Monarch Butterfly (Danaus plexippus) Life‐Stage Risks from Foliar and Seed‐Treatment Insecticides

Abstract

Conservation of North America's eastern monarch butterfly (Danaus plexippus) population would require establishment of milkweed (Asclepias spp.) and nectar plants in the agricultural landscapes of the north central United States. A variety of seed-treatment and foliar insecticides are used to manage early-and late-season pests in these landscapes. Thus, there is a need to assess risks of these insecticides to monarch butterfly life stages to inform habitat conservation practices. Chronic and acute dietary toxicity studies were undertaken with larvae and adults, and acute topical bioassays were conducted with eggs, pupae, and adults using 6 representative insecticides: beta-cyfluthrin (pyrethroid), chlorantraniliprole (anthranilic diamide), chlorpyrifos (organophosphate), imidacloprid, clothianidin, and thiamethoxam (neonicotinoids). Chronic dietary median lethal concentration values for monarch larvae ranged from 1.6 × 10 -3 (chlorantraniliprole) to 5.3 (chlorpyrifos) μg/g milkweed leaf, with the neonicotinoids producing high rates of arrested pupal ecdysis. Chlorantraniliprole and beta-cyfluthrin were generally the most toxic insecticides to all life stages, and thiamethoxam and chlorpyrifos were generally the least toxic. The toxicity results were compared to insecticide exposure estimates derived from a spray drift model and/or milkweed residue data reported in the literature. Aerial applications of foliar insecticides are expected to cause high downwind mortality in larvae and eggs, with lower mortality predicted for adults and pupae. Neonicotinoid seed treatments are expected to cause little to no downslope mortality and/or sublethal effects in larvae and adults. Given the vagile behavior of nonmigratory monarchs, considering these results within a landscape-scale context suggests that adult recruitment will not be negatively impacted if new habitat is established in close proximity of maize and soybean fields in the agricultural landscapes of the north central United States. Environ Toxicol Chem 2021;00:1-17.

Host Plant Species Mediates Impact of Neonicotinoid Exposure to Monarch Butterflies

“…Using the same concentrations from Experiment 2, we conducted a small bioassay on bumblebees to confirm the toxicity of clothianidin and imidacloprid used in our monarch studies (Supplemental Materials). Based partly on findings by Krishnan et al [40,56], we predicted that monarchs would be more tolerant of neonicotinoids (e.g., the lethal dose would be higher) than previous studies indicated, such as Pecenka and Lundgren [41]. In terms of flight performance, we predicted that monarchs exposed to neonicotinoids as larvae would fly shorter distances and fly slower, perhaps because of impaired neurological functions.…”

“…Olaya-Arenas et al [39] found no effect of clothianidin on survival or developmental time below 54 ng/g leaf on A. syriaca. In leaf disc studies, Pecenka and Lundgren [41] found an LC50 of approximately 9.8 ng/g (converted in Krishnan et al [56]) for clothianidin on A. incarnata. Krishnan et al [40] found an acute LC50 of 9.4 ug/g for imidacloprid and 0.8 ug/g for clothianidin on A. curassavica, which was similar to Krischik et al's [42] finding of 6 ug/g imidacloprid on A. curassavica.…”

“…In the current study, we preserved insufficient leaf biomass to comprehensively analyze chemical residues for both neonicotinoids and cardenolides across the separate milkweed species. Importantly, three previous studies [38,39,56] showed similar neonicotinoid effects using different milkweed species, although these studies did not compare multiple host plant species within the same experiment.…”

“…Krishnan et al [40] found an acute LC50 of 9.4 ug/g for imidacloprid and 0.8 ug/g for clothianidin on A. curassavica, which was similar to Krischik et al's [42] finding of 6 ug/g imidacloprid on A. curassavica. See Krishnan et al [56] for further details and summaries of findings from acute and chronic exposure to neonicotinoids.…”

Conservation risks and benefits of establishing monarch butterfly (Danaus plexippus) breeding habitats close to maize and soybean fields in the north central United States: A landscape‐scale analysis of the impact of foliar insecticide on nonmigratory monarch butterfly populations

“…Olaya‐Arenas et al (2020) also did not detect any synergistic effects when monarch larvae were chronically exposed to milkweed leaves treated with a cocktail of insecticides, fungicides, and herbicides at mean and maximum concentrations reported in Indiana agricultural settings. To further explore insecticide‐use patterns on landscape‐scale adult production, we could also incorporate the acute toxicity of foliar applications to eggs, pupae, and second‐generation adults as well as chronic insecticide effects on larvae that survive exposure to spray‐drift residues (Krishnan et al, 2021). However, as noted previously, even when assuming 100% mortality of monarchs downwind of a treated field, landscape‐scale adult productivity is greater when habitat is established within and outside the 38.1‐m buffer zone (Scenario 3) than when new habitat is planted only outside the no‐plant zone (Scenario 4).…”