Modelling thermal reaction norms for development and viability in Drosophila suzukii under constant, fluctuating and field conditions.

Phenological models for insect pests often rely on knowledge of thermal reaction norms. These may differ in shape depending on developmental thermal conditions (e.g. constant vs. fluctuating) and other factors such as life-stages. Here, we conducted an extensive comparative study of the thermal reaction norms for development and viability in the invasive fly, Drosophila suzukii, under constant and fluctuating thermal regimes. Flies, were submitted to 15 different constant temperatures (CT) ranging from 8 to 35 °C. We compared responses under CT with patterns observed under 15 different fluctuating temperature (FT) regimes. We tested several equations for thermal performance curves and compared various models to obtain thermal limits and degree-day estimations. To validate the model's predictions, the phenology was monitored in two artificial field-like conditions and two natural conditions in outdoor cages during spring and winter. Thermal reaction norm for viability from egg to pupa was broader than that from egg to adult. FT conditions yielded a broader thermal breadth for viability than CT, with a performance extended towards the colder side, consistent with our field observations in winter. Models resulting from both CT and FT conditions made accurate predictions of degree-day as long as the temperature remained within the linear part of the developmental rate curve. Under cold artificial and natural winter conditions, a model based on FT data made more accurate predictions. Model based on CT failed to predict adult's emergence in winter. We also document the first record of development and adult emergence throughout winter in D. suzukii. Population dynamics models in D. suzukii are all based on summer phenotype and CT. Accounting for variations between seasonal phenotypes, stages, and thermal conditions (CT vs. FT) could improve the predictive power of the models.

When insect pests build their own thermal niche: The hot nest of the pine processionary moth.

Temperature strongly drives physiological and ecological processes in ectotherms. While many species rely on behavioural thermoregulation to avoid thermal extremes, others build structures (nests) that confer a shelter against climate variability and extremes. However, the microclimate inside nests remains unknown for most insects. We investigated the thermal environment inside the nest of a temperate winter-developing insect species, the pine processionary moth (PPM), Thaumetopoea pityocampa. Gregarious larvae collectively build a silken nest at the beginning of the cold season. We tested the hypothesis that it provides a warmer microenvironment to larvae. First, we monitored temperature inside different types of nests varying in the number of larvae inside. Overall, nest temperature was positively correlated to global radiation and air temperature. At noon, when global radiation was maximal, nest temperature exceeded air temperature by up to 11.2-16.5 °C depending on nest type. In addition, thermal gradients of amplitude from 6.85 to 15.5 °C were observed within nests, the upper part being the warmest. Second, we developed a biophysical model to predict temperature inside PPM nests based on heat transfer equations and to explain this important temperature excess. A simple model version accurately predicted experimental measurements, confirming that nest temperature is driven mainly by radiation load. Finally, the model showed that nest temperature increases at the same rate as air temperature change. We conclude that some pest insects already live in warm microclimates by building their own sheltering nest. This effect should be considered when studying the impact of climate change on phenology and distribution.

Inventory of Varroa destructor susceptibility to amitraz and tau-fluvalinate in France.

Varroa destructor is one of the greatest threats for the European honeybee, Apis mellifera. Acaricides are required to control mite infestation. Three conventional chemical acaricide substances are used in France: tau-fluvalinate, flumethrin and amitraz. Tau-fluvalinate was used for over 10 years before experiencing a loss of effectiveness. In 1995, bioassay trials showed the first mite resistance to tau-fluvalinate. In some countries, amitraz was widely used, also leading to resistance of V. destructor to amitraz. In France, some efficiency field tests showed a loss of treatment effectiveness with amitraz. We adapted the bioassay from Maggi and collaborators to determine mite susceptibility to tau-fluvalinate and amitraz in France in 2018 and 2019. The lethal concentration (LC) which kills 90% of susceptible mite strains (LC90) is 0.4 and 12 µg/mL for amitraz and tau-fluvalinate, respectively. These concentrations were chosen as the determining factors to evaluate mite susceptibility. Some mites, collected from different apiaries, present resistance to amitraz and tau-fluvalinate (71% of the mite samples show resistance to amitraz and 57% to tau-fluvalinate). As there are few active substances available in France, and if mite resistance to acaricides continues to increase, the effectiveness of the treatments will decrease and therefore more treatments per year will be necessary. To prevent this situation, a new strategy needs to be put in place to include mite resistance management. We suggest that a bioassay would be a good tool with which to advise the policymakers.

Bistability induced by generalist natural enemies can reverse pest invasions.

Analytical modeling of predator-prey systems has shown that specialist natural enemies can slow, stop and even reverse pest invasions, assuming that the prey population displays a strong Allee effect in its growth. We aimed to formalize the conditions in which spatial biological control can be achieved by generalists, through an analytical approach based on reaction-diffusion equations. Using comparison principles, we obtain sufficient conditions for control and for invasion, based on scalar bistable partial differential equations. The ability of generalist predators to control prey populations with logistic growth lies in the bistable dynamics of the coupled system, rather than in the bistability of prey-only dynamics as observed for specialist predators attacking prey populations displaying Allee effects. As a consequence, prey control is predicted to be possible when space is considered in additional situations other than those identified without considering space. The reverse situation is also possible. None of these considerations apply to spatial predator-prey systems with specialist natural enemies.

Modelling the impact of Apivar treatment on a Varroa mite population and the influence of resistance.

BACKGROUND: Varroa destructor is a parasite of honeybees. It causes biological damage leading to the colony collapse in the absence of treatment. In recent years, acaricide resistance has emerged in Varroa mites, leading to a decrease in treatment efficacy. We modelled the action of Apivar (amitraz) treatment, using three input parameters: treatment duration, treatment period, and daily mortality due to the treatment. The output parameters were cumulative mite mortality during treatment, the residual number of Varroa mites, and treatment efficacy, expressed as a percentage. RESULTS: The model was validated by monitoring efficacy in the field, in 36 treated hives. According to the model, treatment in the absence of brood is optimal. For a long period without egg laying during the winter, an initial infestation of 100 mites and a start date for treatment of August 7, a minimal treatment efficacy of 98.8% is required for stabilization of the mite population for year to year. More effective treatment is associated with lower cumulative numbers of dead Varroa mites over the entire treatment period. Thus, the total number of dead mites observed during the monitoring of field efficacy provides information about more than just the initial level of colony infestation. The proportion of resistant mites can be modelized by a decrease of daily mortality rate influencing treatment efficacy. Management of the initial Varroa mite infestation of the colony by the beekeeper can compensate for the decrease in treatment efficacy for resistance thresholds of up to 40% of resistant mites. CONCLUSION: Treatment efficacy depends on several parameters, including initial level of infestation, treatment period and the presence of acaricide resistance. Amitraz resistance may lead to treatment failure, even if the beekeeper is able to keep initial infestation rates low. © 2021 Society of Chemical Industry.

The Vulnerability of Tropical Ectotherms to Warming Is Modulated by the Microclimatic Heterogeneity.

Most tropical ectotherms live near their physiological limits for temperature. Substantial ecological effects of global change are predicted in the tropics despite the low amplitude of temperature change. These predictions assume that tropical ectotherms experience air temperature as measured by weather stations or predicted by global circulation models. The body temperature of ectotherms, however, can deviate from ambient air when the organism samples the mosaic of microclimates at fine scales. The thermal heterogeneity of tropical landscapes has been quantified only rarely in comparison to temperate habitats, limiting our ability to infer the vulnerability to warming of tropical ectotherms. Here, we used thermal imaging to quantify the heterogeneity in surface temperatures across spatial scales, from the micro- up to landscape scale, at the top of an Inselberg in French Guiana. We measured the thermal heterogeneity at the scale of Clusia nemorosa leaves, by categorizing leaves in full sun versus leaves in the shade to quantify the microclimatic variance available to phytophagous insects. Then, we measured the thermal heterogeneity at the scales of the single shrub and the landscape, for several sites differing in their orientation toward the sun to quantify the microclimatic heterogeneity available for larger ectotherms. All measurements were made three times per day over four consecutive days. There was a high level of thermal heterogeneity at all spatial scales. The thermal variance varied between scales, increasing from the within-leaf surface to the landscape scale. It also shifted across the day in different ways depending on the spatial scale. Then, using a set of published data, we compared the critical temperature (CTmax) of neo-tropical ectotherms and temperature distributions. The portion of space above the CTmax varied substantially depending on spatial scale and taxa. Insects were particularly at risk at the surface of leaves exposed to solar radiation but not on shaded leaves. By contrast, ants tolerated elevated surface temperatures and can survive almost anywhere in the habitat. We suggest that the fine scale mosaic of microclimates in the tropics modulates the vulnerability of ectotherms to warming. By moving just a few meters, or even a few centimeters, small tropical ectotherms can radically change their microclimatic temperature and escape overheating.

Impulsive spatial control of invading pests by generalist predators.

We model the conditions for pest eradication in a reaction-diffusion system made of a prey and a generalist predator through spatial impulsive control within a bounded domain. The motivating example is the control of the invasive horse chestnut leafminer moth through the yearly destruction of leaves in autumn, in which both the pest and its parasitoids overwinter. The model is made of two integro-partial differential equations, the integral portion describing the within-year immigration from the whole domain. The problem of pest eradication is strongly related to some appropriate eigenvalue problems. Basic properties of the principal eigenvalues of these problems are derived by using of Krein-Rutman's theorem and of comparison results for parabolic equations with non-local terms. Spatial control of the pest can be achieved, if one of these principal eigenvalues is large enough, at an exponential rate. This is true without and with parasitoids, the latter case being of course more rapid. We discuss the possible implementation of these results to the leafminer invasion problem and discuss complementary methods.