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Articles : Status Report on the Monarch Butterfly (Danaus plexippus) in Canada

In October of 1995, three areas in southern Ontario (Point Pelee, Long Point and Prince Edward Point) were designated as Monarch butterfly reserves as part of an international agreement with Mexico. Little protection currently exists elsewhere in Canada with regard to the Monarch and its habitats. Ultimately, the survival of the Monarch depends on the health of the overwintering sites in Mexico and California. However, it is important for Canada to do its part in ensuring that Monarchs returning from the overwintering grounds each year have a place to breed in order to rebuild the population, which may be severely reduced in size by climatic disasters and predation at the overwintering sites. Without ensuring the continued existence of suitable breeding habitat within its own borders, Canada is in no position to offer aid or advice with regard to the current crisis at the Mexican overwintering sites. Efforts to protect the Monarch in Canada should therefore concentrate on maintaining existing habitat for Monarchs, encouraging the creation of new habitat, and on identifying and protecting critical staging areas in southern Ontario where Monarchs congregate in great numbers along their fall migration route.

At present there is abundant habitat for Monarchs in southern Ontario and Quebec, consisting mainly of abandoned farms and roadside verges. This habitat can easily be lost in one of three ways:

  1. Abandoned farms being put into active production.
  2. Succession of abandoned farmlands into wooded and brushy habitats.
  3. Active programs to eliminate milkweed.

The continued availability of milkweeds growing on abandoned farmlands can be ensured by encouraging farmers to cut abandoned fields in the late fall (mid-September to early October) when Monarch breeding has ended. This would restrict the growth of woody plants (trees and shrubs) - which normally crowd out milkweeds in the succession of old fields to brushy and wooded habitats - while allowing milkweeds and nectar-producing wildflowers to continue to thrive each year. Weedy and brushy fields in parks (including provincial and national parks in southern Ontario which contain areas of abandoned farmland) could be maintained in the same way through selective cutting or controlled burning.

Rather than maintaining grasses along the median strips and verges of highways, wildflower mixes including milkweeds could be planted instead, as has been done in some areas of the United States. While the maintenance of grassy borders along roads and highways requires the application of herbicides and repeated mowings, strips of wildflowers would require mowing only once a year, in the late fall, to restrict the growth of woody plants. In addition to creating increased breeding habitat for Monarchs, maintaining wildflowers along highways would be more economical, and would be beneficial to many other species of butterflies and flower-visiting insects. Maintaining powerline corridors through selective cutting rather than herbicide spraying would similarly increase the availability of hostplant and nectar resources for Monarchs.

The creation of butterfly gardens and milkweed areas in communities would also increase the availability of Monarch habitat, and would partially compensate for the loss of habitat resulting from urban sprawl. The creation of butterfly gardens could be undertaken as part of public education and awareness programs in schools and communities, highlighting the uniqueness of the Monarch and the role that community and backyard gardens can play in ensuring its continued survival in North America. There has been a great increase in public awareness of the Monarch and its annual migration in the last few years (see section H) and many communities would probably respond positively if the connection between milkweed in gardens, and the spectacular migration and overwintering in Mexico, were drawn. The increasing popularity of butterfly gardening in North America, and the growing number of seed houses now dealing in wildflowers, suggests that community programs to develop Monarch habitat would be well received.

Milkweed are presently considered noxious weeds in most areas of Canada. The decision to control milkweeds in left to the provinces, and ultimately to regional municipalities. In most areas there appears to be no active program to eliminate milkweeds, except where there are specific complaints. However, lepidopterists contacted in both Manitoba and Nova Scotia reported that milkweeds were currently being sprayed and eliminated in their areas (see section D). The removal of milkweeds from provincial noxious weed acts is a necessary first step to ensuring the continued existence of breeding habitat for the Monarch in Canada. An obvious conflict is created by encouraging communities, farmers and various agencies to maintain milkweed areas and plant butterfly gardens, while officially milkweeds are still recognized as noxious weeds. Milkweeds are included in noxious weed acts because they are considered to be poisonous to livestock. However, the level of toxicity varies among milkweed species, and Common Milkweed, which is one of the least toxic species, was apparently listed by the Ontario Weed Control Act based only on circumstantial evidence (Alex 1992). In many habitats where they occur milkweeds pose no direct threat to livestock, and they could be safely removed from noxious weed acts, while recognizing that in some situations where there is a real concern it may still be necessary to eradicate them.

In addition to ensuring that breeding habitat for Monarchs in Canada remains widely available, consideration should also be given to the importance of certain areas of Canada to the success of the Monarch's annual migration. The main population of Monarchs breeding in eastern Canada funnels through southern and eastern Ontario each year, and the availability of roosting sites and nectar sources is crucial to the success of their fall migration. Staging areas along the north shores of Lake Ontario and Lake Erie where Monarchs gather into migratory swarms and form large aggregations at overnight roosting spots should be recognized and protected. While it is probable that specific trees are not crucial as roosting sites, the presence of both suitable roosting trees and adjacent open areas where the butterflies can forage for nectar is essential. Critical stands of trees where overnight roosts form each year should be identified and protected from disturbance such as the development of campgrounds, picnic grounds, and roadways. Weedy fields adjacent to these areas, that support large numbers of wildflowers suitable for nectaring, should be maintained through selective cutting. Most staging areas in Canada lie within protected areas (e.g. Presquile Provincial Park, Point Pelee National Park, etc.) but this does not ensure the maintenance of suitable habitat for the monarch. For example, most areas of former abandoned fields in parks are rapidly changing to brushy and wooded habitats that are not suitable for nectaring.

Based on estimates of the number of butterflies overwintering in Mexico and California, the eastern population of the Monarch presently numbers in the tens of millions, while the smaller western population numbers in the millions. Historical data indicates that the size of both populations fluctuates regularly, and often dramatically, as a result of winter storm mortality, poor breeding conditions, predation, parasitism, disease and other pressures in combination with each other. Numbers can vary by a factor of 10 over time, and in the past both the eastern and western populations of the Monarch have suffered losses approaching 90% and still recovered when the 10% surviving experienced optimal conditions in the breeding range. However, while a fluctuating population size appears to be the norm for Monarchs, human degradation of the overwintering sites in Mexico is resulting in consistently higher levels of mortality, which may be pushing the population size below a threshold level from which it can recover itself. Widespread and increasing use of herbicides throughout North America may also result in dwindling fall migrations to the overwintering sites in the next few years (Brower 1995), due to the eradication of hostplants and nectar sources in the breeding range. Without real protection of the Mexican overwintering sites, as well as protection of breeding habitats and nectar resources along migration routes in Canada and the United States, the eastern population of the Monarch may become extirpated in North America early in the next century (Brower 1995).

The number of Monarchs reaching Canada each year is entirely dependent on the success of overwintering in Mexico and California, and on weather conditions in the United States during the northward remigration in the spring and early summer. Severe winter storms with freezing temperatures at the Mexican overwintering sites resulted in extremely high mortality in the eastern population of the species in the winter of 1991-92, with 80% mortality being recorded at one site (Brower 1995), and high mortality has also been reported in the recent winter of 1995-96. At the overwintering sites in California, possibly as a result of disease, numbers of western Monarchs crashed in 1992, dropped to the lowest level on record by the winter of 1994-95, then rebounded to near-normal levels in the winter of 1995-96.

Few Monarchs reached Canada in 1992 due to the high mortality at the overwintering sites in Mexico, and numbers remained low over much of the country in 1992 and 1993 due to unusually cool, wet summers. Numbers in Canada increased in 1994 and 1995 to about average. Monarch occurrence in each region of Canada in the last five years is described in more detail below.

British Columbia
Monarchs occur erratically in British Columbia at the best of times and in recent years there have been few reports. In the last five years, breeding has been observed near Lillooet (adults and larvae in numbers) and in the Okanagan Valley (larvae on milkweed).

Prairie Provinces
In Alberta, Monarchs occur very rarely, and breeding usually occurs only in the southern region of the province, and rarely as far north as Edmonton. In 1994 about a dozen Monarchs were found breeding near Taber in the Old Man River area.

In Saskatchewan, where Monarchs are also scarce and irregular, none was recorded in 1993, but in 1994 a small migration occurred with a total of 20 Monarchs being reported in the province (Minno & Minno 1995).

In southern Manitoba, Monarchs occur regularly and in greater numbers than elsewhere in the prairies, although they are less common in the southeastern part of the province, which is more extensively wooded and supports less Monarch habitat. Monarchs were common to abundant in 1991 in southern Manitoba. In 1992 they were uncommon, and weak flights with very few Monarchs being reported occurred in 1993 and 1994. In 1995 Monarchs were again fairly common, but still below the levels of 1991. In southern Manitoba, milkweeds are being sprayed with herbicides in roadside ditches in response to complaints from farmers who were concerned that the plants would spread from roadsides into their fields.

Ontario and Quebec
In southern Ontario and southern Quebec, Monarchs occur annually, and frequently in abundance. In 1991 numbers in Ontario and Quebec were at normal levels with Monarchs being common in most milkweed patches. In 1992, after the disastrous winter kill in Mexico, Monarch numbers were very low over much of Ontario, making it one of the worst years for Monarchs ever recorded (McKown 1993). Monarchs increased to some degree in 1993, and a large migratory flight was noted at Point Pelee (see Species Movement in section F). The Monarch was more common in 1994 and 1995, almost back to normal levels, reflecting the gradual recovery of the species from the 1992 disaster. In spite of generally low numbers in much of Canada over the last few years, Point Pelee in extreme southern Ontario (the primary concentration point for migrating Monarchs in Canada) continued to report large numbers of Monarchs.

Maritime Provinces
In New Brunswick, Monarchs occur irregularly and breeding is largely restricted to a limited area of the province where Common Milkweed grows along the banks of the Saint John River. Dr. Anthony Thomas of Forestry Canada (personal communication) has seen about 100 Monarchs in the province in 20 years of field work; 1991 was a good year for Monarchs in New Brunswick, but only one or two per year were recorded form 1992 to 1995.

In Nova Scotia, Monarchs are never very common; none was encountered in 1994, and only a few were found in 1995. Common Milkweed is rare and localized in Nova Scotia, and officers spray for it when they find it. As a result, Monarch breeding in Nova Scotia does not produce large numbers of adults.

In both Nova Scotia and New Brunswick, limited breeding occurs on localized patches of Swamp Milkweed.

In Canada, suitable breeding habitat for Monarchs exists wherever milkweeds (Asclepias) occur. In eastern Canada, the primary larval hostplant of the Monarch is Common Milkweed (Asclepias syriaca), which accounts for 95% of the milkweed found in the east. Common Milkweed grows widely on abandoned farmland, along roadsides, and in other open areas where weedy species proliferate. Swamp Milkweed (A. incarnata) and Butterfly Weed (A. tuberosa) are also utilized by Monarch in eastern Canada, though to a much lesser extent. Swamp Milkweed occurs in small stands in open wetlands and wet ditches, while Butterfly Weed is largely confined to very dry sandy areas in southern Ontario.

In western Canada, Showy Milkweed (A. speciosa) is the primary larval hostplant. Low Milkweed (A. ovalifolia) and Green Milkweed (A. viridiflora) are also sometimes used by Monarchs in the west (Bird et al. 1995, Klassen et al. 1989). In the Prairie Provinces milkweed is widespread in areas of native short grass and long grass prairie, and also occurs in agricultural areas, particularly along roadsides, river banks and irrigation ditches. In British Columbia milkweed grows at scattered locations in arid valleys and on south-facing hillsides.

Wildflowers, which are particularly prevalent in abandoned farmlands and roadsides, are used as nectar sources by the adult butterflies and are also an important component of Monarch habitats. They are especially important during the fall migration, when sugars obtained from nectar are converted to the fat that is essential for the butterflies to complete their migration and overwinter successfully. Goldenrods (Solidago) and asters (Aster), as well as Purple Loosestrife (Lythrum salicaria) and milkweeds, are the nectar sources used most frequently by Monarchs.

There have been several habitat trends that have affected Monarch breeding range and productivity. There has been a great increase in Monarch habitat due to the clearing of the deciduous forest region of the eastern United States and southeastern Canada. This, in combination with the lost of extensive areas of habitat in the eastern great plains as a result of agriculture, has shifted the overall range of Monarchs into eastern North America (see North American Distribution in section B). During the middle and latter part of the twentieth century it has become increasingly uneconomical to maintain small farms, and as a result there has been a great increase in abandoned farmland in the east, creating suitable areas for Monarch breeding and nectaring. In rural areas adjacent to large cities there also has been a trend for estate housing to occupy large acreages of unused farmland. As a result there is probably more Monarch habitat now than there has ever been. Over the next several decades the trend will be for a lot of this habitat to be lost, as this farmland gives way to the growth of trees and shrubs.

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