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Articles : Title

Status, Distribution, and Potential Impact from Noxious Weed Legislation

ABSTRACT

The present report summarizes status and distributional information on the Milkweeds (Asclepias spp.) that occur in Canada and considers whether noxious weed legislation poses a threat to any of the species. Since the Monarch Butterfly (Danaus plexippus) requires Milkweed as a larval food, this insect could be put at risk due to Milkweed eradication programs.

Of the fourteen species of Milkweed that occur in Canada - all of which are native - Common Milkweed (Asclepias syriaca) is the most widespread and locally abundant. It occurs from southeast Saskatchewan to Prince Edward Island. Swamp Milkweed (Asclepias incarnata) is also fairly common and is found from Prince Edward Island to southeast Manitoba. Showy Milkweed (Asclepias speciosa) is a western species that occurs from southern Manitoba to southern British Columbia. Oval-leafed Milkweed (Asclepias ovalifolia) is also a western species that occurs from extreme northwestern Ontario to southern British Columbia. Green Milkweed (Asclepias viridiflora) is primarily a western species that is found from southern Ontario to southern British Columbia. Whorled Milkweed (Asclepias verticillata) occurs from southern Ontario to southern Saskatchewan. Poke Milkweed (Asclepias exaltata) and Butterfly-weed (Asclepias tuberosa) occur in southern Ontario and southwest Quebec. Milkweed (Asclepias lanuginosa) only occurs in southern Manitoba where it is rare. The following five species occur only in Ontario where they are considered rare: Milkweed (Asclepias hirtella), Purple Milkweed (Asclepias purpurascens), Four-leafed Milkweed (Asclepias quadrifolia), Sullivant's Milkweed (Asclepias sullivantii), and Variegated Milkweed (Asclepias variegata).

Common Milkweed (Asclepias syriaca), although native, has been able to successfully colonize areas that have been disturbed by agriculture or development and in many areas the plant spreads in a "weedy" manner. This weedy behavior has been the main reason that some species of Milkweed have been listed as noxious weeds: Common Milkweed (Asclepias syriaca) is listed in Nova Scotia, Quebec, Manitoba, and Saskatchewan; and Showy Milkweed (Asclepias speciosa) is listed in Manitoba. The Ontario weed act lists all species of Milkweed as noxious weeds, however, the primary concern is for Common Milkweed.

The Monarch Butterfly (Danaus plexippus) occurs across much of Canada as adults, however, its breeding range is mainly restricted to south and central Ontario, and southern Quebec. Although the Monarch larva can feed on a range of Milkweed species, it is largely dependent on Common Milkweed (Asclepias syriaca). From the point of view of the Monarch Butterfly, it is primarily the Ontario populations of Common Milkweed that are important to its breeding in Canada. The enforcing of noxious weed legislation in some provinces-like Nova Scotia where there is currently a very limited population of Common Milkweed-could impact peripheral colonies of Monarch Butterflies. In Ontario, where there is a very abundant and spreading population of Common Milkweed, it is unlikely that there are sufficient resources available to enforce the noxious weed act to the point where the Monarch Butterfly could be put at significant risk.


1.0 INTRODUCTION
The purpose of the present report is to summarize status and distributional information on the Milkweeds (Asclepias spp.) that occur in Canada and consider whether noxious weed legislation poses a threat to any of the species. This information is needed to determine whether the Monarch Butterfly (Danaus plexippus) that requires Milkweed as a larval food, could be put at risk due to Milkweed eradication programs.

All Milkweeds that occur in Canada are native species. Common Milkweed (Asclepias syriaca), although native, has been able to successfully colonize areas that have been disturbed by agriculture or development and in many areas the plant spreads in a "weedy" manner. The behavior of this species has been the main reason that "Milkweeds" have been listed in some noxious weed legislation. Since the Monarch Butterfly (Danaus plexippus) requires Milkweed as a larval food (Urquhart & Urquhart, 1979), this migrating insect could be at risk itself in Canada should Milkweeds be reduced or eliminated in some areas due to the enforcement of noxious weed legislation.

1.1 Milkweeds and the Monarch Butterfly

The Monarch Butterfly (Danaus plexippus) is a very unusual insect in that it spends the spring and summer breeding season in central and eastern North America but migrates to northern Mexico to survive the northern winter as dormant adults (Urquhart & Urquhart, 1979). During the breeding season, Monarchs depend on various species of Milkweed for larval food. In Canada, there are 14 species of Milkweed known, however, one species-Common Milkweed (Asclepias syriaca)-is by far the most important food source for the Monarch (Crolla & Lafontaine, 1996; Urquhart & Urquhart, 1979). In eastern North America, Malcolm et al.., (1989) report that the late summer breeding range of the Monarch coincides with the range of Asclepias syriaca - Milkweed they describe as "extremely abundant". Although adult Monarch Butterflies have been recorded in all Canadian provinces, their breeding is largely restricted to southern and central Ontario, and southern Quebec (Crolla & Lafontaine, 1996; Urquhart & Urquhart, 1979). Breeding records, according to Crolla & Lafontaine (1996), are as follows: British Columbia-rare; Alberta-rare; Saskatchewan-sparse; Manitoba-uncommon; Ontario-very common; Quebec-common; New Brunswick-rare; Nova Scotia-rare; Prince Edward Island-none; Newfoundland-none. The breeding area is also the center of abundance of Common Milkweed, the principle larval food plant (Crolla & Lafontaine, 1996; Urquhart & Urquhart, 1979). Although the Monarch can use various species of Milkweed as larval food and the adult butterfly occurs across much of Canada it is clear that the main food is Common Milkweed (Asclepias syriaca) and the main breeding range is in southern and central Ontario (Crolla & Lafontaine, 1996; Urquhart & Urquhart, 1979). Therefore, from the perspective of the Monarch Butterfly, it is the Ontario populations of Common Milkweed that most are important. For an up-to-date and thorough review of the status of the Monarch Butterfly in Canada, refer to Crolla & Lafontaine (1996).

1.2 Description of the Milkweed Genus

In North America, there are over 100 species of Milkweed that belong to the genus Asclepias (Woodson, 1954; Gleason, 1969). Although the distribution of many species overlaps, hybrids between species are rare (Kephart & Heiser, 1980). There are 14 species of Milkweed known in Canada-refer to Table 1. Milkweeds are perennial herbs that grow each year from a thick root or deep rhizome. The stems are simple or sparingly branched with opposite or whorled leaves [one of the Canadian species-Butterfly-weed (Asclepias tuberosa)- has alternate leaves (Gleason, 1968)]. The flowers are generally small and arranged in clusters at the top of the main stem and sometimes also at the ends of secondary branches. The flowers of all species have a characteristic shape-a ring of five drooping petals overtopped by five erect hood-shaped petals. All species except Butterfly-weed have milky juice-hence the common name "Milkweed".

TABLE 1: Summary of Milkweed Provincial Distribution and Status

Milkweed Species Distribution and Status
BC AB SK MB ON QC NB NS PE NF
Asclepias exaltata - - - - Unc. Rare - - - -
A. hirtella - - - - Rare - - - - -
A incarnata - - - Unc. Com. Com. Unc. Unc Unc -
A. lanuginosa - - - Rare - - - - - -
A. ovalifolia Rare Unc. Unc. Unc. Rare - - - - -
A. purpurascens - - - - Rare - - - - -
A. quadrifolia - - - - Rare - - - - -
A. speciosa Unc. Unc. Unc. Unc. - - - - - -
A. sullivantii - - - - Rare - - - - -
A. syriaca - - Rare Unc. Com. Unc. Unc. Unc. Unc. -
A. tuberosa - - - - Unc. Rare - - - -
A. variegata - - - - Ex. - - - - -
A. verticillata - - Rare Unc. Rare - - - - -
A. virdiflora Unc. Rare Unc. Unc. Rare - - - - -
Total Species 3 3 5 7 12 4 2 2 2 0
AB = Alberta, BC = British Columbia, MB = Manitoba, NB = New Brunswick, NF = Newfoundland, NS = Nova Scotia, ON = Ontario, PE = Prince Edward Island, QC = Quebec, SK = Saskatchewan. Com. = common, Ex = extirpated, Unc. = uncommon. If a cell is empty, it indicates that species does not occur in that province. The status assessments are based on the most current floras and rare plant publications.

Although most mature Milkweed plants have many flowers, only a few of the flowers on a plant will produce fruit (Gleason, 1968). The fruit consists of a long pod containing many flat round seeds attached to silky threads capable of transporting the seeds long distances on the gentlest of breezes. Some species of Milkweed can also reproduce vegetatively by means of shoots that can develop on their extensive roots. The flowers are insect-pollinated and have a very specialized floral structure (Gleason, 1968). The complex flower structures have evolved to facilitate cross-pollination by insects and most species are self-incompatible, however, a few species can produce some viable seed by self-pollination (Kephart, 1981).

1.3 Biological and Economic Significance of Milkweeds

Milkweeds have both positive and negative attributes. North American native people used the seed "floss" and stem fibers (Woodson, 1954). In colonial New England, settlers used the floss for stuffing pillows and cushions (Woodson, 1954). The stem fibers were considered as a substitute for flax and hemp (Woodson, 1954) but it was considered inferior to them. During and shortly after the Second World War, a number of investigations were carried out to determine the usefulness of the latex of various species of Milkweed as a possible source of rubber and the seed floss as a replacement for "kapok" in life preservers (Groh 1943; Groh & Dore, 1945; Moore 1947; Woodson, 1954). The investigations evidently ended sometime after the Second World War when it became apparent the Milkweed could not compete with existing crop plants (Woodson, 1954.)

Milkweeds are a nectar source for the Honeybee (Apis mellifera) (Robinson & Oertel, 1975) and for many other insects (Wilson et al., 1979) including the well-known Monarch Butterfly. Due to their complex flower structure that is regarded as second in complexity only to the Orchid family (Wyatt, 1976), Milkweeds have been the subject of many studies into pollination mechanisms and life history evolution.

On the negative side of the ledger, some Milkweeds - especially Common Milkweed (Asclepias syriaca) - can be aggressive and persistent weeds in agricultural land and the plants are toxic to livestock (Kingsbury, 1964). This situation has resulted in some Milkweeds being declared "noxious weeds" in several jurisdictions.

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