During favorable periods, growth, development and reproduction of D. suzukii life stages depend on temperature, the drying power of the air, and host quality and availability . Zerulla et al. followed via dissection of field collected females the lack of ovary development in D. suzukii during winter and the resumption of ovary development in spring . Plantamp et al. found that cold treatments had a strong impact on adult survival but had no effect on female’s fertility. The magnitude and pattern of age-specific fecundity in D. suzukii contrasts sharply with other drosophilids such as D. melanogaster that have oviposition rates an order of magnitude higher during the first 20 days of adult life, but unlike D. suzukii reproduction falls off dramatically after 40 days . Toxopeus et al. found that acclimated winter morph adults have delayed reproductive maturity, and can remain active at lower temperatures than summer morph adults. These above attributes suggest that D. suzukii is able to maintain population pressures longer and under more adverse conditions than other drosophilids . The above biology is further enhanced in temperate areas by the fly’s ability to attack a wide range of hosts, some of which are available year around . In Europe, Kenis et al. reared SWD from 84 plant species belonging to 19 families, 38 of which are non-native. While development, fecundity and mortality rates vary with hosts , modeling the dynamics of these hosts and their effects on D. suzukii is vexing. In the PBDM, round plastic plant pot we assume hosts are available for D. suzukii reproduction when temperatures are in the favorable range.The recent literature speaks to the urgent need for approaches to estimate the geographic distribution and relative abundance of native and exotic species that may experience novel climate due to range expansion or climate change.

Specifically, Sect. 4.3 by Working Group II in the assessment reports of the Intergovernmental Panel on Climate Change outlines the shortcomings of widely used standard methods based largely on the climate envelope approaches commonly used to assess the impact of climate change on ecosystem. Among the gaps identified in IPCC AR4 were: the ‘‘inability to account for species interactions, the lack of physiological mechanisms, and the inability to account for population processes’’ . Single species and multi-trophic PBDM circumvent some of these problems and often challenge assumptions on how field data should be interpreted , especially where data gaps exists . The polyphagous spotted wing Drosophila, D. suzukii, is a native of Eastern and Southeastern Asia . Phylogenetic analysis by Ometto et al. indicates that the species originated during the late Miocene when its native Asian range was characterized by extended mountain temperate forests. Kimura found that D. suzukii had one of the broadest geographic ranges among the drosophilids of Japan, ranging from Hokkaido in the north to Iriomote Island east of Taiwan. The fly has extended its range in temperate and tropical areas in North America and Europe and the Mediterranean Basin where hosts are abundant , including areas where weather may be only moderately suitable for population development. Several biological attributes appear to enhance the fly’s ability to expand its range. For example, it has a very wide host range ; unlike most drosophilid larvae that develop on relatively protein-rich rotting fruit, D. suzukii larvae uniquely develop on protein-poor, carbohydrate-rich ripening fruit ; and its thermal characteristics are conducive to its range expansion. Specifically, laboratory data on developmental rates were consistent across studies and predicted a lower thermal threshold of 5.95 C and an upper threshold of *31 C .

Ryan et al. estimated a lower threshold of 8.1C and found that no adults emerged above 30.9 C. The differences in threshold estimates likely reflect differences in experimental methods. A low threshold for immature stages enables the species to be active early in the spring and to complete development of immature stages in the fall to produce winter morph adults , while the high threshold for reproduction would appear to delay reproduction until hosts are available in spring. Reproduction potential in D. suzukii is relatively low, approximately half that of D. melanogaster . Reported differences in reproductive rates observed in European and North American populations may be due to experimental conditions or may have some genetic roots . In our analysis, we used the higher fecundity figure, noting that it has little impact on the prospective geographic range and patterns of relative abundance. The effects of temperature on D. suzukii mortality on summer adults were variable among reported studies, especially at low and high temperatures . Tochen et al. characterized the effects of relative humidity and temperature on reproduction and mortality of summer adults, and the combined effects were incorporated in the PBDM . There is general agreement in the literature that cold winter temperatures are the major limiting factor for the fly in north temperate regions. Based on laboratory and field studies, Jakobs et al. concluded that adult D. suzukii phenotypic plasticity alone is insufficient to allow D. suzukii to overwinter in northern temperate areas. Stephens et al. studied cold hardiness of winter-acclimated D. suzukii in the laboratory, and concluded that the species is chill-intolerant, and further stressed that while the winter-morph adult is the most cold-tolerant life stage, both summer and winter-morph adult forms could not overwinter in cold climates without microclimate refuges . Using available data on summer morphs survival with temperature, the PBDM predicts a wide distribution for the fly in the USA and Mexico with highest densities in warmer tropical areas of Mexico = Hawaii [Florida[coastal southern California with lower year round populations in temperate areas of Northern California , and mid- to late-summer populations developing in Oregon and Washington .

Similar mid to late summer populations develop in the north Central USA with highest densities found in woodlands relative to crop fields such as raspberry . Recent, albeit incomplete, studies on winter morph adult survival at 1 and 5C suggest phenotypic plasticity may significantly increase survivorship at low temperatures . Data from Toxopeus et al. on summer and winter morph adult survival at 0 C and after 1 h exposure to temperatures from 0 to -13 C suggest that the mortality function for winter morph adults is displaced approximately 5–6 C to lower temperatures . For example, survivorship of the winter morph at 0 C is about the same as for the summer morph at 5 C. We explored heuristically the effect of shifting mortality Eq. 2i leftward 5.5 C in the PBDM and found that this does not radically change the prospective geographic distribution of D. suzukii in the USA and Mexico, but because more adults survive the winter, densities increase in many areas such as coastal California and in northern areas such Maine and Wisconsin . In south and central Europe and the Mediterranean Basin, fly densities increase more widely to levels predicted for coastal California , and inter annual variability declines . The effect of increased winter morph survival on the density dynamics of egg production and summer and winter morph adults were explored for the cold location of Benton, WA . The results show the increase in winter survival, a modest increase in population densities, but no changes in population phenology. In colder areas such as Benton WA, the favorable season is limited by cool average temperature below *12.75 C that delay reproduction in spring and that induce the development of non-reproducing winter morph adults in fall . Data to characterize fully winter morph adult mortality are currently unavailable, but can easily be incorporated in the PBDM when they become available, and a complete analysis will be performed.Behavioral factors can also affect survival during adverse periods. For example, Tochen et al. posited that behavioral adaptation of short-distance migration enables D. suzukii adults to move towards favorable microclimates that enhance their survival and reproduction in otherwise marginal environments . This point was clearly illustrated by Harris et al. during 2011–2013 using apple cider vinegar traps to monitor D. suzukii adults in multiple crops and associated fruiting plants at the Wolfskill USDA Germplasm Repository at Winters, CA, USA. During spring and summer, D. suzukii adults were trapped in the orchards, 25 liter round pot but during November through April, high trap captures were associated only with citrus, with highest numbers of flies found around a nearby house where fruiting ornamentals such as common myrtle and fire thorn were present and had fruit until April 2013.

Had the trapping occurred only in the orchard area, the results would have suggested that D. suzukii had decreased to very low numbers during winter when in fact adults were active nearly all year around with trap catches near the house during winter being nearly twice as high as those during the season in the orchards . In a mark-recapture study, D. suzukii adults were found to take refuge in wild habitat surrounding a cultivated raspberry field before migrating back to the field when the susceptible crop was present . This information suggests that surrounding habitats with hosts, especially in sheltered areas, should be monitored for D. suzukii adult winter activity and survival . Fly movement makes accurate sampling of adult densities throughout the year vexing. The trapping records of Dalton et al. during 2010 at five sites in Oregon, Washington and California likely had unknown sampling bias. As predicted by the PBDM, D. suzukii adults were found during much of the year at Stockton , CA which is located about 40 miles south of Winters, CA where Harris et al. recorded D. suzukii year around . The climate at both locations has the moderating influence of San Francisco Bay . Specifically, the PBDM predicts that cold weather mortality [i.e., l] had little impact on D. suzukii at Stockton CA, but temperatures were often too low for reproduction . Hot weather during summer caused some suppression of D. suzukii, but the effect was not large. Predicted cold weather mortality rates at Stockton, CA were 1/10th those at Marion and Wasco Counties of Oregon and Benton County in Washington where populations grew only after mid-summer . This delayed phenology is due to cold temperatures in late fall and winter that drive D. suzukii populations to near zero despite the presence of alternate hosts in the area , allowing populations to resurge only in mid- to late-summer. The impact of cold temperatures on reproductive quiescence occurred only during fall in Oregon and Washington, while mortality due to higher temperatures during summer did not appear to have much impact at any of the four western locations studied by Dalton et al. . The delayed phenology of the fly in these colder areas suggests that, as occurs with aphids and other insects carried long distance as aeroplankton , D. suzukii adults may be carried from more favorable nearcoastal areas of the northwest to augment reinvasion of colder areas . This possibly explains the earlier phenology of D. suzukii at Marion Co, OR compared to more inland areas of Wasco Co, OR or Benton Co, WA . Similar scenarios may also occur in the northern reaches of the central USA that may be reinvaded from warmer southern areas, and in Europe into higher latitudes and elevations in mountainous areas . Climatic conditions in the Mediterranean Basin are more favorable and stable for D. suzukii where year-round survival is predicted. In contrast, D. suzukii is limited by high temperatures and low RH in hot dry areas of the southwestern USA and northern Mexico , but cold winter weather can also be a factor in parts of this area. Similarly, high temperatures and low RH would also impact D. suzukii in inland North Africa. In summary, D. suzukii is a cold intolerant invasive species that attacks a wide range of wild and domesticated berries and fruit. The cold resistant winter morph adults can suvive winters and reproduce during spring in marginal north temperate climates . The PBDM gave good predictions of the prospective distribution and relative favorability of climate for D. suzukii in North America, Europe and the Mediterranean Basin. Despite not including the effects of host plants availability and microclimate effects, the PBDM was able to predict the relative dynamics and phenology of adult trapping data at four locations ; data that were likely complicated by sampling bias and fly movement among potential hosts and possibly between sheltered habitats .