Experimental field evidence shows milkweed contaminated with a common neonicotinoid decreases larval survival of monarch butterflies

1. Neonicotinoid insecticides are the most widely used class of insecticides in the world and can have both lethal and sub-lethal effects on non-target organisms in agricultural areas. Monarch butterflies (Danaus plexippus) have experienced dramatic declines in recent decades and, given that a large proportion of milkweed on the landscape grows in agricultural areas, there is concern about the negative effects of neonicotinoids on this non-target insect.

2. In the field, we exposed common milkweed (Asclepias syriaca), an obligate host plant of monarch butterflies, to agriculturally-realistic levels of clothianidin, a widely used neonicotinoid insecticide. We tested whether this treatment influenced the number of eggs laid and larval survival over two years.

3. Milkweeds were transplanted into 60 experimental plots alongside a corn crop planted with a clothianidin seed-coat and 60 control plots alongside an untreated corn crop. The number of eggs, larvae at each stage (first–fifth instar), and presence of other arthropods were recorded weekly from June to the end of August and survival from egg to fifth instar was estimated using a Bayesian state-space statistical model.

4. We counted more eggs in treated plots compared to control plots, suggesting a preference for treated milkweed. The number of plots with arthropods did not differ between treatments, but within treated plots, there was a greater decrease in the number of arthropods throughout the season. There was no evidence that monarchs selected plots with fewer arthropods for oviposition. Larval survival was lower in clothianidin-treated plots compared to control plots.

5. Our results suggest milkweed near clothianidin-treated crops can reduce larval survival of monarch butterflies. While we provide some evidence that clothianidin could also act as an ecological trap for this species, further work is needed to identify additional components of fitness, including individual egg-laying rates and survival beyond the pupal stage. Our findings add to a growing body of evidence that neonicotinoids can negatively affect non-target organisms.