As I write this article, approximately 2.1 million honey bee colonies are pollinating ~1.3 million acres of nut-producing almond trees in California. Since the estimated number of colonies in the United States is 2.92 million according to the USDA National Agricultural Statistics Service, that means ~70% of the country’s managed honey bee colonies are currently in almond orchards in the central valley of California. If we assume each colony is comprised of ~10 frames of bees, that’s around 20 billion bees.
How big of a number is 20 billion bees? Well, last year ~30 million eastern monarch butterflies successfully migrated to their overwintering grounds in the Sierra Madre Mountains of Mexico. The eastern monarch migration is widely regarded as the largest insect migration event in the world. But if the numbers above are correct, over the past month there were ~600 times more honey bees that made an assisted migration to California almond orchards, mostly on the backs of 18-wheel trucks!
All this is to say if we care about honey bee health in the United States, almond pollination is perhaps the single most important time to ensure that bees aren’t exposed to harmful levels of pesticides. But is that the case? Are the pesticides that almond growers apply during bloom safe for bees? What about the “inert” adjuvants in pesticide sprays, are they safe for bees? These are the topics for the fifty-second Notes from the Lab, where I summarize “Pollen treated with a combination of agrochemicals commonly applied during almond bloom reduces the emergence rate and longevity of honey bee (Hymenoptera: Apidae) queens,” written by Dylan Ricke and colleagues and published in the Journal of Insect Science [2021].
Bees are always exposed to a cocktail of pesticides during crop pollination. For example, honey bees are simultaneously exposed to 17 pesticides on average during apple pollination in New York (McArt et al. 2017). During Michigan blueberry pollination, honey bee-collected pollen contains 35 pesticides on average (Graham et al. 2021). My lab has analyzed several samples from beekeepers conducting California almond pollination and we typically find ~20 pesticides in bee bread, trapped pollen, or foraging bees.*
What does this mean for pesticide risk to bees during pollination? It means we need to understand the impact of co-exposures, not just the impact of exposure to a single pesticide. With this goal in mind, Ricke and colleagues set out to understand how co-exposure to several pesticides that are commonly applied during bloom impact development and survival of new queens.
For their study, the authors chose to focus on individual and combined effects of chlorantraniliprole (active ingredient in Altacor), propiconazole (Tilt), an organosilicone and alkyphenol ethoxylate spray adjuvant (Dyn-Amic), and diflubenzuron (Dimilin 2L) as a positive control since the harmful effects of diflubenzuron to honey bee larvae have already been documented (see Wade et al. 2019). Chlorantraniliprole and diflubenzuron are both insecticides, propiconazole is a fungicide, and organosilicone and alkyphenol ethoxylate spray adjuvants are used to improve pesticide performance (wetting, particle size, etc.) during application. All are labeled for use during almond bloom, and tank mixes of the pesticides and adjuvants (i.e., combining multiple products into one spray) are common.
To test the impact of the pesticides and adjuvants on queen development and survival, the authors created swarm boxes that were provisioned with contaminated pollen (Photo 1). Concentrations of the pesticides were chosen to mimic high but field-realistic exposures based on maximum application rates: 40 parts per million (ppm) chlorantraniliprole, 90 ppm propiconazole, 100 ppm diflubenzuron, and 0.8% by weight of the adjuvants.
Each swarm box received contaminated pollen, sucrose, nurse bees, and 30 young larvae grafted into queen cups on a cell bar frame (Photos 1 & 2). After 4 days (96 hrs), samples of pollen, nurse bees, and jelly were taken for ….
