Degree-day , or phenology models, are standard tools for integrated pest management in temperate regions and are used to predict the life stages of pests in order to time management activities and increase the effectiveness of control measures. Degree-day models work best for pests with a high level of synchronicity and few generations . Our data suggest that D. suzukii has short generation times, high reproductive levels, and high generational overlap compared to other dipteran fruit pests. Given this life history, stage-specific population models represent an alternative and potentially more applicable tool for modeling pest pressure. Pest population estimates may be greatly improved by employing additional tools such as mark recapture and analytical or individual-based models. The ability to describe and forecast damaging pest populations is highly advantageous for fruit producers, policy makers, and stakeholder groups. Many such studies have been directed at forecasting populations of medically important insect species. The major factors affecting survival, fecundity and population dynamics of drosophilids include temperature, humidity,dutch bucket for tomatoes and the availability of essential food resources. Therefore, an improved understanding of the role of temperature on D. suzukii may provide for a better understanding its seasonal population dynamics.
In this paper, we present a population model for D. suzukii that represents a novel modification of the classic Leslie projection matrix, which has proven to be one of the most useful age structured population models in ecology, with applications for diverse organisms including plants, animals, and diseases . Our modification accounts for the effect of temperature on the survival and fertility of D. suzukii in calculating population growth of the organism. Typically, researchers have introduced elements of environmental stochasticity to matrix models to study environmental effects on population trajectories. However, our approach relies on temperature-dependent estimations of age specific fecundities and survival that are determined by models fit to life table data generated for multiple temperatures. Our environmentally dependent matrix model is unique in that it does not rely on simulation of environmental effects on populations, but the matrix itself is recalculated at each iteration in direct response to environmental input. Model predictions were run under environmental conditions from different regions to illustrate variation between and within study sites in different years. These simulations make important predictions about age structure and population trends that have implications for pest management both in a broad sense and with regional specificity. This modeling tool may improve current management practices by predicting pest pressure independent of trap catches or samples of infested fruit. We also see potential applications of this model for research in other fields of study and for broadening the understanding of how pests interact with the environment.The population projection model was written in the open source statistical environment R version 3.0.2.
Briefly, the matrix calculations were based on age-specific regressions of temperature-dependent population parameters as highlighted by Tochen et al.. Whereas immature life stages of D. suzukii may experience different environmental conditions than adults because these life stages are completed within the fruit, in this study, ambient air temperatures were used to predict population dynamics for all life stages. To return age-specific population vectors for 50 age-classes of D. suzukii for each test case, a vector of mean daily temperatures for each site was input into the R statistical interface. The bio-fix, or the point where the model began in the spring, was determined using methods described in Tochen et al.. Biofix essentially described the earliest point in the season when the temperature allows the population to increase. Calculations for population estimates were initiated on the bio-fix date of 2 February in Parlier and 1 April in Wilmington and Salem . In Pergine and Sant’Orsola, estimates were initiated on 6 April . The population matrices were initiated with 100 flies in the population vector for 41–50 day-old females based on the assumption that females of this age group represent flies that would be emerging from diapause in spring. The log transformed sum of D. suzukii from all life stages for each day represented the total population estimate except where age distributions are considered. For daily age distribution of D. suzukii from Parlier, Salem and Wilmington during 2013, 1–3 day-old D. suzukii were classified as eggs, 4–7 day-olds were larvae, 8–9 day-olds were pupae, and 10–50 day-olds were classified as adults. Among the most important assumptions of the model are that populations of D. suzukii would not be limited by host availability, are not density dependent, do not exhibit Allee effects, and that response to current temperature is not dependent on previous temperature exposure.
Seasonal weekly trap catches of D. suzukii were recorded in all study sites, except Riva del Garda, but model estimations for this location was included because the climate here is much different from the other locations studied in Italy. Trap counts were pooled data from commercial blueberry fields in Wilmington ; unsprayed apricot, blackberry, blueberry, cherry, peach, and citrus orchards in Parlier ; commercial blueberry fields and surrounding blackberry vegetation in Salem ; strawberry, blackberry, cherry and blueberry fields in Pergine ; and unsprayed strawberry and raspberry fields in Sant’Orsola . In Wilmington and Salem, traps were made of clear plastic cups, ca. 1 liter in volume each. Each trap had 6–15 entrance holes 4.5– 9 mm in diameter. Trap baits in Wilmington consisted of a yeast and sugar water mixture containing 6 g yeast and 40 g sugar dissolved in 710 ml water. In Salem, traps were baited with 100– 200 ml natural apple cider vinegar and 1–3 ml unscented liquid soap to break water surface tension. In Parlier the traps were made to the specifications of the ‘‘Haviland Trap’’ design for D. suzukii monitoring. A 750-ml plastic container served as the basin for each trap. A 7.5-cm diameter hole was cut in the lid, over which a piece of 0.6-cm wire mesh was attached. Each trap was covered with a Pherocon trap cover , which had a built-in wire hanger. Each trap was filled with 250– 300 ml of apple cider vinegar with 15 ml of unscented soap added as a surfactant to each container of vinegar. In Trentino the containers were 1000-ml graduated white polyethylene bottles filled with 200 ml apple cider vinegar . All traps were placed near the fruiting level of host plants or on stable surfaces in shaded areas and were checked weekly. The contents of each trap were collected into a separate container that was taken to the laboratory for processing, and at the same time,blueberry grow pot the traps were refilled with fresh apple cider vinegar and unscented soap, as described above, in the field. The liquid and contents from each trap sample were strained in the laboratory and the numbers of adult SWD collected were recorded by gender. All data from traps were analyzed to display mean weekly D. suzukii per trap for each of the regions. Mean daily temperatures for all seasons are presented together with trap catches.Population estimates using temperature data indicate that D. suzukii populations are able to increase to high levels in all of the studied locations . The population estimates in all regions broadly tracked demographic trends of D. suzukii caught in traps . When comparing early-season population estimates between Wilmington, Parlier, and Salem , the population estimates were highest in 2013 in Wilmington followed by Parlier and then Salem. However, the population estimate for Salem surpassed Wilmington by 15 June and surpassed Parlier on 16 July, as Salem population estimates continued to climb while the latter sites experienced declining populations after reaching the first peak of their bimodal distributions. In Parlier, the early-season population peaked on 16 June, subsequently decreasing to a low on 10 September before increasing to a second population peak on 9 November, then decreasing again as winter progressed.
In Wilmington, the population curve peaked on 21 June, then the population curve declined slightly for an extended period, followed by a second period of population increase beginning on 19 September to a population peak in November. In Salem, populations consistently increased from 25 April to a peak on 22 October, followed by a steep decrease. When comparing population estimates between seasons for the initial harvest period of early- to mid-season blueberries in Salem , the majority of model outputs for this period estimated greater populations for 2013. When comparing populations along the elevation gradient of the three Italian sites, higher early-season populations were predicted at the lowest elevation Riva del Garda, followed by Pergine and then Sant’Orsola . In Pergine, greater population numbers were estimated for the majority of the growing season during 2013 compared to 2012 . In all model predictions, immature life stages comprised by far the majority of the population, except at the beginning or end of the season when adults tended to dominate . One exception was Wilmington, where temperatures remained favorable for reproduction into the late fall so that immature stages remained a majority of the population . In Salem, fall temperatures initially caused cessation of reproduction, leaving a majority of adults, but December temperatures allowed for some reproductive activity to occur . In the early spring, a higher relative percentage of adults occurred due to the overwintering adults that were initiating their first reproduction. In part, this was an artifact of initiating the model with only older adult females. In the fall, environmental conditions became unfavorable for reproduction but may not have had strong effects on adult survival. Overall, no populations reached a completely stable age structure, but the highest relative stability for each site occurred in the middle of the season. Stability of age structure was the highest in Wilmington, followed by Parlier and finally Salem, which had a high degree of instability . Demarcation of distinct generations was very clear for the first part of the season in Parlier and Salem , but during the mid season at these sites and in Wilmington , it was very difficult to distinguish individual generations to distinguish complete generations from partial generations.In Wilmington, D. suzukii counts were first recorded on 5 May 2013 at one fly per trap with an erratic increase to a peak in numbers at 26 flies per trap on 26 July . After this period, the trap numbers gradually decreased to six flies per trap until 4 December, at which point D. suzukii trapping was discontinued. In Parlier, two population peaks were found during the crop season, one during the early part of the season, followed by a long mid-summer period without fly captures, and a second peak during the latter portion of the season . Adult D. suzukii were first caught on 19 March 2013 at one fly per trap and increased to a high of six flies per trap on 16 May, after which they decreased to zero on 27 July. The trap numbers remained at this level until 19 September, after which numbers continued to increase into December. In Salem and Wilmington only one population peak was observed during the summer period . During 2012 in Salem , D. suzukii trap counts consistently increased starting on 5 July from one fly per trap per week to a maximum average of 17 flies per trap on 6September. During 2013 in Salem , the first D. suzukii trap counts were observed on 30 May at an average of three flies per trap per week and gradually increased until 10 September, when a maximum of 27 flies per trap was observed. The first trap counts during 2013 were therefore consistently recorded four weeks before those found in 2012 and higher levels of flies were found in traps during 2013 in Salem. In Pergine and Sant’Orsola, fly counts were first observed 23 June 2013 , and on 7 July 2013 in respectively. In Italy, one population peak was visible each year for Pergine and Sant’Orsola. In 2012 in Pergine, the first flies were trapped on 25 July, approximately four weeks before those caught during 2013. The mean number of flies per trap per week increased to a peak of 4.6 on 10 September 2012 to a peak of 11.3 on 29 August 2013. Infested fruit was first found in 2013 on 24 June in Pergine , and on 19 July in Sant’Orsola . First fruit infestation in Pergine in 2012 was determined on 24 June , compared to 28 July 2013 .