Abstract
Understanding how neonicotinoid insecticides affect non-target arthropods, especially pollinators, is an area of high priority and popular debate. Few studies have considered how pollinators interact and detect neonicotinoids, and almost none have examined for these effects in anthophilous Diptera such as hover flies (Syrphidae). We investigated behavioral responses of two species of hover flies, Eristalis arbustorum L. (Eristalinae) and Toxomerus marginatus Say (Syrphinae), when given a choice between artificial flowers with uncontaminated sucrose solution and neonicotinoid-contaminated (clothianidin) sucrose solution at field-realistic levels 2.5 ppb (average) and 150 ppb (high). We examined for 1) evidence that wild-caught flies could detect the insecticide gustatorily by analyzing amount of time spent feeding on floral treatments, and 2) whether flies could discriminate floral treatments visually by comparing visitation rates, spectral reflectance differences, and hover fly photoreceptor sensitivities. We did not find evidence that either species fed more or less on either of the treatment solutions. Furthermore, T. marginatus did not appear to visit one of the flower choices over the other. Eristalis arbustorum, however, visited uncontaminated flowers more often than contaminated flowers. Spectral differences between the flower treatments overlap with Eristalis photoreceptor sensitivities, opening the possibility that E. arbustorum could discriminate sucrose-clothianidin solution visually. The relevance of our findings in field settings are uncertain but they do highlight the importance of visual cues in lab-based choice experiments involving insecticides. We strongly encourage further research in this area and the consideration of both behavioral responses and sensory mechanisms when determining insecticidal impacts on beneficial arthropods.
Highlights
Much concern has arisen about excessive use of pesticides and the resulting environmental impacts
Region of Practical Equivalence (ROPE) null hypothesis tests were done against the null ROPE interval of (-0.15, 0.15; or 1.16 seconds difference in hover fly feeding response), where smaller ROPE overlap percentages indicate greater confidence in the presence of a difference in response, and ROPE overlaps < 2.5% are the null hypothesis rejection threshold
There was insufficient evidence to determine if E. arbustorum or T. marginatus fed significantly more or less than ±1.16 seconds (i.e., ±0.15 loge seconds–Table 1) between any two flower treatments (Table 1, Fig 1A)
Summary
Two hover fly species common in the Midwestern United States were tested in this experiment: Eristalis arbustorum (Linnaeus 1758 [9]) and Toxomerus marginatus (Say 1823 [28]). Arenas were provisioned with flowers that flies were found commonly feeding upon in the field: Plantago major (Linnaeus 1753 [35]) flowers for T. marginatus, and Daucus carota (Linnaeus 1753 [35]) flowers for E. arbustorum. The purpose of this was to induce feeding behavior in the flies, which preliminary trials deemed necessary. After the two-hour acclimation period, real flowers were replaced with experimental, artificial flowers and the trial was commenced
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