Monarchs in the Classroom

One factor likely to regulate the abundance of monarchs is their interaction with natural enemies. Many predators, parasites, and parasitoids attack monarch eggs, larvae, and adults. Predation occurs when one organism (a predator) kills and consumes another organism (a prey) to obtain energy and nutrients. Predators such as spiders and ants attack eggs and young larvae feeding on milkweed, whereas birds and wasps have been observed preying on adult monarch butterflies. Parasitoids are specialized insects such as small flies and wasps that lay eggs on other insects. Parsitoid larvae then eat their prey from the inside out, usually emerging from the prey carcass as a pupa or adult. Smaller organisms that live and multiply inside their hosts, taking nutrients and resources, are called parasites. Parasites can be unicellular microbes such as viruses and bacteria, or larger organisms like mites and nematodes.

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 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.

Monarch larvae and adults display bright warning coloration as a signal to potential predators.

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, and together they kill 7-44% of the total monarchs in any one colony (reference).

Orioles (L) are avian predators of monarchs overwintering in Mexico. Many dead monarchs litter the forest floor in Mexico (R), such as these victims of bird predation.

Invertebrate predators such as ants, spiders, and wasps attack monarch larvae on milkweed plants. Less is known regarding invertebrate predation in monarchs, but 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.

An assassin bug (L) pierces the cuticle of a monarch larvae and draws out the inner fluids and tissues. Ants (R) attack a fourth instar larva that crawled onto the wrong leaf.

Parasitoids

A few species of parasitoids lay their eggs on monarch larvae, including tachinid flies and braconid wasps. Tachinid fly larvae feed on monarch caterpillars, but usually don’t kill their hosts until just before the caterpillars pupate. When a parasitized caterpillar hangs upside down in the pre-pupal "J"-shape, several fly maggots will emerge from the dorsal anterior end and drop to the ground on long, gelatinous tendrils. Braconid wasp parasitism is less common, but as many as 32 tiny adult wasps may emerge from a single monarch carcass. Very little is known about variation in rates of parasitism and predation by invertebrates throughout the monarchs' range.

Monarch larva parasitized by tachinid flies (L). Parts of its body turned brown and transparent shortly before fly maggots emerged. Tachnid fly maggots (R) leave long tendrils hanging from the body of their host.

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 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).

A fifth instar larva (L) showing signs of bacterial decay shortly after death. The nuclear polyhedrosis virus that attacks monarchs has been called the "black death" (R) because infected larvae and pupae turn black and disintegrate following infection.