Status, Distribution & Conservation

The Monarch Butterfly, Danaus plexippus L., in Canada

General Biology


Population Dynamics

Borkin (1982) found that only 12% of immatures survived from egg to pupation in Wisconsin suggesting that egg and larval mortality are moderate to extreme at least in the northern part of the range. She reports that there is an initial "pulse" of larvae from immigrants followed by variation in local breeding yielding successive broods (Borkin 1982). Potentially the most limiting factor in butterfly populations is the ability of females to fly to find mates and then to lay their complement of eggs (Ehrlich 1984; Dennis 1993). A large suite of potential mortality sources - climatic events, predators, parasitoids, parasites, fungi and diseases (Dempster 1984) - can affect adult population levels. For example, female mortality in western populations appears to be more common than that experienced by males since more males are commonly found at the overwintering roosts (Frey & Leong 1993).

The vast majority of butterflies found at the eastern overwintering site, 85%, have been shown, via cardenolide amounts and types present in the butterflies, to have developed on the Common and Showy Milkweeds, A. syriaca and A. speciosa, which only grow north of 35° N latitude (Seiber et al. 1986; Malcolm 1987). Malcolm et al. (1987) have shown that Milkweeds in the south senesce and die in June at 30° N which pushes the Monarchs further north (see also Cockrell et al. 1993). Similarly, using wing wear, cardenolide profiles and amounts, Malcolm et al. (1993) have shown that re-colonization of eastern North America is by a series of successive broods, although Urquhart & Urquhart (1979b) have shown that Monarchs can migrate all the way from Mexico to latitudes similar to those found in southern Ontario.


Eastern and western populations are likely distinct (there are differences in behaviour, size, colour, and palatability; Marriott 1994a; 1994b) throughout their range with little transfer of individuals and genes across the dividing mountain ranges (Brower & Boyce 1991; Brower, 1995; Brower et al. 1995 but see Urquhart & Urquhart, 1977 and Urquhart 1987 for a differing opinion). A risk to the eastern population at present, if gene flow exists, or inter-populational transfers of western individuals to the eastern range are made by man, is the current high infestation (55-65% of all butterflies; Leong et al. 1992) of the Neogregarine protozoan parasite, Ophryocystis electroscirrha (Leong et al. 1992; Marriott 1993; Brower et al. 1995). Both populations are sensitive to large scale weather patterns such as storm and cold fronts which can result in mortality from drowning (Brown 1992) or freezing, despite increased cold-hardiness of the migrating brood (Masters et al. 1988; Anderson & Brower, 1993), during migration. Also, as stated above, variable, fluctuating populations may be more at risk from such events.

Brower (1995) suggests that high predation rates being experienced by overwintering butterflies at the Mexican roosts are due to a historical land use shift which allowed for the expansion of Milkweeds which contain lower amounts and emetic potencies of cardenolides. If this is true then changes in land use patterns in Canada have been both a boon (allowing for significant range expansion) and a bane (leaving butterflies less protected against predators) for the Monarch. Brower (1995) also expresses concern over the widespread use of herbicides and pesticides in eastern North America to "control" Milkweeds and other organisms which are perceived as pests and predicts that the breeding range of the eastern population will decline markedly over the next 10 years if such use continues.