Status, Distribution & Conservation

The Monarch Butterfly, Danaus plexippus L., in Canada


The Monarch butterfly, Danaus plexippus (L.) (Lepidoptera: Danaidae), is a migrant which colonizes Canada each year in the late spring to breed, re-populating its range from its overwintering grounds. There are two distinct populations: a small western one found in Canada only in British Columbia, which overwinters in California, and a much larger eastern population, found east of the Rocky Mountains throughout the rest of Canada, which overwinters in central Mexico. The Monarch varies greatly in abundance and distribution from place to place and from year to year in Canada but, since the Monarch breeds in Canada and the northern U.S., having from one to three generations depending on latitudinal location and other edaphic factors, it generally becomes more common as the summer progresses. Monarchs are most often noticed when they assemble to begin the fall migration south. The large eastern population is most abundant in Ontario due in large part to the low latitude and modifying effect of the Great Lakes on the climatic regime. Eastern Canada provides a significant proportion of the fall migrants that overwinter in Mexico.

The distribution of the Monarch in Canada is determined largely by the distribution of its obligate larval hostplants, the Milkweeds (Asclepias sp.; Asclepiadaceae). There is little doubt that land use changes over the last 200 years have greatly aided the expansion of milkweeds, especially the Common Milkweed, A. syriaca, in the east and Showy Milkweed, A. speciosa, in the west, thus increasing the available range of the Monarch. For example, there has been a demonstrable increase in the abundance and distribution of Monarchs in Ontario over the last 5 decades. A number of conservation issues may affect this distribution greatly in the coming years. These issues include further land use changes, global warming and the concomitant increase in carbon dioxide and low-level ozone, weed control legislation and programs which have direct effects on hostplant availability as well as indirect effects on the habitat as a whole due to herbicide and pesticide spraying, and the spread of invasive alien plant species which alter open habitats and may compete as inappropriate larval hosts.

Of major importance also is the ongoing degradation and destruction of the overwintering roosts in Mexico, as well as future conservation efforts by the United States. All of the breeding Monarchs which make their way to eastern Canada each year have bred in the southern U.S. - the butterflies which reach Canada are one or two generations removed from those that overwintered in Mexico. International cooperation between these three nations is essential to encourage the continuation of the "endangered phenomenon" that is the migration of the Monarch butterfly. International cooperation can provide benefits through protection of the overwintering sites in Mexico and California and through protection of the spring and autumn migratory pathways through the United States. Their protection has a direct impact on the numbers of Monarchs that return to Canada each spring, in turn, Canadian conservation efforts have far-reaching effects on recruitment to the overwintering roosts.