Comparing the effectiveness of different insecticide application orders for suppressing Drosophila suzukii Matsumura (Diptera: Drosophilidae) infestation: experimental and modeling approaches.

Drosophila suzukii Matsumura (Diptera: Drosophilidae) is a key pest of soft-skinned fruit such as blackberry and blueberry. Differing seasonal spray regimes are expected to have variable effects on D. suzukii populations. Semi-field cage trials were performed at three locations in the United States (Georgia, Oregon, and North Carolina) on blueberry and blackberry crops to evaluate this hypothesis. Insecticides with different efficacy rates (ZC - zeta-cypermethrin, SPI - spinetoram, CYAN - cyantraniliprole) were applied during field experiments conducted within large cages. Treatment schedules consisted of two insecticide applications which performed over three weeks. Seasonal treatment schedules were applied in the following order: ZC-CYAN and CYAN-ZC in rabbiteye and highbush blueberry with the addition of a ZC-SPI treatment applied in blackberry. In addition, a population model was applied to simulate the relative efficacy of the insecticide schedules in Oregon on D. suzukii population model based on previously published efficacy, biological, and weather parameters. Overall, all schedules resulted in reduced D. suzukii infestation compared to untreated control (UTC) treatments, with statistical differences in all three locations. The numerically lower infestation was found in some cases in ZC-CYAN schedule. Population modeling conducted exclusively for blueberry, and the simulations indicated no discernible differences between the two respective schedules (ZC-CYAN vs CYAN-ZC). The present study demonstrates that seasonal infestation of D. suzukii could be suppressed irrespective of application order. Additional research is required to assess the optimal timing and sequence of insecticide applications for controlling seasonal populations of D. suzukii in fruit crops. Such information could be invaluable for growers who are seeking to strategize their insecticide applications.

Temperature-Dependent Life Table Parameters of Brown Marmorated Stink Bug, Halyomorpha halys (Stål) (Hemiptera: Pentatomidae) in the United States.

Brown marmorated stink bug, Halyomorpha halys (Stål) (Hemiptera: Pentatomidae), is a generalist pest that causes serious injury to a variety of crops around the world. After the first detection in the USA, H. halys became a serious threat to growers resulting in significant crop damage. Understanding the effect of temperature on H. halys development will help to achieve successful control by predicting the phenological timing of the pest. Here, life table parameters (survival, development, reproduction, and daily mortality) of H. halys were evaluated for New Jersey and Oregon populations in the US. Parameters were determined from field-collected and laboratory-reared individuals. The results indicated that New Jersey populations had higher levels of egg-laying than Oregon populations and exhibited higher and earlier fecundity peaks. Survival levels were similar between populations. Linear and nonlinear fit were used to estimate the minimum (14.3 °C), optimal (27.8 °C), and maximum (35.9 °C) temperatures where development of H. halys can take place. An age-specific fecundity peak (Mx = 36.63) was recorded at 936 degree days for New Jersey populations, while maximum fecundity (Mx = 11.85) occurred at 1145 degree days in Oregon. No oviposition was recorded at the lowest (15 °C) or highest (35 °C) trialed temperatures. Developmental periods increased at temperatures above 30 °C, indicating that such higher temperatures are suboptimal for H. halys development. Altogether the most optimal temperatures for population increase (rm) ranged from 25 to 30 °C. Survival rates of H. halys at suboptimal low temperatures of 8 °C (i.e., 61%) is comparable to previous reports. The present paper provides additional data and context from a range of experimental conditions and populations. Such temperature-related H. halys life table parameters can be used to provide determine the risk to susceptible crops.

Open field trials of food-grade gum in California and Oregon as a behavioral control for Drosophila suzukii Matsumura (Diptera: Drosophilidae).

The invasion of Drosophila suzukii, spotted-wing drosophila, across Europe and the US has led to economic losses for berry and cherry growers, and increased insecticide applications to protect fruit from damage. Commercial production relies heavily on unsustainable use of conventional toxic insecticides. Non-toxic insecticide strategies are necessary to alleviate the disadvantages and non-target impacts of toxic conventional insecticides and improve Integrated Pest Management (IPM). A novel food-grade gum deployed on dispenser pads (GUM dispensers) was evaluated to mitigate D. suzukii crop damage in five commercial crops and nine locations. Trials were conducted at a rate of 124 dispensers per hectare in cherry, wine grape, blueberry, raspberry, and blackberry in California and Oregon, USA during 2019 and 2020. The majority of trials with the food-grade gum resulted in a reduction of D. suzukii egg laying in susceptible fruit. In some cases, such damage was reduced by up to 78%. Overall, results from our meta-analysis showed highly significant differences between GUM treatments and the untreated control. Modeling simulations suggest a synergistic reduction of D. suzukii damage when used in combination with Spinosad (Entrust SC) insecticide. These data illustrate commercial value of this tool as a sustainable alternative to manage D. suzukii populations within a systems approach.

Analysis of a mechanistic model of corals in association with multiple symbionts: within-host competition and recovery from bleaching.

Coral reefs are increasingly experiencing stressful conditions, such as high temperatures, that cause corals to undergo bleaching, a process where they lose their photosynthetic algal symbionts. Bleaching threatens both corals' survival and the health of the reef ecosystems they create. One possible mechanism for corals to resist bleaching is through association with stress-tolerant symbionts, which are resistant to bleaching but may be worse partners in mild conditions. Some corals have been found to associate with multiple symbiont species simultaneously, which potentially gives them access to the benefits of both stress-sensitive and -tolerant symbionts. However, within-host competition between symbionts may lead to competitive exclusion of one partner, and the consequences of associating with multiple partners simultaneously are not well understood. We modify a mechanistic model of coral-algal symbiosis to investigate the effect of environmental conditions on within-host competitive dynamics between stress-sensitive and -tolerant symbionts and the effect of access to a tolerant symbiont on the dynamics of recovery from bleaching. We found that the addition of a tolerant symbiont can increase host survival and recovery from bleaching in high-light conditions. Competitive exclusion of the tolerant symbiont occurred slowly at intermediate light levels. Interestingly, there were some cases of post-bleaching competitive exclusion after the tolerant symbiont had helped the host recover.

Timescale separation and models of symbiosis: state space reduction, multiple attractors and initialization.

Dynamic Energy Budget models relate whole organism processes such as growth, reproduction and mortality to suborganismal metabolic processes. Much of their potential derives from extensions of the formalism to describe the exchange of metabolic products between organisms or organs within a single organism, for example the mutualism between corals and their symbionts. Without model simplification, such models are at risk of becoming parameter-rich and hence impractical. One natural simplification is to assume that some metabolic processes act on 'fast' timescales relative to others. A common strategy for formulating such models is to assume that 'fast' processes equilibrate immediately, while 'slow' processes are described by ordinary differential equations. This strategy can bring a subtlety with it. What if there are multiple, interdependent fast processes that have multiple equilibria, so that additional information is needed to unambiguously specify the model dynamics? This situation can easily arise in contexts where an organism or community can persist in a 'healthy' or an 'unhealthy' state with abrupt transitions between states possible. To approach this issue, we offer the following: (a) a method to unambiguously complete implicitly defined models by adding hypothetical 'fast' state variables; (b) an approach for minimizing the number of additional state variables in such models, which can simplify the numerical analysis and give insights into the model dynamics; and (c) some implications of the new approach that are of practical importance for model dynamics, e.g. on the bistability of flux dynamics and the effect of different initialization choices on model outcomes. To demonstrate those principles, we use a simplified model for root-shoot dynamics of plants and a related model for the interactions between corals and endosymbiotic algae that describes coral bleaching and recovery.

Once a reservoir, always a reservoir? Seasonality affects the pathogen maintenance potential of amphibian hosts.

Host species that can independently maintain a pathogen in a host community and contribute to infection in other species are important targets for disease management. However, the potential of host species to maintain a pathogen is not fixed over time, and an important challenge is understanding how within- and across-season variability in host maintenance potential affects pathogen persistence over longer time scales relevant for disease management (e.g., years). Here, we sought to understand the causes and consequences of seasonal infection dynamics in leopard frogs (Rana sphenocephala and Rana pipiens) infected with the fungal pathogen Batrachochytrium dendrobatidis (Bd). We addressed three questions broadly applicable to seasonal host-parasite systems. First, to what degree are observed seasonal patterns in infection driven by temperature-dependent infection processes compared to seasonal host demographic processes? Second, how does seasonal variation in maintenance potential affect long-term pathogen persistence in multi-host communities? Third, does high deterministic maintenance potential relate to the long-term stochastic persistence of pathogens in host populations with seasonal infection dynamics? To answer these questions, we used field data collected over 3 years on >1400 amphibians across four geographic locations, laboratory and mesocosm experiments, and a novel mathematical model. We found that the mechanisms that drive seasonal prevalence were different from those driving seasonal infection intensity. Seasonal variation in Bd prevalence was driven primarily by changes in host contact rates associated with breeding migrations to and from aquatic habitat. In contrast, seasonal changes in infection intensity were driven by temperature-induced changes in Bd growth rate. Using our model, we found that the maintenance potential of leopard frogs varied significantly throughout the year and that seasonal troughs in infection prevalence made it unlikely that leopard frogs were responsible for long-term Bd persistence in these seasonal amphibian communities, highlighting the importance of alternative pathogen reservoirs for Bd persistence. Our results have broad implications for management in seasonal host-pathogen systems, showing that seasonal changes in host and pathogen vital rates, rather than the depletion of susceptible hosts, can lead to troughs in pathogen prevalence and stochastic pathogen extirpation.

Niche expansion via acquired metabolism facilitates competitive dominance in planktonic communities.

Acquired phototrophs, organisms that obtain their photosynthetic abilities by hosting endosymbionts or stealing plastids from their prey, are omnipresent in aquatic ecosystems. This acquisition of photosynthetic metabolism allows for niche expansion, and can therefore influence competition outcomes by alleviating competition for shared resources. Here, we test how acquired metabolism alters competitive outcomes by manipulating light availability to control the energetic contribution of photosynthesis to acquired phototrophs. Using freshwater protists that compete for bacterial prey, we demonstrate light-dependent competition outcomes of acquired phototrophs (Paramecium bursaria) and strict heterotrophs (Colpidium sp.) in laboratory model experiments. We then synthesize these findings using a series of mathematical models, and show that explicitly accounting for resource competition improves model fits. Both empirical and mathematical models predict that the acquired phototroph should increase in competitive dominance with increasing light availability. Our results highlight the importance of acquired metabolism to community dynamics, highlighting the need for more empirical and theoretical studies of this mechanism for niche expansion.

Fertilization by coral-dwelling fish promotes coral growth but can exacerbate bleaching response.

Many corals form close associations with a diverse assortment of coral-dwelling fishes and other fauna. As coral reefs around the world are increasingly threatened by mass bleaching events, it is important to understand how these biotic interactions influence corals' susceptibility to bleaching. We used dynamic energy budget modeling to explore how nitrogen excreted by coral-dwelling fish affects the physiological performance of host corals. In our model, fish presence influenced the functioning of the coral-Symbiodiniaceae symbiosis by altering nitrogen availability, and the magnitude and sign of these effects depended on environmental conditions. Although our model predicted that fish-derived nitrogen can promote coral growth, the relationship between fish presence and coral tolerance of photo-oxidative stress was non-linear. Fish excretions supported denser symbiont populations that provided protection from incident light through self-shading. However, these symbionts also used more of their photosynthetic products for their own growth, rather than sharing with the coral host, putting the coral holobiont at a higher risk of becoming carbon-limited and bleaching. The balance between the benefits of increased symbiont shading and costs of reduced carbon sharing depended on environmental conditions. Thus, while there were some scenarios under which fish presence increased corals' tolerance of light stress, fish could also exacerbate bleaching and slow or prevent subsequent recovery. We discuss how the contrast between the potentially harmful effects of fish predicted by our model and results of empirical studies may relate to key model assumptions that warrant further investigation. Overall, this study provides a foundation for future work on how coral-associated fauna influence the bioenergetics of their host corals, which in turn has implications for how these corals respond to bleaching-inducing stressors.

Comparative Insecticide Application Techniques (Micro-Sprinkler) Against Drosophila suzukii Matsumura (Diptera: Drosophilidae) in Highbush Blueberry.

Drosophila suzukii (Matsumura), spotted-wing drosophila, is a major pest in small fruit crops including highbush blueberry. Controlling D. suzukii is challenging and chemical control is the main method to manage D. suzukii populations. Growers have expressed interest in using micro-sprinklers as an alternative method to apply insecticides. The current study aimed to evaluate if insecticide applications using micro-sprinklers can be used as an alternative method to protect the fruit from D. suzukii egg-laying. Modeling was used as an additional tool to parameterize the relative insecticide efficacy on oviposition. Field measurements of different treatments were conducted over periods of eleven days on commercial-standard highbush blueberry. Cyantraniliprole and spinetoram were applied using both a micro-sprinkler and a backpack sprayer. Treatments of Chromobacterium subtsugae and zeta-cypermethrin were only applied using a backpack sprayer. Both cyantraniliprole and spinetoram treatments resulted in moderate suppression of D. suzukii egg-laying. No statistical significance was found between micro-sprinkler and backpack sprayer applications for these two insecticides. Zeta-cypermethrin treatments using a backpack sprayer resulted in the most significant suppression of D. suzukii egg-laying over eleven days, while C. subtsugae was less effective at preventing D. suzukii egg-laying. Modeling simulations estimate the impact of the control methods on D. suzukii populations dynamics and simulation outputs indicated that backpack sprayers reduced D. suzukii populations at more pronounced levels compared to micro-sprinkler applications. The present study indicates that there is an underlying value of micro-sprinkler systems as an alternative and rapid spray application technique to help suppress D. suzukii pest populations during high-pressure periods in highbush blueberry production.

A time-since-infection model for populations with two pathogens.

The pioneering work of Kermack and McKendrick (1927, 1932, 1933) is now most known for introducing the SIR model, which divides a population into discrete compartments for susceptible, infected and removed individuals. The SIR model is the archetype of widely used compartmental models for epidemics. It is sometimes forgotten, that Kermack and McKendrick introduced the SIR model as a special case of a more general framework. This general framework distinguishes individuals not only by whether they are susceptible, infected or removed, but additionally tracks the time passed since they got infected. Such time-since-infection models can mechanistically link within-host dynamics to the population level. This allows the models to account for more details of the disease dynamics, such as delays of infectiousness and symptoms during the onset of an infection. Details like this can be vital for interpreting epidemiological data. The time-since-infection framework was originally formulated for a host population with a single pathogen. However, the interactions of multiple pathogens within hosts and within a population can be crucial for understanding the severity and spread of diseases. Current models for multiple pathogens mostly rely on compartmental models. While such models are relatively easy to set up, they do not have the same mechanistic underpinning as time-since-infection models. To approach this gap of connecting within-host dynamics of multiple pathogens to the population level, we here extend the time-since-infection framework of Kermack and McKendrick for two pathogens. We derive formulas for the basic reproduction numbers in the system. Those numbers determine whether a pathogen can invade a population, potentially depending on whether the other pathogen is present or not. We then demonstrate use of the framework by setting up a simple within-host model that we connect to the population model. The example illustrates the context-specific information required for this type of model, and shows how the system can be simulated numerically. We verify that the formulas for the basic reproduction numbers correctly specify the invasibility conditions.

Integrating Infection Intensity into Within- and Between-Host Pathogen Dynamics: Implications for Invasion and Virulence Evolution.

AbstractInfection intensity can dictate disease outcomes but is typically ignored when modeling infection dynamics of microparasites (e.g., bacteria, virus, and fungi). However, for a number of pathogens of wildlife typically categorized as microparasites, accounting for infection intensity and within-host infection processes is critical for predicting population-level responses to pathogen invasion. Here, we develop a modeling framework we refer to as reduced-dimension host-parasite integral projection models (reduced IPMs) that we use to explore how within-host infection processes affect the dynamics of pathogen invasion and virulence evolution. We find that individual-level heterogeneity in pathogen load-a nearly ubiquitous characteristic of host-parasite interactions that is rarely considered in models of microparasites-generally reduces pathogen invasion probability and dampens virulence-transmission trade-offs in host-parasite systems. The latter effect likely contributes to widely predicted virulence-transmission trade-offs being difficult to observe empirically. Moreover, our analyses show that intensity-dependent host mortality does not always induce a virulence-transmission trade-off, and systems with steeper than linear relationships between pathogen intensity and host mortality rate are significantly more likely to exhibit these trade-offs. Overall, reduced IPMs provide a useful framework to expand our theoretical and data-driven understanding of how within-host processes affect population-level disease dynamics.

Drosophila suzukii (Diptera: Drosophilidae): A Decade of Research Towards a Sustainable Integrated Pest Management Program.

Drosophila suzukii (Matsumura) (Diptera: Drosophilidae) also known as spotted-wing drosophila (SWD), is a pest native to Southeast Asia. In the last few decades, the pest has expanded its range to affect all major European and American fruit production regions. SWD is a highly adaptive insect that is able to disperse, survive, and flourish under a range of environmental conditions. Infestation by SWD generates both direct and indirect economic impacts through yield losses, shorter shelf life of infested fruit, and increased production costs. Fresh markets, frozen berries, and fruit export programs have been impacted by the pest due to zero tolerance for fruit infestation. As SWD control programs rely heavily on insecticides, exceedance of maximum residue levels (MRLs) has also resulted in crop rejections. The economic impact of SWD has been particularly severe for organic operations, mainly due to the limited availability of effective insecticides. Integrated pest management (IPM) of SWD could significantly reduce chemical inputs but would require substantial changes to horticultural management practices. This review evaluates the most promising methods studied as part of an IPM strategy against SWD across the world. For each of the considered techniques, the effectiveness, impact, sustainability, and stage of development are discussed.

Canopy spray deposition and related mortality impacts of commonly used insecticides on Drosophila suzukii Matsumura (Diptera: Drosophilidae) populations in blueberry.

BACKGROUND: Insecticide applications in blueberry production systems play a crucial role in the control of Drosophila suzukii populations. Here, quantitative spray deposition patterns were obtained under replicated field experiments in blueberry during two field seasons with three sprayers, i.e. cannon, electrostatic, and air-blast. Seven insecticides were tested (at 6 hours using a Potter spray tower) to determine the mortality data for adult D. suzukii. Spray deposition and mortality data for adult D. suzukii were used to create model simulations for insect populations. Model simulations included field deposition rates of sprayers and insecticide mortality as factors. Simulations were applied in different combinations with five applications over a 6-week period. RESULTS: Relative deposition rates for the cannon sprayer were elevated in the upper zones of the canopy, whereas for the air-blast sprayer, deposition was greater in the bottom zones. Electrostatic spray deposition was relatively uniform within the six canopy zones. Clear trends in D. suzukii laboratory mortality were found with lowest to highest mortality recorded for phosmet, spinetoram, spinosad, malathion, cyantraniliprole, zeta-cypermethrin, and methomyl respectively. Maximum D. suzukii population impacts, as shown by model outputs, were observed with air-blast sprayers together with zeta-cypermethrin. CONCLUSION: The electrostatic sprayer had the least variable canopy deposition among the three types of spray equipment, and the air-blast sprayer had the highest overall deposition rates. This study provides new hypotheses that can be used for field verification with these spray technologies and insecticides as key factors. © 2019 Society of Chemical Industry.

Multiple coexistence equilibria in a two parasitoid-one host model.

Briggs et al. (1993) introduced a host-parasitoid model for the dynamics of a system with two parasitoids that attack different juvenile stages of a common host. Their main result was that coexistence of the parasitoids is only possible when there is sufficient variability in the maturation delays of the host juvenile stages. Here, we analyze the phenomenon of coexistence in that model more deeply. We show that with some distribution families for the maturation delays, the coexistence equilibrium is unique, while with other distributions multiple coexistence equilibria can be found. In particular, we find that stable coexistence does not necessarily require mutual invasibility.

Reversible phenotypic plasticity with continuous adaptation.

We introduce a novel model for continuous reversible phenotypic plasticity. The model includes a one-dimensional environmental gradient, and we describe performance of an organism as a function of the environmental state by a Gaussian tolerance curve. Organisms are assumed to adapt their tolerance curve after a change of the environmental state. We present a general framework for calculating the genotype fitness if such adaptations happen in a continuous manner and apply the model to a periodically changing environment. Significant differences of our model with previous models for plasticity are the continuity of adaptation, the presence of intermediate phenotypes, that the duration of transformations depends on their extent, fewer restrictions on the distribution of the environment, and a higher robustness with respect to assumptions about environmental fluctuations. Further, we show that continuous reversible plasticity is beneficial mainly when environmental changes occur slow enough so that fully developed phenotypes can be exhibited. Finally we discuss how the model framework can be generalized to a wide variety of biological scenarios from areas that include population dynamics, evolution of environmental tolerance and physiology.