Status, Distribution & Conservation

The Monarch Butterfly, Danaus plexippus L., in Canada


Trends in Habitat Quality, Quantity and Protection

Urban sprawl is altering the amount of habitat available in the interior of B.C. (C. Guppy, pers. comm.), in southern Ontario (pers. obs.) and no doubt in other provinces as well. Other factors which may affect habitat availability include global climate warming (likely to increase the range of highly successful hostplants such as A. syriaca in eastern Canada but may make areas of the central prairies unsuitable), pollution (increasing carbon dioxide levels, low-level ozone) and widespread herbicide and pesticide use (including the use of "biological" controls lacking in sufficient specificity, such as Bt (Bacillus thuringiensis), to control lepidopterous pests). The recent spread of the alien invasive weeds, Swallow-worts or Dog Strangling Vines (Vincetoxicum rossicum, V. nigrum and V. hirundinaria; Asclepiadaceae) in southern Ontario (see Moore 1959; Pringle 1973; McNeil 1981; Kirk 1985 and Kubisz 1992) is especially worrisome. These vines can be described as the "purple loosestrife of dry fields" due to their clonal nature, habit of choking out native vegetation forming large monoclonal stands and rapid reproduction. A further problem, specific to the Monarch, may be presented by the relatively close relationship of the acceptable Asclepias sp. and the invasives - both are members of the Milkweed family. Female butterflies have been observed to oviposit on Vincetoxicum in eastern Toronto but it is not known whether larvae are capable of surviving on the plants (T. Mason, pers. comm.).

A current problem, especially in Ontario, but also in Saskatchewan (B. Walters, pers. comm.) and in other provinces, is the existence of weed control legislation for "the control of noxious weeds that interfere with land used for agricultural or horticultural purposes" (Anon. 1990). Using Ontario as an example, this legislation (Anon. 1993) lists all Asclepias sp. as "noxious" weeds which landowners are obliged by law to remove from their lands in both agricultural and urban areas. Since these plants are the sole host of Monarchs this legislation can only be perceived as a direct threat to their habitat (Cundiff 1993). Problems with the current legislation include: rare plants (8 of the 12 Asclepias sp. which occur in Ontario are listed as rare, Argus & Keddy, 1984; M. Oldham, pers. comm.) listed as "noxious" weeds, the lack of a specific definition of what constitutes a noxious weed, enforcement by several levels of weed inspectors is often inconsistent from one area to another (in one example a superintendent of a "natural environment" provincial park was instructed to remove the Milkweeds from the park despite the existence of large numbers of plants just outside the park boundaries; D. Davis, pers. comm.), the use of a section of the act which enables Municipalities to designate their own "noxious weeds" (enabling one Guelph municipality to designate "goldenrods" as noxious in the mistaken belief that they cause hayfever; D. Davis, pers. comm.), the unknown impact of herbicide use and plant removal on pollinators, and conflicts between the mandates of different government Ministries (Schappert 1992; Ferris 1994; Schneider 1994).

A number of private and public organizations have been attempting to have the Weed Control Act in Ontario reviewed with the intent of obtaining a clear definition of what constitutes a noxious weed, the listing of specific species instead of wholescale listing of entire genera (7 of 24 taxa listed on the schedule of noxious weeds are for entire genera), the exclusion of native and rare species which are non-invasive and pose no defined threat to agriculture, a means of ensuring that the act does not become a barrier to naturalization projects, clarification of exemptions to the act and consistent application of the act across the entire province (Schappert 1992; Ferris 1994; Schneider 1994). At a symposium held in November of 1994 at Guelph all of these points were presented to the Ministry responsible for the administration of the Act, but as of this writing no formal report to the Ministry has been submitted (H. Lang, pers. comm.).