When humans alter nectar chemistry, what happens to bees?

Sarah K. Richman, Isabelle M. Maalouf, Angela M. Smilanich, Denyse Marquez Sanchez, Sharron Z. Miller, Anne S. Leonard

This is a plain language summary of a Functional Ecology research article which is published here.

Narrative images of the study. Top row: Representative genera of plants containing the focal NSMs tested in the study. From left: Thymus, containing thymol; Digitalis, containing digoxin; Citrus, containing caffeine. Bottom row: Study components. From left: Bombus impatiens workers inside the nest, workers housed inside individual chambers with nectar feeder access, preparation for phenoloxidase (PO) activation assay. Photo credits: Anne Leonard (top row), Sarah Richman (bottom row).

Given current insect declines and growing awareness of the threats facing wild and managed pollinators, neonicotinoid pesticides have received much attention for their capacity to alter bee physiology and behavior. Applied in agricultural and residential settings, these systemic pesticides leach into floral nectar of target plants as well as into wildflowers and hedgerow plants growing near the application site. Much is known about these chemicals’ effects on bee health; however, the already complex nature of nectar chemistry may complicate the story. Floral nectar is rich in naturally occurring secondary metabolites: chemicals that have their own powerful effects on bees. For example, caffeine in nectar can improve bees’ ability to learn floral scents and colors. Other chemicals improve bees’ immune systems. Given these chemicals’ wide-ranging and potent effects on bee biology, we asked how they interacted with pesticide exposure to affect survival, behavior and immune function in bumblebees (Bombus impatiens, the Common Eastern Bumblebee).

Our study design involved maintaining captive bees on nectar diets of either plain sugar water, or sugar water supplemented with one of three naturally occurring nectar chemicals (the alkaloid caffeine, the phenolic thymol, or the cardiac glycoside digoxin). We then gave bees a single, acute dose of nectar laced with Imidacloprid (a popular neonicotinoid pesticide), or a control dose of plain sugar water. The resulting diets allowed us to test the separate effects of naturally occurring and pesticide chemicals, their effects in combination, and the effects of no chemicals at all. After feeding bees the different dietary combinations, we observed their activity, measured their immune system functioning, and quantified their lifespans. Given the wide diversity of chemicals we tested, we found a similarly diverse array of responses. In some cases, a single brief exposure to the pesticide washed out the effect of the naturally occurring chemical. When the naturally occurring chemical was beneficial on its own, the washing out of effects may harm bees in the long run. In other cases, the naturally occurring chemical offset the negative effects of the pesticide. Our study shows that even simple dietary manipulations can have strong effects on this ecologically and commercially important pollinator species.