Monarchs become toxic to predators by sequestering toxins from the milkweed they ingest as larvae, and are brightly colored in both the larval and adult stages to warn predators of this toxicity. Despite the fact that milkweeds are assumed to convey some degree of protection from generalist predators and parasitoids, monarchs of all life stages are vulnerable to predation and disease.
Monarch eggs and small larvae face considerable dangers of predation, and only about 5% of monarchs reach the last larval instar. Ants, spiders, true bugs, beetles, and lacewing larvae are some known predators of monarch eggs and larvae. In a laboratory experiment, one lacewing larva was observed consuming 40 monarch eggs. Chinese mantids and paper wasps have also been observed preying on immature monarchs. Adults face less danger of being eaten by predators during the breeding season, but there is a much greater risk of being eaten by bird predators in overwintering locations. In Mexico, the black-headed grosbeak, black-backed oriole, and Scott’s oriole are responsible for much of the predation of overwintering monarchs, with some additional predation by mice. In California, Rufous-sided towhees consume adult monarchs in overwintering clusters.
Parasitoids develop by feeding in or on a host organism, causing its eventual death. Monarch parasitoids are reported to include 12 species of tachinid flies and at least one braconid wasp. The best-studied monarch parasitoid is the tachnid fly Lespesia archippivora, which attacks larvae, resulting in the death of late-instar larvae or pupae. Some sites where tachinid fly parasitism has been studied have found parasitism rates of up to 90%, but the average rate is between 10 and 20% in the wild. Recent studies have documented a pupal parasitoid of monarchs, Pteromalus cassotis. These tiny wasps lay eggs inside a monarch chrysalis, which emerge as adult wasps from the monarch pupa casing a few weeks later. More research is needed to understand P. cassotis and the effects this species has on monarch populations. Parasitoids, such as those mentioned here, are often introduced as biological control agents to rid an area of unwanted pests. Bio-control agents often have harmful non-target effects on beneficial species, like monarchs or other pollinators.
Monarch larvae are generally found singly on milkweed plants, unlike the large aggregations of adults in overwintering clusters. Lower larval density in milkweed patches reduces the chance of diseases, such as nuclear polyhedrosis virus and Pseudomonas bacteria, spreading between larvae. These diseases are often fatal to monarchs.
Perhaps the most-studied parasite of monarchs is a protozoan parasite called Ophryocystis elektroschirra (OE). This parasite cannot be transferred between larvae or adults simply by contact. To become infected, a larva must ingest dormant OE spores that fall from the abdomen of an infected adult to the surface of milkweed leaves. While it is often not fatal, OE can have negative effects on survival, mass, and life span of monarchs. There is a higher occurrence of this parasite in populations that do not migrate, such as the one in southern Florida. The eastern migratory population has the lowest occurrence of OE, likely due to the fact that infected monarchs are less likely to make it to their overwintering destinations in Mexico and therefore will not reproduce and spread the parasite. Recent studies about OE and exotic milkweed describe how the year-round presence of tropical milkweed in some parts of the US may be facilitating the spread of this parasite. For more information, please read our Potential Risks of Growing Exotic Milkweeds for Monarchs flyer.