Uptake and toxicity of clothianidin to monarch butterflies from milkweed consumption

Abstract

Recent concern for the adverse effects from neonicotinoid insecticides has centered on risk for insect pollinators in general and bees specifically. However, natural resource managers are also concerned about the risk of neonicotinoids to conservation efforts for the monarch butterfly (Danaus plexippus) and need additional data to help estimate risk for wild monarch butterflies exposed to those insecticides. In the present study, monarch butterfly larvae were exposed in the laboratory to clothianidin via contaminated milkweed plants from hatch until pupation, and the effects upon larval survival, larval growth, pupation success, and adult size were measured. Soils dosed with a granular insecticide product led to mean clothianidin concentrations of 10.8–2,193 ng/g in milkweed leaves and 5.8–58.0 ng/g in larvae. Treatment of soils also led to clothianidin concentrations of 2.6–5.1 ng/g in adult butterflies indicating potential for transfer of systemic insecticides from the soil through plants and larvae to adult butterflies. Estimated LC50s for total mortality (combined mortality of larvae and pupae) and EC50 for larval growth were variable but higher than the majority of concentrations reported in the literature for clothianidin contamination of leaves.

Larval pesticide exposure impacts monarch butterfly performance

“…However, clothianidin had relatively weak effects at both concentrations for all variables measured, with a moderate and only marginally significant reduction in survival at the maximum concentration tested. While our statistical power to tease apart relatively minor differences in mortality was low, this outcome corresponds closely with data reported by Bargar et al 47 who calculated clothianidin LC 50 values ranging from 47 to 205 ng/g for monarchs experiencing chronic exposure over their entire larval period, and Krishnan et al 45 who reported a range of 800-7,800 ng/g for acute exposure across instars. Given that 56.55 ng/g was the maximum value we ever recorded in > 1,000 milkweed leaf samples at multiple sites over 2 years 52 and other studies reported even lower maximum concentrations (4.02 ng/g in field samples in South Dakota 46 ), we consider it unlikely that clothianidin is a major driver of monarch declines.…”

“…Overall, our data point to several key messages. First, based on the results from this experiment combined with several other recently published studies 45,47,52,75 , it seems unlikely that neonicotinoids are one of the primary drivers of monarch declines. This is not to say that neonicotinoids are completely innocuous, but compared with other ongoing threats (e.g., loss of milkweed host-plants and overwintering habitat), they likely play a secondary role.…”

“…4A vs. B), this prior work also found that negative effects on adult traits were relatively insensitive to wide variation in concentration beyond the untreated control. Other studies testing monarch reaction to pesticides similarly show poor correspondence between dose and response 46,47 , even though a linear relationship is often assumed. The range of values chosen in our study, while seemingly high (i.e., the max.…”

Optimization of QuEChERS Method for Simultaneous Determination of Neonicotinoid Residues in Pollinator Forage

“…The proposed method allows for the quantification of the compounds in a single run at sub-ng/g concentrations using a faster and/or more sensitive method than those found in the literature [12][13][14]17,22,23,[26][27][28]. The method can quantify specific analytes in a mixture of neonicotinoids at trace levels in small quantities of milkweed leaf and pollen to facilitate exposure assessment for honey bees, native bees, and monarch butterflies [12,18,19,29].…”

“…To date, extraction of neonicotinoids from plant leaf tissues, specifically milkweed leaf tissue, is labor-intensive and involves complicated clean-up steps [16,[18][19][20][21]. These methods use various components, such as Celite, C 18 cartridges, concentration steps, sodium chloride, anhydrous magnesium sulfate, filtration, and solvent exchange.…”