et="_self">Figure 5).

4. Discussion

There is no doubt that the host range of SWD is vast [9] [12] - [14] [24] . 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 [1] [9] [25] . 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fruit and larvae over time even at a time when the number of adults observed in baited traps was increasing (CRP unpublished data).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 [19] [26] - [28] . In addition, Different varieties of crops contain differing amounts of resources, such as soluble sugar content (brix) which likely effects varietal preference [9] [19] [28] (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 [29] - [31] . 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 [32] [33] . 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.

5. Conclusions

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.

Acknowledgements

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.

Cite this paper
Sward, G. , Glass, S. and Philips, C. (2016) The Phenology of Infestations and the Impacts of Different Varieties of Cold Hardy Red Raspberries on Drosophila suzukii. Advances in Entomology, 4, 183-190. doi: 10.4236/ae.2016.43019.
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