gyne production to become noticeable.

Background According to the classical model of Macevicz and Oster, annual eusocial insects should show a clear dichotomous "bang-bang" strategy of resource allocation; colony fitness is maximised when a period of pure colony growth (exclusive production of workers) is followed by a single reproductive period characterised by the exclusive production of sexuals. However, in several species graded investment strategies with a simultaneous production of workers and sexuals have been observed. Such deviations from the "bang-bang" strategy are usually interpreted as an adaptive (bet-hedging) response to environmental fluctuations such as variation in season length or food availability. To generate predictions about the optimal investment pattern of insect colonies in fluctuating environments, we slightly modified Macevicz and Oster's classical model of annual colony dynamics and used a dynamic programming approach nested into a recurrence procedure for the solution of the stochastic optimal control problem. Results 1) The optimal switching time between pure colony growth and the exclusive production of sexuals decreases with increasing environmental variance. 2) Yet, for reasonable levels of environmental fluctuations no deviation from the typical bang-bang strategy is predicted. 3) Model calculations for the halictid bee Lasioglossum malachurum reveal that bet-hedging is not likely to be the reason for the graded allocation into sexuals versus workers observed in this species. 4) When environmental variance reaches a critical level our model predicts an abrupt change from dichotomous behaviour to graded allocation strategies, but the transition between colony growth and production of sexuals is not necessarily monotonic. Both, the critical level of environmental variance as well as the characteristic pattern of resource allocation strongly depend on the type of function used to describe environmental fluctuations. Conclusion Up to now bet-hedging as an evolutionary response to variation in season length has been the main argument to explain field observations of graded resource allocation in annual eusocial insect species. However, our model shows that the effect of moderate fluctuations of environmental conditions does not select for deviation from the classical bang-bang strategy and that the evolution of graded allocation strategies can be triggered only by extreme fluctuations. Detailed quantitative observations on resource allocation in eusocial insects are needed to analyse the relevance of alternative explanations, e.g. logistic colony growth or reproductive conflict between queen and workers, for the evolution of graded allocation strategies.

1. Host plant finding in walking herbivorous beetles is still poorly understood. Analysis of small-scale movement patterns under semi-natural conditions can be a useful tool to detect behavioural responses towards host plant cues. 2. In this study, the small-scale movement behaviour of the monophagous leaf beetle Cassida canaliculata Laich. (Coleoptera: Chrysomelidae) was studied in a semi-natural arena (r = 1 m). In three different settings, a host (Salvia pratensis L., Lamiales: Lamiaceae), a non-host (Rumex conglomeratus Murr., Caryophyllales: Polygonaceae), or no plant was presented in the centre of the arena. 3. The beetles showed no differences in the absolute movement variables, straightness and mean walking speed, between the three settings. However, the relative movement variables, mean distance to the centre and mean angular deviation from walking straight to the centre, were significantly smaller when a host plant was offered. Likewise, the angular deviation from walking straight to the centre tended to decline with decreasing distance from the centre. Finally, significantly more beetles were found on the host than on the non-host at the end of all the trials. 4. It is concluded that C. canaliculata is able to recognise its host plant from a distance. Whether olfactory or visual cues (or a combination of both) are used to find the host plant remains to be elucidated by further studies.

BackgroundSocial insects show considerable variability not only in social organisation but also in the temporal pattern of nest cycles. In annual eusocial sweat bees, nest cycles typically consist of a sequence of distinct phases of activity (queen or workers collect food, construct, and provision brood cells) and inactivity (nest is closed). Since the flight season is limited to the time of the year with sufficiently high temperatures and resource availability, every break reduces the potential for foraging and, thus, the productivity of a colony. This apparent waste of time has not gained much attention.ResultsWe present a model that explains the evolution of activity breaks by assuming differential mortality during active and inactive phases and a limited rate of development of larvae, both reasonable assumptions. The model predicts a systematic temporal structure of breaks at certain times in the season which increase the fitness of a colony. The predicted pattern of these breaks is in excellent accordance with field data on the nest cycle of the halictidLasioglossum malachurum.ConclusionActivity breaks are a counter-intuitive outcome of varying mortality rates that maximise the reproductive output of primitively eusocial nests.

We present the results of individual‐based simulation experiments on the evolution of dispersal rates of organisms living in metapopulations. We find conflicting results regarding the relationship between local extinction rate and evolutionarily stable (ES) dispersal rate depending on which principal mechanism causes extinction: if extinction is caused by environmental catastrophes eradicating local populations, we observe a positive correlation between extinction and ES dispersal rate; if extinction is a consequence of stochastic local dynamics and environmental fluctuations, the correlation becomes ambiguous; and in cases where extinction is caused by dispersal mortality, a negative correlation between local extinction rate and ES dispersal rate emerges. We conclude that extinction rate, which both affects and is affected by dispersal rates, is not an ideal predictor for optimal dispersal rates.