monarch-esa-petition-final_61585

Mice also kill large numbers of wintering monarchs. Mice are estimated to kill about 5 percent of
butterflies in a given overwintering colony (Brower et al. 1985, Glendinning et al. 1988). One
species in particular, the black-eared mouse (Peromyscus melanotis), preys extensively on
monarchs, establishing residency inside monarch colonies, and feeding on live, moribund, and
recently dead monarchs on the forest floor (Glendinning 1993, p. 324). In cold conditions,
monarchs fall to the ground at night, and though some re-animate once the sun rises, those that
have fallen to the ground are exceedingly vulnerable to predation. Overwintering adults are also
subject to predation from wasps (Leong et al. 1990). Monarchs are susceptible to very high
levels of predation when they are clustered during the winter, but adults also face a number of
predators during migration and the breeding season including birds, wasps, spiders, mantids, and
dragonflies (Smithers 1973, White and Sexton 1989, in Prysby 2004, p. 27).
Monarch caterpillars and eggs are also subject to extremely high levels of both predation and
parasitism. A large suite of invertebrate predators including ants, spiders, crab spiders, and wasps
prey on developing monarch larvae, and several species of flies and wasps parasitize larvae.
Monarch toxins do not stave off the very high levels of predation and parasitism from
invertebrate natural enemies (Prysby 2004, p. 36). Only approximately 8 to 12 percent of
monarch eggs and larvae survive through metamorphosis (Borkin 1982, Oberhauser et al. 2001,
Prysby 2004, p. 27), indicating that a large population size is required to maintain population
growth.
Twelve species of tachinid flies, and brachonid and pteromalid wasps are known to parasitize
monarch caterpillars, with the tachinid fly Lespesia archippivora (Order Diptera) being a
primary predator responsible for high rates of parasitism (Oberhauser 2012, p. 20). A single
monarch pupa can host up to ten tachinid fly maggots (Altizer and de Roode 2010, p. 20).
Studies of parasitism rates from tachinid flies have been reported from many regions and include
mortality rates in the study area of one percent in southern Ontario, 12 percent in Wisconsin, 42
percent in Hawaii, 43 percent in Texas and Louisiana, 70 – 98 percent in central Mexico, and
100 percent in study sites in Australia (Prysby 2004, p. 28). Parasitoid flies alone comprise a
very significant source of mortality for monarch caterpillars, and have been identified as a major
factor regulating wild monarch populations (Altizer and de Roode 2010, p. 20). In addition to
parasitoid flies, developing monarchs have numerous other predators (Prysby 2004, p. 35,
Oberhauser et al. 2007, Oberhauser 2012, p. 20).
Spiders and ants have also been identified as contributing to high levels of early mortality, with
ants being a significant predator during the egg stage (Lynch and Martin 1993, Prysby 2004, p.
36). Calvert (1996) reported 100 percent mortality of monarch eggs and larvae in a one-hectare
Texas prairie due likely to predation from fire ants (Solenapsis invicta Buren, Formicidae) (p.
149).
The high rates of mortality of monarch eggs, caterpillars, and adults from predation underline the
importance to the long-term survival of the species of having a very large population size, and
magnify the threat posed to the long-term survival of the species of recent population declines of
more than 50 percent from the 17-year average in the west and more than 90 percent from the
20-year average east of the Rockies. Monarch reproductive success is dependent on large
Monarch ESA Petition 78