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most commonly used adulticides are organophosphate (e.g., malathion, naled) and pyrethroid
(e.g., pyrethrin, permethrin, resmethrin, sumithrin, prallethrin) insecticides (Mazzacano and
Black 2013). These compounds have broad -spectrum toxicity and can cause severe impacts to
non-target animals including butterflies.
General losses of biodiversity in insect communities that affect a wide range of orders and
families have been noted by some researchers in areas where mosquito adulticides are sprayed
(Eliazar and Emmel 1991, Kwan et al. 2009). Multiple studies have also shown negative impacts
of mosquito treatments specifically on butterfly populations. Barrier treatments, in which
pesticide applied as a spray to foliage forms a coating that kills adults that come into contact with
it, can have lethal and sub-lethal effects on adult or immature butterflies.
The decline of the federally endangered Schaus swallowtail butterfly (Heraclides aristodemus
ponceanus), endemic to southern Florida, has been linked to pesticide applications for mosquito
control (Eliazar and Emmel 1991), as has the decline of the Miami blue butterfly (Cyclargus
thomasi bethunebakeri) (USFWS 2012, 77 FR 20948). Mosquito-control pesticides are also
considered to be a contributing factor in the extinctions of the Florida zestos skipper (Epargyreus
zestos) and rockland grass skipper (Hesperia meskei pinocayo) (see:
http://www.fws.gov/southeast/news/2013/032.html ).
In areas that are being treated for mosquitoes, monarchs can be exposed to the insecticides at the
larval stage if the insecticides are over-sprayed or drift onto milkweed hosts, or as adults if
butterflies are flying or visiting nectar plants during or after applications.
Researchers at the University of Minnesota investigated toxicity to monarchs of two insecticides
often used to control mosquitos (Oberhauser et al. 2006, Monarch Joint Venture 2014), and
found that monarchs did have higher mortality when exposed:
The University of Minnesota conducted research on how monarch larvae and adults were
affected by exposure to insecticides commonly used in mosquito control (resmethrin and
permethrin). These pyrethroids can be sprayed as ultra-low volume treatments or as barrier
treatments. Ultra-low volume treatments intended to affect insects as they are flying, whereas the
barrier treatments remain on leaves, providing a barrier to mosquitoes that may not be out
foraging during the day. Both the ultra-low resmethrin study and the barrier permethrin study
showed negative impacts on monarch larvae and adults. Leaves from the barrier treatments
resulted in higher mortality to monarch larvae than control leaves up to 3 weeks after the initial
application (Monarch Joint Venture 2014).
Increased mortality of monarchs from mosquito control programs is thus a significant potential
threat, although impacts have not been assessed.
Grasshopper Control on Rangelands in the Western United States
Insecticides are commonly used in rangeland areas across many western states to control native
grasshoppers that compete with cattle for forage. When grasshopper numbers are high, the
Animal Plant Health Inspection Service (APHIS) routinely facilitates spraying of insecticides to
control Mormon crickets and grasshoppers on public and private lands. APHIS lists three
Monarch ESA Petition 99