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Ieder voorjaar inventariseert Wageningen Marine Research in opdracht van het ministerie van Landbouw, Natuur en Voedselkwaliteit het kokkelbestand in de Waddenzee, Oosterschelde en Westerschelde. Op basis van de uitkomsten van de voorjaarssurvey wordt met behulp van groei- en sterftefactoren een schatting gemaakt van het kokkelbestand dat op 1 september aanwezig zal zijn (najaarsbestand). Op basis van dit najaarsbestand wordt door de vergunningverlener vastgesteld hoeveel kokkels opgevist mogen worden. In de berekening van het najaarsbestand wordt gewerkt met gemiddelde waarden voor groei en sterfte tijdens de zomermaanden. Aangenomen wordt dat in de periode van 1 mei tot 1 september 28% van de kokkels een natuurlijke dood sterft. In de zomer van 2018 en 2019 is echter grootschalige kokkelsterfte opgetreden in de Oosterschelde en Waddenzee. Vermoedelijk had dit te maken met het optreden van hoge temperaturen op de droogvallende platen waar kokkels zich ingraven en overtijen tijdens laagwater. In 2019 is dit project gestart met als doel het verbeteren van de schatting van het najaarsbestand op basis van de voorjaarsbemonstering. In 2020 is het doel aangepast naar het onderzoeken van zomersterfte onder kokkels in relatie tot de droogvalduur en gemeten temperaturen in het sediment. Om de relatie met temperatuur te kunnen onderzoeken, is het project gekoppeld aan een ander project (BO43 NAGW ZWD 2020, deelproject Hittestress) dat WMR uitvoert in opdracht van het Ministerie van Landbouw, Natuur en Voedselkwaliteit en waarbinnen onderzoek is gedaan aan het verloop van temperaturen in de bodem, in relatie tot droogvalduur, de luchttemperatuur, instraling door de zon en sedimenttype. In voorliggend rapport zijn de methoden die in 2019 en 2020 zijn toegepast voor het meten van sterfte in de Oosterschelde vastgelegd en de gemeten sterftecijfers gerapporteerd. Daarnaast is een literatuuronderzoek uitgevoerd naar wat bekend is over kokkelsterfte in relatie tot hoge temperaturen. In 2019 was de sterfte gemiddeld genomen voor de 1-jarige kokkels in de gehele Oosterschelde 5%, voor de 2-jarige 23% en voor de meerjarige 87%. In 2020 is kokkelsterfte over de zomer uitzonderlijk hoog gebleken. Met een sterfte van 95% onder de 1-jarige kokkels en 81% onder de 2-jarige kokkels, lag de sterfte onder 1- en 2-jarigen hoger dan voor meerjarige kokkels (sterfte van 68%) en hoger dan de gemiddelde zomersterfte in de Oosterschelde (28%, over een heel jaar is de gemiddelde sterfte ca. 60%). De locatie Roggenplaat vertoonde de laagste sterftecijfers, terwijl locaties Oesterdam en Dortsman beide hoge sterftecijfers vertoonden, ondanks hun geografisch contrasterende ligging, sedimentsamenstelling en daarmee samenhangende temperatuurverloop. Mogelijk heeft dit te maken met het aantal uur dat de temperatuur ’s nachts onder de 23°C kwam tijdens de hittegolf. Dit aantal uur ligt veel hoger op de Roggenplaat (202 uur) dan op de andere locaties (83 uur voor Dortsman en 134 uur voor Oesterdam). Met de huidige klimaatverandering zullen luchttemperaturen stijgen en worden extremere temperaturen en meer hittegolven verwacht. Als gevolg zal de watertemperatuur ook stijgen en mogelijk ook al eerder in de zomer zijn afkoelende werking verliezen. Daardoor zouden kokkels langer blootgesteld zijn aan voor hen suboptimale temperaturen, waarbij ze verminderd actief zijn en wat mogelijk hun fitness vermindert. Dit effect zal dan het grootst worden geacht in het oostelijke deel van de Oosterschelde, waar het water langer verblijft, eerder en hoger opwarmt en meer slibrijke sedimenten te vinden zijn. Met de huidige kennis is echter niet te voorspellen hoe de kokkelpopulatie in de Oosterschelde zal veranderen als gevolg van klimaatverandering.

Understanding the relationship between an insect's developmental rate and temperature is crucial to forecast insect phenology under climate change. In the winter moth Operophtera brumata timing of egg-hatching has severe fitness consequences on growth and reproduction as egg-hatching has to match bud burst of the host tree. In the winter moth, as in many insect species, egg development is strongly affected by ambient temperatures. Here we use laboratory experiments to show for the first time that the effect of temperature on developmental rate depends on the stage of development of the eggs. Building on this experimental finding, we present a novel physiological model to describe winter moth egg development in response to temperature. Our model, a modification of the existing Sharpe-Schoolfield biophysical model, incorporates the effect of developmental stage on developmental rate. Next we validate this model using a 13-year data-set from winter moth eggs kept under ambient conditions and compared this validation with a degree day model and with the Sharpe-Schoolfield model, which lacks the interaction between temperature and developmental stage on developmental rate. We show that accounting for the interaction between temperature and developmental stage improved the predictive power of the model and contributed to our understanding of annual variation in winter moth egg phenology. As climate change leads to unequal changes in temperatures throughout the year, a description of insect development in response to realistic patterns of temperature rather than an invariable degree-day approach will help us to better predict future responses of insect phenology, and thereby insect fitness, to climate change.This article is protected by copyright. All rights reserved.

Heritable microbial symbionts can have important effects on many aspects of their hosts' biology. Acquisition of a novel symbiont strain can provide fitness benefits to the host, with significant ecological and evolutionary consequences. We measured barriers to horizontal transmission by artificially transferring facultative symbionts from the grain aphid, Sitobion avenae, and five other aphid species into two clonal genotypes of S. avenae. We found the symbiont Hamiltonella defensa establishes infections more easily following a transfer from the same host species and that such infections are more stable. Infection success was also higher when the introduced symbiont strain was more closely related to the strain that was originally present in the host (but which had previously been removed). There were no differences among successfully established symbiont strains in their effect on aphid fecundity. Hamiltonella defensa did not confer protection against parasitoids in our S. avenae clones, although it often does in other aphid hosts. However, strains of the symbiont Regiella insecticola originating from two host species protected grain aphids against the pathogenic fungus Pandora neoaphidis. This study helps describe the extent to which facultative symbionts can act as a pool of adaptations that can be sampled by their eukaryote hosts.

Under changing climatic conditions, species need to adapt to their new environment. Genetic adaptation is crucial to prevent population extinction(1) but examples where climate change leads to genetic changes in wild populations have been few(2,3). The synchronization between the timing of egg hatching of a herbivorous insect, the winter moth (Operophtera brumata), and the seasonal bud burst of its food plant, oak (Quercus robur), has been disrupted by climate change(4) and a quantitative genetic model predicts that selection will delay the egg hatching date(5). Here we show, using both long-term observational data and experiments, that the egg hatching date has changed genetically, resulting in closer synchrony with oak bud burst. The observed rate of change matches the predicted rate of change of one day per year. Hence, altered selection pressures, caused by environmental change, result in a rapid adaptive response in insect phenology. These genetic changes in a key life-history trait in this herbivorous insect therefore seem to be fast enough to match the climate-change-induced advancement of their host phenology.

The importance of microbial facultative endosymbionts to insects is increasingly being recognized, but our understanding of how the fitness effects of infection are distributed across symbiont taxa is limited. In the pea aphid, some of the seven known species of facultative symbionts influence their host's resistance to natural enemies, including parasitoid wasps and a pathogenic fungus. Here we show that protection against this entomopathogen, Pandora neoaphidis, can be conferred by strains of four distantly related symbionts (in the genera Regiella, Rickettsia, Rickettsiella and Spiroplasma). They reduce mortality and also decrease fungal sporulation on dead aphids which may help protect nearby genetically identical insects. Pea aphids thus obtain protection from natural enemies through association with a wider range of microbial associates than has previously been thought. Providing resistance against natural enemies appears to be a particularly common way for facultative endosymbionts to increase in frequency within host populations.

Summary 1. Maternal effects may play an important role in shaping the life history of organisms. Using an insect herbivore, the winter moth (Operophtera brumata) feeding on oak (Quercus robur), we show that maternal effects can affect seasonal timing of egg hatching in an herbivore in an adaptive way. 2. Winter moth egg-hatching needs to coincide with oak bud opening, as only freshly emerged leaves are suitable as food for the caterpillars. However, there is spatial variation in the timing of bud opening among oaks to which the winter moth needs to adapt. 3. We show experimentally that the generation time between the mother’s and her offsprings’ hatching dates was shorter for mothers who hatched late relative to bud opening of the tree they had to feed on (and hence had to feed on older leaves) than for mothers’ who hatched on time. Maternal feeding conditions affected both the larval and the pupal development time of the mother as well as the egg development time of her offspring: at all three stages developmental time was shorter for the mistimed treatment. 4. We thus show that adaptation to spatial variation may be achieved via maternal effects. While this is a mechanism selected to adapt to spatial variation, it may now also play a role in adaptation to climate change induced shifts in host phenology, and allow insect herbivores to adapt to changes in the seasonal timing of their food availability without the need for genetic change.

Climate change has been shown to affect the phenology of many organisms, but interestingly these shifts are often unequal across trophic levels, causing a mismatch between the phenology of organisms and their food.We consider two alternative hypotheses: consumers are constrained to adjust sufficiently to the lower trophic level, or prey species react more strongly than their predators to reduce predation. We discuss both hypotheses with our analyses of changes in phenology across four trophic levels: tree budburst, peak biomass of herbivorous caterpillars, breeding phenology of four insectivorous bird species and an avian predator.In our long-term study, we show that between 1988 and 2005, budburst advanced (not significantly) with 0.17 d yr(-1), while between 1985 and 2005 both caterpillars (0.75 d year(-1)) and the hatching date of the passerine species (range for four species: 0.36-0.50 d year(-1)) have advanced, whereas raptor hatching dates showed no trend.The caterpillar peak date was closely correlated with budburst date, as were the passerine hatching dates with the peak caterpillar biomass date. In all these cases, however, the slopes were significantly less than unity, showing that the response of the consumers is weaker than that of their food. This was also true for the avian predator, for which hatching dates were not correlated with the peak availability of fledgling passerines. As a result, the match between food demand and availability deteriorated over time for both the passerines and the avian predators.These results could equally well be explained by consumers' insufficient responses as a consequence of constraints in adapting to climate change, or by them trying to escape predation from a higher trophic level, or both. Selection on phenology could thus be both from matches of phenology with higher and lower levels, and quantifying these can shed new light on why some organisms do adjust their phenology to climate change, while others do not.

For many leaf-feeding herbivores, synchrony in phenology with their host plant is crucial as development outside a narrow phenological time window has severe fitness consequences. In this review, we link mechanisms, adaptation, and population dynamics within a single conceptual framework, needed for a full understanding of the causes and consequences of this synchrony. The physiological mechanisms underlying herbivore and plant phenology are affected by environmental cues, such as photoperiod and temperature, although not necessarily in the same way. That these different mechanisms lead to synchrony, even if there is spatial and temporal variation in plant phenology, is a result of the strong natural selection acting on the mechanism underlying herbivore phenology. Synchrony has a major impact on the population densities of leaf-feeding Lepidoptera, and years with a high synchrony may lead to outbreaks. Global climate change leads to a disruption of the synchrony between herbivores and their host plants, which may have major impacts for population viability if natural selection is insufficient to restore synchrony.

Heritable microbial symbionts can have important effects on many aspects of their hosts' biology. Acquisition of a novel symbiont strain can provide fitness benefits to the host, with significant ecological and evolutionary consequences. We measured barriers to horizontal transmission by artificially transferring facultative symbionts from the grain aphid, Sitobion avenae, and five other aphid species into two clonal genotypes of S. avenae. We found the symbiont Hamiltonella defensa establishes infections more easily following a transfer from the same host species and that such infections are more stable. Infection success was also higher when the introduced symbiont strain was more closely related to the strain that was originally present in the host (but which had previously been removed). There were no differences among successfully established symbiont strains in their effect on aphid fecundity. Hamiltonella defensa did not confer protection against parasitoids in our S. avenae clones, although it often does in other aphid hosts. However, strains of the symbiont Regiella insecticola originating from two host species protected grain aphids against the pathogenic fungus Pandora neoaphidis. This study helps describe the extent to which facultative symbionts can act as a pool of adaptations that can be sampled by their eukaryote hosts.

Climate change has led to an advance in phenology in many species. Synchrony in phenology between different species within a food chain may be disrupted if an increase in temperature affects the phenology of the different species differently, as is the case in the winter moth egg hatch-oak bud burst system. Operophtera brumata (winter moth) egg hatch date has advanced more than Quercus robur (pedunculate oak) bud burst date over the past two decades. Disrupted synchrony will lead to selection, and a response in phenology to this selection may lead to species genetically adapting to their changing environment. However, a prerequisite for such genetic change is that there is sufficient genetic variation and severe enough fitness consequences. So far, examples of observed genetic change have been few. Using a half-sib design, we demonstrate here that O. brumata egg-hatching reaction norm is heritable, and that genetic variation exists. Fitness consequences of even a few days difference between egg hatch and tree bud opening are severe, as we experimentally determined. Estimates of genetic variation and of fitness were then combined with a climate scenario to predict the rate and the amount of change in the eggs' response to temperature. We predict a rapid response to selection, leading to a restoration of synchrony of egg hatch with Q. robur bud opening. This study shows that in this case there is a clear potential to adapt - rapidly - to environmental change. The current observed asynchrony is therefore not due to a lack of genetic variation and at present it is unclear what is constraining O. brumata to adapt. This kind of model may be particularly useful in gaining insight in the predicted amount and rate of change due to environmental changes, given a certain genetic variation and selection pressure.