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.