A Fungicide Commonly Used During Crop Pollination is Acutely Toxic to Honey Bee Larvae

Captan is the most-used fungicide during New York apple pollination, with over 307,000 kg applied in 2018 (Wieben, 2021). In fact, the next five most-used fungicides (mancozeb, copper sulfate, thiophanate-methyl, cyprodinil, and myclobutanil) combined for only 207,000 kg of use, meaning captan is not only the most-used fungicide, it’s by far the most-used fungicide.
Countrywide use of captan is more variable, but it’s safe to say captan is among the most-used fungicides in the USA. For example, captan is the fungicide used on the most acres of California strawberries (California Department of Pesticide Regulation, 2023) and it’s among the most-used fungicides on a variety of fruits and vegetables throughout the USA (Wieben, 2021). Unfortunately, the most recent publicly available data on pesticide use in the USA is from 2018-2019, so it’s impossible to know precisely which pesticides have been used for the past five years. This is a pretty serious issue for anyone who wants to understand recent patterns of pesticide use in the USA. But that’s a topic for a different column.
For this month’s column, we’re going to focus on the fungicide captan, which is sprayed on crops while bees actively gather pollen and nectar from those crops (Photo 1). Does exposure to captan impact growth and survival of developing honey bee larvae? Does co-exposure with the neonicotinoid insecticide thiamethoxam increase or decrease the toxicity of captan to honey bee larvae? And do the effects on larvae scale up to whole colonies in a year-long field trial? These are the topics for the seventy-ninth Notes from the Lab, where I summarize “Acute toxicity of the fungicide captan to honey bees and mixed evidence for synergism with the insecticide thiamethoxam,” published in Scientific Reports [2024] and written by Daiana De Souza and colleagues at the Dyce Lab for Honey Bee Studies at Cornell University. Full disclosure, I am a coauthor on this study.
For our study, De Souza and colleagues combined laboratory larval rearing trials with a year-long field trial …