Drosophila suzukii (Diptera: Drosophilidae) is a devastating pest that attacks thin-skinned stone and small fruit in all major production regions of the United States and worldwide. D. suzukii first appeared in the United States in 2008 and is believed to be native to Southeast Asia. This vinegar fly has a unique ovipositor that is serrated allowing oviposition in fully intact fruit. The larvae that emerge from the eggs damage fruit through feeding rendering them unmarketable. Typical management practices include increased pesticide applications, primarily targeting gravid females.
The first objective of this research was to determine the impact of light, temperature, and forced egg retention of D. suzukii under different diurnal regimes. These regimes included cool (10-23°C) and warm (12-27°C) daily cycles with a 16:8 (light: dark), 24h light, and 24h dark cycles. Under all regimes, the oviposition peaked during 14h00-17h00. Under dark conditions, egg-laying levels were elevated compared to normal daily light regimes. Under 24h light conditions, oviposition was reduced to virtually zero. When flies were forced to retain their eggs, oviposition occurred at elevated levels compared to treatments where there was no forced egg retention. In this treatment, oviposition started directly after the oviposition medium was offered.
The second objective was to evaluate the survival of D. suzukii larvae when exposed to temperatures on a thermogradient bench ranging from 29-49°C for 60 minutes. Lower temperatures resulted in the highest survival levels. At 38-45°C, survival was significantly reduced to virtually zero. Four-day-old larvae were additionally exposed to ‘heat conditioning treatments’ to determine if pre-conditioning would allow increased larval survival at temperatures above 38°C. Larval ‘heat therapy treatments’ included 30, 60, and 90 min at 35°C. All larvae were then allowed to cool to room temperature (22°C) for 60 mins prior to being exposed to thermogradient temperatures within a thermal heat bench. Larval survival for no heat therapy and 90-minute heat therapy treatments (no and less optimal heat therapy) were statistically similar. Larval survival for more optimal heat therapy treatments (30 and 60 minute treatments) were statistically similar. More flies emerged in optimal heat therapy treatments compared to the less optimal and no heat therapy treatments. At 41-48°C, the more optimal heat therapy treatments resulted in adult D. suzukii emergence and showed higher emergence trends compared to the no heat therapy and less optimal treatments.
This research contributed to the understanding of D. suzukii oviposition behavior and ability to adapt to heat extremes as described above. This information can be incorporated into pest management strategies to reduce pesticide reliance and lead to better control methods. Knowing the heat necessary to restrict larval survival may be incorporated by increasing the heat surrounding the fruiting plant. Using black weed mat while pruning potentially could increase heat. This could be paired with adding lighting during the vulnerable growing period when D. suzukii are likely to attack. Both recommendations need to be researched in an outside growing environment before industry implantation.