Similarly, it will be pertinent to explore the effects of heattolerant microbes like Acinetobacter on pollinator health and behaviour in the face of climate warming. Our results are a product of the microbe taxa we chose for our synthetic community and the plant species inoculated. However, our findings are likely generalisable to other bioregions, given the widespread geographic distributions of our focal microbes and the inclusion of both native and non-native plant species. Regardless, future research will benefit from incorporating additional plant and nectar traits and increasing replication within clades of plant taxa to clarify mechanisms influencing nectar microbe ecology.Drosophila suzukii Matsumura causes economic damage to susceptible small and stone fruit in North America, Asia, and Europe . Adult female flies oviposit in fruit and developing larvae render the high-value fresh fruit unmarketable and reduce processed fruit quality. Damage from D. suzukii in Western U.S.A. production areas may cause up to $500 million in annual losses assuming 30 % damage levels , and $207 million losses in Eastern U.S.A. production regions . Worldwide, the potential economic impacts due to D. suzukii damage are significant. In any integrated pest management system, it is important to use multiple strategies to manage key pests. For D. suzukii, some of these strategies include monitoring, fruit sampling, and direct control methods . For example, in Trento Province, Northern Italy, prior to the adoption of IPM, the potential losses to D. suzukii were about 13 % of the berry industry’s revenue, plastic planters bulk while after the implementation of an IPM strategy including mass trapping, field sanitation, and insecticide programs, the sum of losses and associated control costs decreased to about 7 % .
The ability to describe, forecast, and more effectively manage damaging pest populations can benefit producers, extension agents, and practitioners . Phenology models based on accumulation of heat units or degree days have become the standard method for determining when to treat crops for pests. These DD accumulation models can be used to predict important life history events based on pest development rates . With phenology models, a specific life stage of a pest, such as adults, can be targeted for management, maximizing efficacy of insecticides. DD phenology models tend to work best for pests with a high level of synchrony and few, non-overlapping generations . Previous data have shown that D. suzukii moves through generations rapidly, and has high reproductive levels and overlapping generations . This suggests that limited benefits are to be gained from a traditional DD phenology model. However, insect population models can also be helpful to predict impending damage of agriculturally and medically important insect pest populations . The major factors affecting population dynamics of D. suzukii include temperature, humidity , and the availability of essential food resources . Although DD phenology models may have limited application for a pest such as D. suzukii, accumulation of heat units can play an important role in predicting population dynamics. Temperature-dependent developmental, survival, and reproductive data are available for all life stages of D. suzukii . Recent D. suzukii modeling has used a combination of mean temperature and calendar-based matrices . The two published demographic models for D. suzukii include a discrete-time stage-specific Leslie matrix model, which did not estimate transition between different life stages for D. suzukii , and a physiologically based demographic model featuring distributed maturation time .
Asplen et al. used D. suzukii physiological data and included non-linear sub-models to capture temperature-dependent developmental rates and survivorship. Neither model takes into consideration winter survival, early-season reproductive potential, or host availability . However, attempts to model insect survival and fecundity using physiological time and matrices have been conducted successfully for other insects . Management strategies for D. suzukii include chemical , biological , and cultural controls. Additional control strategies may include genetic techniques such as RNAi biopesticides . Little information is available at the population level on the impact of insecticide sprays. Insecticides are typically targeted against specific life stages of D. suzukii and result in differential levels of mortality on the different life stages. Currently, calendar-based insecticide spray intervals are focused on preventing oviposition by D. suzukii , but their impacts on populations over a larger spatial scale are unknown. Organophosphate, pyrethroid, carbamate, spinosyn, and some diamide insecticides show efficacy against D. suzukii adults . Residual activity of currently available insecticides is between 5 and 10 days but can be shorter due to rainfall . There is increasing evidence that some insecticides that are active on adult D. suzukii, including spinosad family compounds, which, may also achieve control through mortality of egg and larval life stages . Biological control agents known to attack D. suzukii have been identified in areas of recent pest invasion . However, parasitoid success appears generally lower in these regions compared to levels observed in the indigenous range of the pest . In North America and Europe, specialist parasitoid species are absent.
Field studies indicate that natural populations of generalist species are not having a meaningful effect on populations of this pest; however, in the scope of an IPM program, a conservation biological control approach using these agents may contribute to an overall reduction in local D. suzukii populations . The complex of biological control agents for D. suzukii includes predators and pathogens ; however, parasitic hymenoptera have been the primary focus of current research. Numerous parasitoid species are known to attack frugivorous drosophilids and most attack larvae or pupae in decaying fruits on the ground . Recent studies in the U.S.A. and Europe found that most resident larval drosophila parasitoids were unable to develop on D. suzukii , but in Asia, several parasitoid species of Asobara, Ganaspis, and Leptopilina can attack and develop from larvae of D. suzukii . Collection trips to South Korea in 2013 and 2014 and China in 2013 yielded parasitoid species that readily attack D. suzukii larvae and pupae . Given the increasing availability of D. suzukii physiology data, the goal of this paper is to provide key insights into how physiological time can be utilized to integrate survival, development, and reproductive data from diverse environments. We demonstrate how physiological time is appropriate to describe population dynamics over the growing season. We also demonstrate how the physiological time concept breaks down during overwintering by examining how extreme temperatures cause mortality in non-acclimated D. suzukii at both high and low temperatures. D. suzukii enters reproductive diapause in November/ December in parts of the U.S.A. , and phenotypic changes among individuals in the population can affect winter survival . We focused on the latter portion of winter and spring to determine if DD accumulation could estimate female reproductive potential. Finally, we examined a cohort-level population model based on accumulation of DD utilizing daily high and low temperatures from different field sites to estimate DD for conditions within known thermal thresholds. These data were used to consider the impacts of current and possible future IPM with the cohort DD population model at the field level. Materials and methods The environmental factors described below illustrate the impacts of environmental conditions within and outside of known temperature thresholds of D. suzukii. Additionally, we describe the role of DD accumulation for estimating sexual maturity of reproductive flies collected during the late dormant period. During late winter and early spring, collection pot there is a transition from temperatures outside of thermal thresholds to conditions falling within thermal thresholds. The D. suzukii population model was used to demonstrate how management practices could affect populations on a relative scale.
We examined survival trends of D. suzukii populations under cold and warm temperature extremes outside the developmental and reproductive thresholds. Populations are expected to decrease substantially after exposure to extremes ; however, even after extended periods of unfavorable conditions and lack of suitable reproductive hosts, D. suzukii are known to respond to traps, indicating persistence of populations . We describe the impacts of such unfavorable conditions on population structure by plotting D. suzukii pupal and adult survival levels at extreme low and extreme high temperatures. Survival was fitted in this case with a Gompertz distribution over calendar days because no DD are accumulated at the extremes. Currently, we lack field data to illustrate the role of such environmental conditions on populations. We do not include these parameters in the model described below as this paper focuses on seasonal population fluctuation only.In this analysis, we examined how warming temperatures at the onset of the growing season affect female reproductive potential of field-collected D. suzukii. The goal was to determine if DD accumulation could be used to estimate reproductive potential of flies, and to determine whether laboratory-generated reproduction data are supported by field observations. Collections of females were conducted using established methods and the late dormant reproductive potential of D. suzukii females was classified by dissection of females under magnification to determine whether mature eggs were present and if they were in the ovaries or free in the abdomen . Collections from Seattle, Washington, U.S.A. were made from March 2011 to February 2012, and collections from Corvallis, Oregon, U.S.A. were made from April 2011 to June 2013 . Flies were collected using container traps baited with apple cider vinegar or yeast-sugar solution. Collections in Italy utilized container traps baited with 200 ml of the liquid bait Droskidrink , composed of 3 parts apple cider vinegar to 1 part red wine, with 4 g raw brown sugar dissolved into the mixture. In all sites, the total numbers of females dissected per location and date were used to calculate the percentage of females containing mature eggs. The percentage of females containing mature eggs was plotted over the midpoint for the time period in DD calculated from the daily high and low temperature using the single sine method. Temperature data originated from weather stations proximate to collection sites representing the regions where collections were made. In all regions, the relationship between accumulated DD and reproductive potential was determined with multiple regression .For the model runs, we used two of the temperature datasets originally from Wiman et al. . The first was from the 2013 growing season in Salem, Oregon, U.S.A. and the second from 2013 in Parlier, California, U.S.A. Mortality factors simulating management activities were applied to select life stages for the periods outlined below. Model runs started early in the season because the population structure during the beginning of the growing season was composed of mostly adults. This timing allows us to see how pesticides targeting adult or immature life stages perform in relative terms. For California, we assume that adults colonize blueberry fields to oviposit on ripening fruit on April 1. Whereas growers would likely apply insecticides more than one time per season, for simplicity, hypothetical insecticides were applied one time at the beginning of the season. Two insecticides with different effects on specific life stages were independently input into the model to compare population-level impacts. The two compounds represented active ingredients that control both adults and immature stages of D. suzukii at different levels . Insecticide A elicited an adult mortality factor of 95 % and an immature mortality factor of 5–100 %. Insecticide B caused 60 % adult mortality and 60–95 % mortality of immature stages. These mortality factors included a range of efficiency in order to simulate reduced residual activity over time. This technology has undergone major advances as a tool for pest management. Double-stranded RNA is administered to targeted insects by genetic modification of the crop, or synthesized in vitro and topically applied to host plants . Murphy et al. described a novel dsRNA delivery system in which researchers genetically engineered yeast to produce dsRNA that knocks down genes that are predicted to be critical for D. suzukii fitness. The yeast biopesticide, Insecticide C, was shown to decrease larval survivorship and to reduce adult locomotor activity and reproductive output. Using these findings, we applied realistic mortality levels as highlighted by Murphy et al. , assuming efficient delivery and persistence, in which D. suzukii egg production and egg viability was 63.2 % lower , and 22 % of the larvae were killed for a period of 7 days. The mortality factors for each class of toxicant were applied using weather data from Parlier, California, U.S.A. for 20–30 April 2013 using these treatment scenarios.