Oberhauser KS, Solensky MJ  (2012)  Monarch Butterfly Ecology.  ECOLOGY.INFO 28

Monarch Butterfly Ecology

Karen S. Oberhauser
Department of Fisheries, Wildlife and Conservation Biology
University of Minnesota,
St. Paul, Minnesota, USA

Michelle J. Solensky
Department of Biology,
The College of Wooster
Wooster, Ohio, USA

Note: This online review is updated and revised continuously, as soon as results of new scientific research become available.  It therefore presents state-of-the-art information on the topic it covers.

Monarch butterflies (Danaus plexippus) are native to North and South America.  In the 1800's, however, they spread to other parts of the world. Monarchs were first seen on Hawaii in the 1840's, and then throughout many South Pacific islands in the 1850's and 1860's (Ackery and Vane-Wright 1984). In the early 1870's, the first monarchs were reported in Australia and New Zealand (Gibbs 1994). It is not clear exactly how and why this emigration occurred. One possibility is that monarchs were transported by ships, either as larvae that found their way onboard from shipyard milkweed plants, or as adult monarchs that happened to land on ocean-going vessels. It is most likely that humans were involved in the process, but it is not known to what extent. Because North American monarchs often fly over 2,200 km during their migration, it is always possible that some made the journey on their own (Vane-Wright 1993).

In the western hemisphere, there are two subspecies of the monarch butterfly: (1) Danaus plexippus plexippus of southern Canada, the USA, Mexico, most Caribbean Islands, Central America and northern South America; and (2)  Danaus plexippus erippus of Peru, Bolivia, Chile, Argentina, Uruguay, Paraguay and eastern Brazil. Neither subspecies has been recorded in northwestern Brazil.

The foci of this review are the North American migratory populations, i.e. those monarchs that migrate to overwintering grounds in central Mexico and the west coast of the USA. The review is 6 pages long and is divided into five parts:

Monarch Breeding Ecology
Fall Migration Ecology
Monarch Overwintering Ecology
Spring Migration Ecology
References and Contact Information

The first section, Monarch Breeding Ecology, is two pages long and begins below.  The other sections are each one page long.

Monarch Breeding Ecology

Overview: Monarch larvae are specialist herbivores, consuming only host plants in the milkweed family (Asclepiadacea). They utilize most of the over 100 North American species (Woodson 1954) in this family, breeding over a broad geographical and temporal range that covers much of the United States and southern Canada. In a typical year, one generation is produced in the southern part of this range by returning migrants, and two to three generations are produced in the northern part.

Monarchs and Milkweed. Milkweed provides monarchs with an effective chemical defense against many predators. Monarchs sequester cardenolides (also called cardiac glycosides) present in milkweed (Brower and Moffit 1974), rendering them poisonous to most vertebrates. However, many invertebrate predators, as well as some bacteria and viruses, may be unharmed by the toxins or able to overcome them. The extent to which milkweed protects monarchs from non-vertebrate predators is not completely understood, but a recent finding that wasps are less likely to prey on monarchs consuming milkweed with high levels of cardenolides suggests that this defense is at least somewhat effective against invertebrate predators (Rayor 2004).

Benefits gained by monarchs from cardenolides are not without cost. Milkweed plants between and even within species vary a great deal in cardenolide concentration, and both the toxin and the sticky latex produced by the plants provide defenses against herbivores. Monarchs appear to be negatively affected by consuming plants with high cardenolide levels, and may actually starve to death when their mandibles are glued together by the latex or if their bodies become mired in a drop of latex formed when the plant is injured (Zalucki and Brower 1992; Malcolm and Zalucki 1996; Zalucki and Malcolm 1999; Zalucki et al. 2001).  Larger larvae reduce this risk by chewing a notch at the base of the milkweed leaf midvein, cutting off the flow of sticky latex to the rest of the leaf and allowing more efficient eating (see Figure 1).

Like other plants, milkweed quality as a host for insects varies. Many insects are nitrogen limited (McNeil and Southwood 1978, Mattson 1980, Scriber 1984, Slansky and Scriber 1985, White 1993). They must consume large quantities of their host plants to accumulate enough nitrogen for growth and development, since animal tissue generally consists of 7-14% nitrogen by dry weight (dw) and plants consist of 0.03-7.0% nitrogen dw (Mattson 1980). Leaf nitrogen levels vary within a season, as plant tissue ages and as plants allocate more resources to reproductive tissue. In addition, plants grow in habitats with different levels of available soil nitrogen. Lavoie and Oberhauser (2004) studied the response of monarch larvae to plants manipulated through fertilizer treatments to contain varying leaf nitrogen levels, and found that they compensated for low nitrogen leaves by consuming more plant tissue per day. If increased consumption makes them more vulnerable to predation or plant defenses, this could result in decreased fitness levels.

The most important northern host plant is Asclepias syriaca (common milkweed - Figure 2), but a number of other species are used as well, including A. incarnata (swamp milkweed - Figure 3) and A. tuberosa (butterfly weed - Figure 4). Central Plains host plants include the vine Cynanchum laeve (sand or honey vine). A northeastern invasive plant in the same genus (C. nigrum) has spread west as far as Wisconsin. This species is attractive to ovipositing females, but monarch larvae do not survive on it (Haribal 1998). In the south, the most important host plants are probably Asclepias oenotheroides (zizotes milkweed), A. viridis (spider milkweed) and A. asperula (antelope horn milkweed), all fairly common throughout Texas and other southern US states.

Egg production. It is difficult to tell just how many eggs female butterflies lay during their lives, but the average in the wild is probably 300 to 400. Captive monarch butterflies average about 700 eggs per female over 2 to 5 weeks of egg laying, with a record of 1179 eggs (Oberhauser 1997). Monarch eggs hatch about 4 days after they are laid, but the rate of development in this stage, like all other stages, is temperature dependent, with individuals in warmer environments developing more rapidly (Zalucki 1982). The proteins that are an important constituent of eggs must either be derived from nutrients ingested during the larval stage or obtained from males during mating (Boggs and Gilbert 1979, Oberhauser 1997). While an individual monarch egg (Figure 5) weighs only about 0.460 mg, about 1/1000 the adult mass, females often lay more than their own mass in eggs throughout their lives.

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