Sugar content of diet does not buffer against chronic oral imidacloprid exposure in the alfalfa leafcutting bee (Hymenoptera: Megachilidae)

Abstract

Bees are economically critical pollinators, but are declining broadly due to several stressors, including nontarget exposure to insecticides and deficiencies in nutrition. Understanding the simultaneous impact of stressors, particularly interactions between them, is critical to effectively conserving bees. Although behavioral effects of pesticides like neonicotinoids have received some attention in solitary bees, our understanding of how they are modulated by diet quality is limited. Furthermore, scarce data exist on what concentrations of orally ingested neonicotinoids elicit mortality in solitary bees. In a controlled exposure laboratory experiment, we investigated how diet quality, as sugar concentration, and chronic oral exposure to imidacloprid affect adult alfalfa leafcutting bees, Megachile rotundata (Fabricius). We provided individuals ad libitum with either 20 or 50% (m/m) sucrose syrups containing either 0, 30, or 300 ppb imidacloprid (measuring 0, 27, and 209 ppb via an ELISA assay). Over 5 wk, we tracked behavior and survivorship of individuals. Imidacloprid decreased survivorship in a dose-dependent fashion, but sucrose content did not affect survivorship, even in bees not fed imidacloprid. In the high imidacloprid treatment, 45% of bees were observed in a motionless supine position while still alive, with this effect appearing to be buffered against by the higher sucrose diet. Our results suggest that diets higher in sugar concentration may prevent an intermediate stage of poisoning, but do not ultimately extend longevity. In devising risk assessments for bees, it is important to consider that interactions between stressors may occur in the stages leading up to death even if survivorship is unaffected.

A novel insecticide impairs bumblebee memory and sucrose responsiveness across high and low nutrition

“…Poor nutrition can make bees more vulnerable to pesticide exposure, but most previous research has focused on the interactions between pesticides and pollen intake, or floral diversity [ 24 – 27 , 84 – 86 ]. We found that flupyradifurone impaired sucrose responsiveness and long-term memory when bumblebees were nutritionally stressed and healthy, indicating that higher carbohydrate intake did not buffer the negative consequences of pesticide exposure [ 30 ]. This is not surprising as negative effects of pesticides on bees regularly occur even when bees are fed high sucrose concentrations [ 41 , 45 , 87 ].…”

“…The interactions between these stressors can be: (i) additive, where the two stressors are equal to their combined individual effect; (ii) synergistic, where the two stressors exacerbate one another and have a greater than additive effect; or (iii) antagonistic, where the impact is lower than the predicted additive effect [23]. Previous studies have found that most interactions between pesticides and poor nutrition are additive [23][24][25][26] or synergistic [13,14,[27][28][29][30]. The underlying mechanism that determines the type of interaction (synergistic, additive or antagonistic) between pesticides and poor nutrition is poorly understood and requires further investigation [23].…”

Pollinators and plant nurseries: how irrigation and pesticide treatment of native ornamental plants impact solitary bees

“…In general, granular formulations are understudied relative to seed and foliar applications [43]. Furthermore, field studies on neonicotinoids and ALCBs are surprisingly uncommon relative to laboratory exposure trials [44][45][46][47]. Field enclosure experiments with closely related solitary bees (Osmia) and neonicotinoid-treated crops, albeit using different application methods, found varying effects on bee reproduction [48,49].…”

“…This is supported by our occasional observations of dead bees on flowers only in imidacloprid cages. While oral LD 50 and LC 50 values of imidacloprid for ALCBs are not well established ( [63], but see [64]), Cecala et al [47] documented 29% and 68% reductions in adult ALCB longevity from ingesting 30 and 200 ppb sucrose syrups, respectively. In their experiment, survivorship fell to 50% after 6-12 days, depending on dosage.…”