Airborne particulate matter and health effects on bees: A correlation does not indicate causation

Thimmegowda et al. (1) analyzed the sublethal effects of exposure of the honey bee Apis dorsata to high levels of airborne particulate matter (PM) in polluted areas in Bangalore (India). Airborne PM is a mixture of chemicals; its solid components are commonly classified by size, ranging from several micrometers (PM10) to a few nanometers (PM0.1). It is well established that the size and composition of PMs determine their adverse health effects in humans (2). Bees are exposed to pollutants, especially airborne particles, during foraging (3⇓–5); thus, studies on the effect of PM on the health of pollinators are important. Existing studies on toxic effects of PM on bees are limited and carried out under controlled conditions (6, 7). Thimmegowda et al. (1) correlated airborne PM levels in urban sites with the differences in behavior, physiology, and gene expression in bees. Furthermore, field-exposed fruit flies showed similar health outcomes. Although interesting, the study has some major flaws and the conclusions are not convincing.