Monarch Predators and Pathogens
Common Monarch Predators
Unavoidable Cannibalism: When larvae emerge from their egg they often eat the egg and begin eating leaf tissue. If there is another egg nearby, the larva will inevitably eat the egg. By controlling the number of eggs oviposited on each leaf will reduce the mortality rate of emergence.
Few Enemies: There are few predators for eggs of monarchs. Ants, mites, and spiders can sometimes be harmful but easily controlled by removing them by hand or a small paint brush.
Ants: Many genera of ants (Formicidae) feed on monarch larvae, especially when they are young. Fire ants (Solenopsis geminata) are the most devastating. The basic solution to controlling ants would be to put ant poison near the plants, but not next to the plants.
Wasps: Large wasps from the Vespidae family often depend on caterpillars for food. Paper Wasps (Polistes spp.) are notorious for locating and capturing monarch larvae on milkweed plants to feed their young.
Flies: Several species of flies from the Tachinidae family parasitize larger monarch larvae with two or three eggs. The emerging fly maggots feed as internal parasites and do not kill the monarch larvae. After pupating, the maggots exit the monarch pupa on a silk thread and they pupate nearby on the ground. Tachinid problems may be avoided by rearing larvae indoors. If the monarch larvae are on milkweed outside, collect them when they are young and bring them indoors to rear.
Spiders: Small species of leaf-rolling spiders can be a constant pest. It sows the edges of a milkweed leaf together with silk to make a nest. It usually comes out at night and feeds on young monarch larvae. These small spiders can simply be pinched when they are inside the nest. Larger monarch larvae have few spider predators.
Assassin Bugs: Assassin bugs from the family Reduviidae are known to feed on monarch larvae. It thrusts its cutting beak into the victim, injects an immobilizing digestive agent, then sucks out the body liquids.
Birds: There are many larvae feeding birds that are not affected by the toxic cardiac glycosides in milkweed plants when they consume monarch larvae. A good example is the California towhee (Pipilo crissalis) which has been reported to consume a large number of monarch larvae in a short period of time.
Few Enemies: Most problems in the pupae stage originate from parasitoids and pathogens the larvae acquire. Occasionally wasps from the family Braconidae parasitize the pupae. Tiny wasps from the family Chalcididae unsuccessfully penetrate the pupa casing therefore leaving a small hole. The pupa begins to turn dark and dies.
Spiders: There are few spider webs above the ground that will capture a monarch, because of its size and strength. However, large orb weaver spiders, Araneus species, is a common predator at overwintering sites. It only feeds in the autumn and makes a thick sticky web at the level of height where the monarchs roost and fly.
Birds: There are many species of birds that consume adult monarchs. Their digestive tracks tolerate high levels of the poisons from cardenolides in milkweed that the larvae consume. Several bird genera are predators at California monarch overwintering sites and can reduce the population by a high percentage in a short period of time. The most common birds of prey are the Cassin’s kingbird (Tyrannus vociferans), rufous-sided towhee (Pipilo erythrophthalmus), chestnut-backed chickadees (Parus rufescens), starings (sturnus vulgaris), and scrub jays (Aphelocoma coerulescens).
Baculoviruses are pathogens, like some human viruses, which attack insects and arthropods. Many butterfly species, especially monarchs, are targets for the Baculovirus species nucleopolyhedroviruse, commonly called NPV. The virus is a tiny particle that enters through spiracles (air passages) or is eaten by larvae. Selected butterflies have DNA codes for genes needed for virus reproduction. The cells of the larvae produce more virus particles until it “melts” with black liquid, dies, and releases more virus particles into the immediate environment destroying all butterfly larvae. One larva may produce billions of the virus polyhedral.
Ultraviolet rays and bleach help control the spread of the virus. Leaves with infected larvae should be carefully removed (including larvae on the soil) and placed in the garbage. Place the contaminated soil from potted plants outside in a garden where there is full sun. Clean cages/containers and table area with 15% bleach with water, and rinse after 30 minutes. Place the cages in full sun for several days. Try not to overcrowd the larvae in a single cage or environment. To see excellent photographs of monarch larvae infected with NPV go to: www.huffingtonpost.com/2011/09/14/zombie-caterpillars-virus_n_962256.html
Neogregarin Ophryocystis elektroscirrha
This pathogenic protozoan is a tissue specific parasite from the family Neogregarinidae, commonly called O.e. When the larvae eat the spores, the disease infects the hypodermal tissue, remains in micronuclear schizogony until after pupation of the monarch, and then rapidly completes morphogenesis in the tissue that becomes the scales of the adult. The adult carries the protozoa spores externally and transfers them to the surface of the egg and plant. Due to this life-history strategy of the parasite, premature killing of its host is not beneficial, as it would not be able to propagate itself.
Nevertheless, monarchs that are heavily infected often die before they mate, don’t live long, and are smaller than normal. If the parasites are going to be more benign in any population, it would be the eastern population because the monarchs fly the farthest distances, and parasites which kill their hosts during this long journey would not produce offspring. Overwintering monarchs in California have a high level of infection because many come from milkweed colonies that produce several generations of monarchs. Four hundred and eighteen abdominal samples taken in the autumn of 1996 from eleven overwintering sites, from Marin County to Ensenada, Baja California, Mex., revealed that monarchs had a gradual decrease in spore loads from 85% to 8% from Santa Barbara to Ensenada. This suggests that many monarchs in Santa Barbara may be residents with an origin from local milkweed stands. Monarchs south of Santa Barbara that are less infected are probably from Arizona and Baja California where dense summer colonies of monarchs are uncommon.
After years of experiments to try and control O.e., there has been only one method that works. If a gravid female is spore free and placed in a small netted enclosure with a few potted milkweed plants, she will lay eggs that are spore free. This new generation, and others, will be “clean” if they are not contaminated by mating with monarchs outside of the gene pool. The spores are easy to detect by rubbing their abdomen on a clear piece of tape, putting it on a 4X5 white card, and placing it under a microscope with 100 power with using light from a lamp shinning down from above. Images of O.e. spores may be seen at www.butterflyfunfacts.com/oe.php.