Non-Target Effects of Clothianidin on Monarch Butterflies

This study determined the lethal and sublethal doses of clothianidin for monarch larvae (Danaus plexippus) based on a 36-hour exposure scenario. The authors also quantified clothianidin levels found in milkweed leaves adjacent to maize fields. The monarch larvae were fed clothianidin for 36 hours in order to reflect the hypothesized shorter pulses of clothianidin exposure they may encounter in the field (expected to be coincidental with planting or maize anthesis).

Larvae were fed clothianidin in nine treatments (1000, 500, 100, 50, 25, 10, 5, or 1 ppb) or assigned to two controls (water or unfed) in the lab. Fed larvae received the clothianidin or water on a leaf disc of swamp milkweed (Asclepias incarnata). For the clothianidin analysis in field, the presence of clothianidin was tested in milkweed (Asclepias syriaca) plants a mean of 1.47±0.39 meters from maize fields in Brookings County, SD, soon after planting (mid-June – late due to a wet spring) and when monarch populations were most abundant (mid-July). 

Based on the dose-response experiment, the LC doses were determined to be: LC10 – 7.72 ppb; LC20 – 9.89 ppb; LC50 – 15.63 ppb; and LC90 – 30.70 ppb. Significant differences were seen in development time, body length, and weight for newly eclosed second instars. Body length and head capsule width of newly eclosed third instars were also significantly affected by treatment. Mean clothianidin found per plant in the field study was 0.58±0.07 ppb, with a maximum of 4.02 ppb in one plant. Twice the proportion of plants per site were contaminated in July compared to June. 

This study documents sublethal effects of clothianidin at exposure levels observed under field conditions, indicating that neonicotinoids could negatively affect larval monarch populations. Only two individuals fed 50 ppb of clothianidin survived, and none at higher doses, so the sublethal effects analyses focused on individuals fed 25 ppb or less. Smaller individuals are more susceptible to predation and other stressors, which combined or individually may contribute to population declines. In the field, clothianidin was found more often in milkweed when monarch larvae are typically at their peak numbers than earlier in the season near planting. This suggests that clothianidin persists in the milkweed and larvae are likely exposed to it throughout the larval stages rather than only for a short period of time as reflected in the 36-hour assay. Therefore, the 36-hour exposure is likely an underestimate of the length of exposure in the field. The authors note that A. syriaca has thicker leaves than A. incarnata that was used in the assay, which could affect the final dose consumed by the larvae in the field. While preliminary, this study shows that monarch larvae are exposed to clothianidin in the field at potentially harmful doses.