Status, Distribution & Conservation

The Monarch Butterfly, Danaus plexippus L., in Canada


Habitat Requirements


The distribution of Milkweeds in Canada, as previously mentioned, corresponds quite closely to the Dfb zone (Humid microthermal climatic zone) of the Köppen climate and vegetation classification scheme described by Ackerman (1941; see also Fig. 3.1, pp. 89 of Scudder 1978; Appendix A). The distribution and density of all of the species of Milkweeds found in Canada is given in Figure 2.

Interestingly, the more common species (A. syriaca, A. incarnata, A. tuberosa, and A. speciosa) have low quality cardenolides (i.e. less protective to the butterflies) while much rarer species have high quality cardenolides (Duffey & Scudder 1972; Roeske et al. 1976). The Common (A. syriaca), Swamp (A. incarnata) and Showy Milkweeds (A. speciosa) are, in most other respects, excellent quality hosts, however, the Butterfly-weed or Orange Milkweed (A. tuberosa) is utilized less often due to its low nitrogen and water content (Ericson 1973). Some reports have shown that the importance of isolated hostplants, or small patches (less than 1 plant per m2) is distinct from that of large patches since females prefer the former for oviposition but require the latter to find males which congregate in larger patches (Suzuki & Zalucki 1986; Zalucki & Suzuki 1987; Zalucki 1993).

Recent reports have suggested that the Swamp Milkweed is utilized to a larger extent than previously thought (Layberry 1995) possibly due to protection from ground-based predators which cannot access plants in standing water (Eickwort 1977). As mentioned previously, up to 85% of the overwintering butterflies tested at Mexican roosts are from the Common and Showy Milkweeds. Contrary to past literature (e.g. Scott 1986), it has recently been shown that Monarch larvae cannot subsist on Dogbanes, Apocynum sp. (Borkin, 1993).

Nectar Sources

The fuel for the migratory flight machinery is nectar obtained from fall wildflowers, especially Goldenrods (Solidago sp.) and Asters (Aster sp.). The dependence of the butterflies on large lipid reserves (up to 45% of dry weight with more than 90% stored in the abdominal fat body; Brown & Chippendale, 1974) makes the availability of adequate nectar resources an absolute requirement in potential habitats (Urquhart 1987). The availability of good quality and abundant nectar sources likely plays an important role in migratory staging area habitat selection (Urquhart & Urquhart 1979a; Ehrlich 1984).