There is no doubt that the host range of SWD is vast   -   . Nevertheless, D. suzukii appears to exhibit some level of varietal preference within host species, or certain varieties may be more suitable, leading to increased adult populations. However, there may be other factors that are influencing the infestation patterns observed. Consistent with other studies, we found that there are higher levels of infestations in some varieties, and that some varieties produced more larvae    . We also observed a significant negative correlation in both of these factors as the harvest season progressed.
Somewhat puzzling was the observed decrease in infested fruit and larvae over time even at a time when the number of adults observed in baited traps was increasing (CRP unpublished data). This trend could indicate dispersal of SWD to neighboring crop hosts or alternative hosts in the environment. One possible explanation for this could be due changes in host that modify a key biological trigger that reduces the olfactory attractiveness of the natal food source. Numerous studies have shown that many different species of insects, including some drosophilids, use olfactory cues to locate and ultimately utilize host plants   -  . In addition, Different varieties of crops contain differing amounts of resources, such as soluble sugar content (brix) which likely effects varietal preference    (Table 1). Slight changes in fruit chemistry can cause greater attractiveness to female SWD, through volatiles released or via better nutrition for developing larvae.
Timing is important when dealing with agricultural pests, and local climactic conditions or microclimates may also influence SWD population dynamics  -  . Pest development time and the time that susceptible fruit are available play an essential role in the severity of an infestation. In addition to understanding how climate impacts nutritional values and attractiveness of varieties, this information may also be useful in evaluating the
Figure 3. Average larvae per fruit for each variety of Red Primocane-Fruiting raspberries evaluated for spotted-wing drosophila infestations in 2015.
Figure 4. Average number of larvae per fruit over time by varieties of Red Primocane-Fruiting raspberries evaluated for spotted-wing drosophila infestations in 2014 and 2015. Dates marked with an asterisk are significantly different.
Table 2. Varieties of Red PrimocaneFruiting raspberries evaluated for spottedwing drosophila infestations in 2014 and 2015.
Figure 5. Regression of the average number per berry over time for the varieties of Red Primocane-Fruiting raspberries evaluated for spotted-wing drosophila infestations in 2015.
potential to provide a phenological mismatch between host and pest. Further exploration into varietal susceptibility is needed to determine what characteristic might make varieties more or less susceptible to SWD infestations.
What remains to be determined is what constitutes optimal development for SWD. Previous assumptions describe fitness for SWD as larger, heavier individuals, faster development time, and greater numbers of eggs laid. It has not been thoroughly explored if fitness truly is defined by those parameters. For example, greater egg numbers may not be the more fit option if few of them survive. The varietal differences observed though fits with the ecological concept of ideal free distribution   . A less optimal environment may be selected by the adult female if the competition in the more optimal environment yields detrimental characteristics (such as competition) to the survival of offspring. Changes in host preference and decreasing larval populations, while trap populations increase (CRP unpublished data), support this idea. As yet, the ideal free distribution has not been explored nor discussed in previous research as it relates to SWD. Future studies should focus on evaluating ideal free distribution for SWD including: determining impacts of host plant and variety on fitness, the effects of competition on fitness, and the combination of these two factors.
One of the keys to development of an IPM program for SWD remains understanding host preference and varietal susceptibility of preferred host crops. For cold hardy raspberries, it is clear that there is a varietal preference occurring. Understanding the host selection and varietal preference opens the door to further understand the biology and ecology of this pest. Key questions remain unanswered, but additional studies can add to our understanding of where and when outbreaks and infestations will occur which will ultimately allow a more integrated approach in managing SWD.
This study was conducted with the support of Dan Braaten, Chuck Heggedahl, Doug Hendrickson, Derek Leppanen, Bob Smith, Keith Mann, Joe Benes and Rob Nalder at the North Central Research as well as the entomology and wild rice summer field crew. This research was partially funding by the University of Minnesota Rapid agricultural response fund.
 Lee, J.C., Bruck, D.J., Curry, H., Edwards, D.L., Haviland, D.R., Van Steenwyk, R. and Yorgey, B. (2011) The Susceptibility of Small Fruits and Cherries to the Spotted Wing Drosophila, Drosophila suzukii. Pest Management Science, 67, 1358-1367. http://dx.doi.org/10.1002/ps.2225
 Calabria, G., Máca, J., Bachli, G., Serra, L. and Pascual. M. (2012) First Records of the Potential Pest Species Drosophila suzukii (Diptera: Drosophilidae) in Europe. Journal of Applied Entomology, 136, 139-147.
 Deprá, M., Poppe, J.L., Schmitz, H.J., De Toni, D.C. and Valente, V.L. (2014) The First Records of the Invasive Pest Drosophila suzukii in the South American Continent. Journal of Pest Science, 87, 379-383.
 Kinjo, H., Kunimi, Y. and Nakai, M. (2014) Effects of Temperature on the Reproduction and Development of Drosophila suzukii (Diptera: Drosophilidae). Applied Entomology and Zoology, 49, 297-304.
 Asplen, M.K., Anfora, G., Biondi, G., Choi, D.S., Chu, D., Daane, K.M., Gibert, P., Gutierrez, A.P., Hoelmer, K.A., Hutchison, W.D., Isaacs, R., Jiang, Z.L., Kárpáti, Z.L., Kimura, M.T., Pascual, M., Philips, C.R., Plantamp, C., Ponti, L., Vétek, L., Vogt, H., Walton, V.M., Yu, Y., Zappalà, L. and Desneux, N. (2015) Invasion Biology of Spotted Wing Drosophila (Drosophila suzukii): A Global Perspective and Future Priorities. Journal of Pest Science, 88, 469-494. http://dx.doi.org/10.1007/s10340-015-0681-z
 Bolda, M., Goodhue, R. and Zalom, F.G. (2010) Spotted Wing Drosophila: Potential Economic Impact of a Newly Established Pest. Giannini Foundation of Agricultural Economics, University of California, Berkley, CA, Vol. 13, 5-8.
 Walsh, D.B., Bolda, M.P., Goodhue, R.E., Dreves, A.J., Lee, J, Bruck, D.J., Walton, V.M., O’Neal, S.D. and Zalom, F.G. (2011) Drosophila suzukii (Diptera: Drosophilidae): Invasive Pest of Ripening Soft Fruit Expanding Its Geographic Range and Damage Potential. Journal of Integrated Pest Management, 2, G1-G7. http://dx.doi.org/10.1603/IPM10010
 Burrack, H.J., Smith, J.P., Pfeiffer, D., Koehler, G. and LaForest, J. (2012) Using Volunteer-Base Networks to Track Drosophila suzukii (Diptera: Drosophilidae) an Invasive Pest of Fruit Crops. Journal of Integrated Pest Management, 3, B1-B5. http://dx.doi.org/10.1603/IPM12012
 Burrack, H.J., Fernandez, G.E., Spivey, T. and Kraus, D.A. (2013) Variation in Selection and Utilization of Host Crops in the Field and Laboratory by Drosophila suzukii Matsumara (Diptera: Drosophilidae), an invasive Frugivore. Pest Management Science, 69, 1173-1180. http://dx.doi.org/10.1002/ps.3489
 Bellamy, D.E., Sisterson, M.S. and Walse, S.S. (2013) Quantifying Host Potentials: Indexing Postharvest Fresh Fruits for Spotted Wing Drosophila, Drosophila suzukii. PLoS ONE, 8, e61227.
 Kimura, M.T., Toda, M.J., Beppu, K. and Watabe, H. (1977) Breeding Sites of Drosophilid Files in and Near Sapporo, Nomherrr Japan, with Supplementary Notes on Adult Feeding Habits. Kontyû, 45, S71-S82,
 Nishiharu, S. (1980) A Study of Ecology and Evolution of Drosophilid Files with Special Regard to Imaginal and Larval Feeding Habits and Seasonal Population Fluctuations. Doctor of Science Thesis, Tokyo Metropolitart University, Tokyo.
 Mitsui, H., Beppu, K. and Kimura, M.T. (2010) Seasonal Life Cycles and Resource Uses of Flower- and Fruit-Feeding Drosophilid Flies (Diptera: Drosophilidae) in Central Japan. Entomological Science, 13, 60-67. http://dx.doi.org/10.1111/j.1479-8298.2010.00372.x
 Steffan, S.A., Lee, J.C., Singleton, M.E., Vilaire, A., Walsh, D.B., Lavine, L.S. and Pattern, K. (2013) Susceptibility of Cranberries to Drosophila suzukii (Diptera: Drosophilidae). Journal of Economic Entomology, 106, 2424-2427. http://dx.doi.org/10.1603/ec13331
 Gutierrez, A.P. and Ponti, L. (2013) Eradication of Invasive Species: Why the Biology Matters. Environmental Entomology, 42, 395-411. http://dx.doi.org/10.1603/EN12018
 Kimura, M.T. (2004) Cold and Heat Tolerance of Drosophilid Flies with Reference to Their Latitudinal Distributions. Oecologia, 140, 442-449. http://dx.doi.org/10.1007/s00442-004-1605-4
 Shearer, P., West, J., Walton, V., Brown, P., Svetec, N. and Chiu, J. (2016) Seasonal Cues Induce Phenotypic Plasticity of Drosophila suzukii to Enhance Winter Survival. BMC Ecology, 16, 11.
 Diepenbrock, L.M., Swoboda-Bhattarai, K.A. and Burrack, H.J. (2016) Ovipositional Preference, Fidelity, and Fitness of Drosophila suzukii in a Co-Occurring Crop and Non-Crop Host System. Journal of Pest Science, 1-9.
 Haviland, D.R., Caprile, J., Rill, S., Hamby, K.A. and Grant, J.A. (2014) Varies by Season, Crop and Nearby Vegetation. California Agriculture, 70, 24-31. http://dx.doi.org/10.3733/ca.v070n01p24
 Klick, J., Yang, W.Q., Walton, V.M., Dalton, D.T., Hagler, J.R., Dreves, A.J., Lee, J.C. and Bruck, D.J. (2016) Distribution and Activity of Drosophila suzukii in Cultivated Raspberry and Surrounding Vegetation. Journal of Applied Entomology, 140, 37-46. http://dx.doi.org/10.1111/jen.12234
 Lee, J.C., Dreves, A.J., Cave, A.M., Kawai, S., Isaacs, R., Miller, C., Van Timmeren, S. and Bruck, D.J. (2015) Infestation of Wild and Ornamental Noncrop Fruits by Drosophila suzukii (Diptera: Drosophilidae). Annals of the Entomological Society of America, 108, 117-129. http://dx.doi.org/10.1093/aesa/sau014
 Kinjo, H., Kuimi, Y., Ban, T. and Nakai, M. (2013) Oviposition Efficacy of Drosopihla suzukii (Diptera: Drosophilidae) on Different Cultivars of Blueberry. Horticultural Entomology, 106, 1767-1771.
 Hoffman, A.A. (1985) Interspecific Variation in the Response of Drosophila to Chemicals and Fruit Odors in a Wind Tunnel. Australian Journal of Zoology, 33, 451-460. http://dx.doi.org/10.1071/ZO9850451
 Keesey, I.W., Knaden, M. and Hansson, B.S. (2015) Olfactory Specialization in Drosophila suzukiisupports an Ecological Shift in Host Preference from Rotten to Fresh Fruit. Journal of Chemical Ecology, 41, 121-128. http://dx.doi.org/10.1007/s10886-015-0544-3
 Yu, D., Zalom, F.G. and Hamby, K.A. (2013) Host Status and Fruit Odor Response of Drosophila suzukii (Diptera: Drosophilidae) to Figs and Mulberries. Journal of Economic Entomology, 106, 1932-1937.
 Tochen, S., Dalton, D.T., Wiman, N.G., Hamm, C., Shearer, P.W. and Walton, V.M. (2014) Temperature-Related Development and Population Parameters for Drosophila suzukii (Diptera: Drosophilidae) on Cherry and Blueberry. Environmental Entomology, 43, 501-510. http://dx.doi.org/10.1603/EN13200
 Rogers, M.A., Burkness, E.C. and Hutchison, W.D. (2015) Evaluation of Covered Tunnels for Management of Drosophila suzukii in Fall-Bearing Red Raspberries: Potential for Reducing Insecticide Use. Journal of Pest Science, 1-7.
 Wiman, N.G., Dalton, D.T., Anfora, G., Biondi, A., Chiu, J.C., Daane, K.M., Gerdeman, B., Gottardello, A., Hamby, K.A., Isaacs, R., Grassi, A., Ioriatti, C., Lee, J.C., Miller, B., Rossi Stacconi, V., Shearer, P.W., Tanigoshi, L., Wang, X. and Walton, V.M. (2016) Drosophila suzukii Population Response to the Environment and Management Strategies. Journal of Pest Science, 1-13.
 Fretwell, S.D. and Lucas, H.L. (1970) On Territorial Behaviour and Other Factors Influencing Habitat Distribution in Birds. I. Theoretical Development. Acta Biotheoretica, 19, 16-36.