Physiological effects of non-nutritive sugars on the survivorship and fecundity of Drosophila suzukii (Diptera: Drosophilidae)

Graduation date: 2018Spotted wing drosophila (SWD), Drosophila suzukii, is an economically damaging pest on small fruits. The estimated economic impact is hundreds of millions of dollars annually in the U.S. alone, and increasing every year. Current control of SWD relies heavily on chemical insecticides which have many negative impacts on environmental and human health. Therefore, this should be replaced or at least complemented with biologically-based alternatives. The current project has focused on investigating non-nutritive sugars including erythritol as an environmentally-friendly control agent against SWD.\ud The study found a potential insecticidal effect from non-nutritive sugars and sugar alcohols, erythritol and erythrose, to decrease the survivorship of D. suzukii. In a dose-dependent feeding, erythritol and erythrose significantly reduced the fly longevity with 1 M - 0.05M doses for 7 days. When sugar solutions were provided separately to flies, there was no effect on survivorship regardless of erythritol concentrations. However, with a serial combination of sucrose and erythritol solutions, fly survivorship was significantly decreased. In a no-choice feeding assay, the fly ingested more erythritol than sucrose or water, and erythritol and sucrose-fed flies gained more weight than water-fed flies. However, in two-choice assays, the amount of erythritol ingested was less than sucrose or water.\ud Total sugar and glycogen levels among the body of erythritol and erythrose-fed flies were significantly less than flies fed nutritive sugars of mannitol, sorbitol, and xylitol. The result indicates that the two non-nutritive sugars can't be used as a substrate for enzymes involved in sugar metabolism. Although the metabolism of erythritol and erythrose is unknown in insects, the mortality of D. suzukii flies ingesting these sugars might be caused by two potential physiological changes: 1) starvation from the feeding of non-metabolizable erythritol and erythrose; and 2) abnormal osmotic pressure increased in the hemolymph with erythritol transported from the midgut.\ud Chapter 3, sucrose and erythritol were applied to blueberries and effects of these combinations on fly mortality and fecundity were monitored in the lab and greenhouse. In the lab test, two sucrose/erythritol combinations (0.5M sucrose/2M erythritol, 1M sucrose/2M erythritol) resulted in the highest mortality and the lowest fecundity in SWD adults. Two combinations, therefore, were selected for further evaluation with blueberry bushes and fruits in the greenhouse. The fly fed on 0.5M sucrose/2M erythritol significantly decreased their survivorship than 1M sucrose/2M erythritol-fed flies in the greenhouse. This result indicates that flies could move more in the bigger cage accelerating the exhaustion of energetic reserves in the body.\ud The presence of erythritol in the hemolymph and frass was determined to investigate the nutritional metabolism and absorption of erythritol in D. suzukii. Unlike sucrose, a large amount of erythritol was observed in the hemolymph of the fly ingested the 0.5M sucrose/0.5M erythritol. Not sucrose, erythritol was found in the frass in the same fly. The results imply that erythritol might be directly transported from the midgut without being metabolized and stored, but is accumulated in the hemolymph which in turn elevates the osmotic pressure in the fly hemolymph.\ud Overall, this research found the sucrose/erythritol combination would be more effective than erythritol alone for practical application, because the combination tastes sweeter to elicit more feeding. This erythritol formulation can be a potential insecticide used alone or as a delivery agent combined with conventional or\ud biological insecticides to enhance their efficacy. While the present research focuses on D. suzukii, it can be expanded to other Dipteran pests as well