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2012, Nuyttens et al. 2013). This dust can contact monarch larvae and adults directly
during the planting operation. Dust also settles on plants, soil and water in and around
fields, where it can expose monarchs (both larvae and adults) after planting.
Unharvested plant material from crops that have taken up neonicotinoids from foliar
sprays, seed treatments, soil drenches, or other application methods contains residues and
metabolites that can be released into soil (Hopwood et al. 2013) and water as the roots,
stalks, and other plant parts decay, adding to environmental contamination.
Given the widespread use of neonicotinoid insecticides, especially as seed treatments, it can be
assumed that milkweeds and nectar plants in monarch habitats could be contaminated with
neonicotinoids. According to Goulson (2013, p. 981): “Given their persistence and accumulation
in soils, we might predict hedgerow plants and trees, field margin vegetation and naturally
regenerating fallows to take up neonicotinoids.”
Very low concentrations of neonicotinoids and other pesticides have been shown to cause sublethal
effects in other insects. For example, studies show sub-lethal impacts of pesticides on bee
species that include changes in foraging behavior, navigation ability, reduced reproduction, and
many other processes (Blacquière et al. 2012, Bryden et al. 2013, Goulson 2013), as well as
increased susceptibility to pathogens (Pettis et al. 2013). Sub-lethal effects of neonicotinoids on
fruit flies include a disruption of mating behavior (Charpentier et al. 2014). These types of
effects are of obvious relevance to monarch populations.
Several studies suggest that levels of neonicotinoids in milkweed and nectar plants that have
been exposed to contaminated soil, water, dust, or spray drift may reach concentrations that are
toxic to monarchs in some situations, based on studies so far:
The evidence presented here [in this review] suggests that the annually increasing use of
neonicotinoids may be playing a role in driving these declines [of farmland insects and
other taxa]. The concentrations accumulating in soil (1 to >100 ppb), waterways (often in
excess of 1 ppb, sometimes up to 200 ppb), field margin plants (1–9 ppb) and nectar and
pollen of flowering crops (1–50 ppb) exceed levels in crop tissues needed to control pest
insects (5–10 ppb) and overlap with LC50 values for a range of non-target insects. They
would appear to be sufficient to cause both direct mortality in the more sensitive
nontarget species and chronic sublethal effects in many more. The groups most at risk are
likely to include soil-dwelling insects, benthic aquatic insects, granivorous vertebrates
and pollinators. Herbivorous insects feeding on field margin and hedgerow plants may
also be exposed (Goulson 2013, p. 985).
Monarchs are in the “pollinator” risk category as adults, and the “herbivorous insects feeding on
field margin and hedgerow plants” risk category as larvae. Milkweeds have largely been
eradicated from corn and soybean fields, but thelands enrolled in the Conservation Reserve
Program and roadsides where common milkweed now hosts most monarch larvae and where
nectaring occurs are largely within agroecosystems where neonicotinoids are widely used.
Monarch ESA Petition 97