This spring, I received several phone calls from beekeepers, growers and pesticide applicators regarding the risk posed to honey bees from insect growth regulators (abbreviated as “IGRs”). These chemicals are used as insecticides to control insect pests, especially in agricultural settings. And since many of them are developed to be insect-specific (e.g., they target leaf-chewing caterpillars, not bees), they can potentially minimize risk to the friendly insects while controlling the pests that cause crop damage.
Due to the flurry of interest in this topic that was crossing my desk, I was pleased to see a new cutting-edge publication on IGRs and bees. So, for our eighth “Notes from the Lab,” it’s time to delve into the risks posed to honey bees from IGRs, where we highlight “The effects of the insect growth regulators methoxyfenozide and pyriproxyfen and the acaricide bifenazate on honey bee (Hymenoptera: Apidae) forager survival,” written by Adrian Fisher and colleagues and published in the Journal of Economic Entomology [111:510-516 (2018)].
Before we get into their study, a quick analogy. I’m sure everyone is familiar with the hormone called testosterone. If you’ve lived through puberty yourself, or you’ve seen junior high school boys go through puberty, you know that hormones can greatly influence behavior and physique!
Well, think how testosterone influences adolescent males and multiply that by 100. Now you’re starting to understand how juvenile hormone influences insects. If some insects are exposed to too much juvenile hormone, they get stuck between being a larva and pupa and never complete development. Or their eggs can become sterile. Or they face several other scenarios that can lead to a slow or quick death. In other words, if you spray juvenile hormone on some insect pests, it could be a really effective chemical that kills the bad guys!
OK, now for the study. The authors observed that several chemicals were being sprayed on California almonds during bloom, including the IGRs pyriproxyfen and methoxyfenozide and the miticide bifenazate. None of these chemicals were developed to harm bees, which is why they were being sprayed during bloom while the bees were out pollinating the almonds.
So… were the chemicals safe for bees? To answer this question, the authors set up a nice assay that mimicked how bees would be exposed to the chemicals during pollination. They collected several forager bees, put them in a wind tunnel (basically, just a fancy box that you can blow air through), and sprayed them with several doses of the IGRs or the miticide: 1/2x, 1x, 2x, and 3x the recommended dose that pesticide applicators would use. Importantly, they found that foraging bees were about 5 times more likely to die when exposed to each of the chemicals compared to controls (bees sprayed with water only).
Wow, 5 times more likely to die… but I thought IGRs and miticides weren’t supposed to hurt bees? Unfortunately, before this study, there was actually very little known about the effects of pyriproxyfen, methoxyfenozide and bifenazate on honey bees.
Let’s take methoxyfenozide, for example. Methoxyfenozide is known to be quite effective at controlling lepidopteran pests (caterpillars and moths) by lowering their fertility and interfering with development. Thus, it’s a relatively popular pesticide that’s sold under the trade name Intrepid®. But is it safe for bees?
In methoxyfenozide’s Preliminary Environmental Fate and Ecological Risk Assessment (September 2016), the US Environmental Protection Agency (EPA) stated that while submitted field studies did not indicate adverse effects on the brood, a recently submitted lab study indicated….