Parasites & Natural Enemies
Research Projects
Monarchs have many natural enemies—predators, parasitoids, and parasites can harm
monarch eggs, larvae, pupae, and adults. Predators are organisms that kill and consume
other organisms (prey) to obtain energy and nutrients. Predators
such as spiders and ants attack eggs and young larvae feeding on milkweed, whereas
birds and wasps can prey on adult monarch butterflies. Parasitoids
are specialized insects such as small flies and wasps that lay eggs on or inside
other insects. Parasitoid larvae then eat their prey from the inside out, usually
emerging from the prey carcass as a pupa or adult. Parasites
are smaller organisms that live and multiply inside their hosts, taking nutrients
and resources. Parasites can be unicellular microbes such as viruses and bacteria,
or larger organisms like mites and nematodes.
Check out the following subtopics on this page:
Monarch Defenses and Warning Coloration
Many prey species have mechanisms to avoid predation, including camouflaged coloration
or bright eye-spots to confuse predators. Bright coloration in insects and other
animals (typically yellow, orange, or red) can act as a signal, warning other animals
that they are poisonous or distasteful. Such color patterns are called
aposematic. When an animal attacks, eats, or encounters such a brightly colored
animal and gets stung, bitten, or poisoned, it learns to associate these warning
colors with a bad experience. Monarchs have a chemical defense that is toxic to
many natural enemies -- they can sequester poisonous compounds from milkweed called
cardenolides, or cardiac glycosides
(Zalucki et al. 1990, Ritland and Brower 1993, Brower et al. 1994, Frick and Wink
1995). Thus, when an animal eats a monarch and gets sick, it learns to avoid potential
prey with similar coloration. However, research has shown that these toxins break
down over time in adult monarchs, and by several weeks of age the butterflies are
much more palatable to predators (Fink and Brower 1981, Brower and Calvert 1985,
Brower 1988, Alonso M. and Brower 1994, Sakai 1994). In addition, the role of sequestered
chemicals in defending monarchs against parasitoids and pathogens has not been explored.
Predation
Birds such as black-beaked orioles and black-headed grosbeaks are common predators
at monarch overwintering sites. These species can eat large quantities of monarchs
without getting poisoned. This may result from the decay of toxins inside the monarchs’
bodies during the many months of migration and overwintering, or from the specific
feeding behavior of the birds. Orioles slit open the monarchs’ abdomens before
feeding, avoiding most of the toxin-rich cuticle. Grosbeaks, which eat the entire
abdomen, can tolerate higher levels of cardenolides in their digestive tracts. Research
has shown that predation by these two bird species accounts for over 60% of the
total monarch mortality during overwinter (Calvert et al. 1979). In some colonies,
up to 9% of the butterflies are eaten by birds during the winter, and this number
can be up to 15% when the forest is disturbed by logging, making it easier for the
birds to reach the branches on which monarchs cluster.
Invertebrate predators such as ants, spiders, and wasps attack monarch larvae on
milkweed plants (Prysby 2004). Wasps have been observed feeding on monarch abdomens
at a California overwintering site (D. Frey, personal communication), and fire ants
have been suggested as a major predator of monarch larvae in Texas (Calvert 1996).
Other research suggests that wasp predators may be sensitive to the chemical defenses
of monarch larvae, and that wasps fed monarch larvae with high cardenolide concentrations
had lower reproductive potential and more deformities in their nests (L.S. Rayor,
personal communication) than wasps that preyed upon less toxic caterpillars.
Parasitoids
Both fly and wasp parasitoids lay their eggs on monarch larvae, but the most important
larval parasitoid is probably a fly species in the family Tachinidae. This
family includes about 10,000 species, most of which parasitize Lepidoptera
(butterflies and moths), although they also parasitize Hymenoptera (ants
and bees), Heteroptera (true bugs and their relatives), Coleoptera
(beetles), Diptera (flies and mosquitoes), Dermaptera (earwigs),
Orthoptera (grasshoppers and crickets), Chilopoda (centipedes), as
well as some scorpions and spiders. Research in the Monarch Lab suggests that the
species Lespesia archippivora (La) is the most important monarch tachinid
parasitoid. It is widespread throughout North and Central America, has been found
in Brazil, and was purposely introduced into Hawaii for biocontrol in 1898. Monarch
larvae in the continental US and Hawaii are frequently parasitized by La,
and the Monarch Larva Monitoring Project documented
an overall parasitism rate of ~13% (Oberhauser et al. 2007). For information on
how you can contribute data that will aid in our understanding of this important
monarch enemy, visit the Monarch
Larva Monitoring Project website.
Female La lay eggs on the host integument (skin), and the fly larvae hatch
and bore into the host soon after oviposition. La complete their larval development
within the host, the maggots emerge from late larvae or pupae, and then pupate in
leaf litter and eclose within ~10-14 days. Fly maggots drop to the ground on long,
gelatinous tendrils that look like white strings hanging from the monarch.
Less is known about the extent to which other parasitoids attack monarchs, but at
least one wasp in the family Braconidae has been reported in monarchs (Arnaud
1978). The closely-related queen, Danaus gilippus is parasitized by two Chalcid
wasps, Brachymeria annulata and B. ovata (Prudic and Olson 2005),
as well as L. archippivora (Arnaud 1978). Current research in the Monarch
Lab demonstrates that the wasp Pteromalus puparum (in the family Pteromalidae
and the same superfamily, Chalcidoidea, as the two Chalcid wasps found in
queens) could be an important pupal parasitoid (Oberhauser et al. in preparation).
Pteromalus puparum wasps are tiny, and over 200 can emerge from one monarch
pupa.
Parasites and Diseases
Parasites are small organisms that complete most or all of their life cycle within
a host, and many are capable of a high degree of within-host replication. Not all
parasites kill their hosts, but parasites almost always have negative effects on
host survival and reproduction. Many parasites and disease-causing pathogens are
known to attack insects, including viruses, bacteria, fungi, protozoans, nematodes,
and mites. Several viral and bacterial pathogens can infect monarchs, including
a nuclear polyhedrosis virus and Pseudomonas bacteria (Brewer and Thomas
1966, Urquhart 1987). Protozoan parasites such as Ophryocystis elektroscirrha
and a microsporidian Nosema species have also been identified in wild and
captive monarchs (McLaughlin and Myers 1970, Leong et al. 1992;1997, Altizer and
Oberhauser 1999, O. Taylor, personal communication). The infective stages of most
insect parasites must be consumed orally, although some can invade though pores
or membranous joints in the insect cuticle. Many researchers are currently exploring
the role of parasites and infectious diseases in regulating insect population size
(E.G. Faeth and Simberloff 1981, Bowers et al. 1993, Jaenike 1998).
Several Monarch Larva Monitoring Project volunteers have reported a horsehair worm
(similar to a nematode) in monarchs collected in the southern US.
Ophryocystis elektroscirrha
Ophryocystis elektroscirrha is a protozoan parasite that was first recovered
from monarch and queen butterflies in Florida in 1966 (McLaughlin and Myers 1970).
New infections occur when larvae ingest parasite spores as they feed on contaminated
egg shells or milkweed leaves. Most spores are transmitted from infected adults
to their offspring (vertical transmission), although horizontal transmission may
also occur. Following ingestion, spores lyse in larval guts. Emerging sporozoites
then penetrate the intestinal wall, enter the hypoderm, and undergo two phases of
vegetative, asexual replication. After host pupation, the parasite undergoes sexual
reproduction and forms dormant spores around the scales of the developing adult
butterfly (McLaughlin and Myers 1970). Most spores form on the adult abdomen, although
spores also develop on the wings, head, and thorax (Leong et. al. 1992; S.M. Altizer,
personal observation).
Heavily infected adults have difficulty emerging from their pupal cases and expanding
their wings, although adults with low parasite loads appear normal (McLaughlin and
Myers 1970; Leong et al. 1992). High parasite doses decrease larval survivorship
from hatching to eclosion, and heavily captive adults are smaller and shorter-lived
than uninfected adults (Altizer and Oberhauser 1999). Researchers in Sonia Altizer’s
lab at the University of Georgia are studying rates of parasitism by Oe and
its effects on monarchs. For more information about this disease and how you can
join in this research, visit monarchparasites.org.
References
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in monarch butterflies. Experientia (Basel) 50(2). 176-181.
- Altizer, S. and Oberhauser, K. 1999. Effects of the protozoan parasite, Ophryocystis
elektroscirrha, on the fitness of monarch butterflies (Danaus plexippus).
Journal of Invertebrate Pathology. In press.
- Arnaud, P.H. Jr. (1978) A host-parasite catalog of North American Tachinidae
(Diptera). U.S. Department of Agriculture Miscellaneous Publication No. 1319,
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- Brower, L. 1988. Avian predation on the monarch butterfly and its implications
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