3 years ago



to erratic climate

to erratic climate conditions because their successful survival, metamorphosis and migration are dependent on appropriate temperature and moisture regimes. In the broadest sense, monarch habitat is defined by the distribution of suitable species of milkweeds and their abundance and condition. Milkweeds contain species-specific suites of toxic secondary compounds used for defense against herbivores that include cardiac glycosides such as cardenolides, and various alkaloids. Monarchs use the toxic chemicals in milkweeds for their own defense, and generally will not lay eggs on any other species; nor will caterpillars eat leaves of other plants (Brower 1984). Milkweeds are in the family Apocynaceae (dogbane family), subfamily Asclepiadoideae (Rapini et al. 2007). Milkweeds used by monarchs are in the tribe Asclepiadeae, subtribe Asclepiadinae (Nazar et al. 2013). Migrating monarchs evolved in North America using milkweeds in the exclusively American genus Asclepias (Fishbein et al. 2011), and also some related vine milkweeds in other genera that most likely dispersed northward from South America (e.g. Cyanchum, Funastrum, and Matelea). Although D. p. plexippus can and does thrive on some African milkweed species in non-native habitats (e.g. Gomphoscarpus and Calotropis species), it did not encounter African milkweeds until the plants were widely dispersed pan-tropically by human colonists, and became naturalized in the 1800s (see Appendix A). Of the 130 species of milkweed in the genus Asclepias in North America, including the Caribbean and Mexico (Woodson 1954, Fishbein et al. 2011), monarch larvae have been observed feeding on 34 of these species (Malcolm and Brower 1986, Lynch and Martin 1993). In addition, monarchs have been observed successfully developing on some species of milkweed vines in related genera, such as Cynanchum laeve (honeyvine or blue vine milkweed), Funastrum (formerly Sarcostemma) crispum (wavyleaf twinevine), F. cynanchoides (fringed twinevine) and some species in the genus Matelea (Lynch and Martin 1993). Only a few of the milkweed species that monarchs use, however, are abundant, widely-distributed enough, and of sufficient quality at the right season to maintain large butterfly populations throughout their yearly cycles. The eastern range of D. p. plexippus during breeding is mainly coincident with the distribution of the most abundant and widely dispersed milkweeds—the northern species A. syriaca (common milkweed) and the southern species A. asperula (antelope horn milkweed), A. viridis (green or spider milkweed), and A. humistrata (pinewoods milkweed) (see Fig.1 in Malcolm et al. 1993). By far the most abundant milkweed species in the northern breeding areas is common milkweed (A. syriaca) which is found from southern Canada to Virginia in the east, throughout the Midwest, and west to Kansas and the Dakotas (Woodson 1954, Woods et al. 2012). Common milkweed has recently expanded southward into Georgia, the Carolinas, and Louisiana (Wyatt et al. 1993, Wyatt 1996), and has also become naturalized in parts of the Pacific Northwest. Common milkweed inhabits places that have experienced soil disturbance, such as some cultivated fields, crop fields that have been abandoned or are fallow, pastures, logged land, riparian zones, suburban and urban vacant lots and waste areas, and along trails, railroad tracks, and roadways. It is also intentionally planted in gardens. Monarch ESA Petition 28

Cardenolide fingerprinting of monarchs in their Mexican winter roosts has shown that the majority of the butterflies that migrated there in the fall were raised on A. syriaca. Thin-layer chromatography studies found that 85 percent (Seiber et al. 1986) and 92 percent (Malcolm et al. 1993) of nearly 400 monarchs fingerprinted in Mexico in winter had fed as larvae on common milkweed (Brower et al. 2012a, p. 97). Although A. syriaca, A. asperula, a viridis, and A. humistrata are the most important species for eastern monarchs, the butterflies also use other milkweed species as they spread throughout their breeding range. In the western portion of the range of eastern monarchs, the butterflies use A. speciosa (showy milkweed) and A. incarnata (swamp milkweed). In Texas, three of the most important milkweed species for monarchs are antelope horn milkweed, green milkweed, and Zizotes milkweed (A. oenotheroides). In eastern Louisiana and other Gulf states, pinewoods milkweed is a common monarch host. Non-native A. curassavica (bloodflower, or tropical milkweed) is now a common host in Texas and the southeast, in part due to the intentional planting of this species in gardens. Other southern milkweed vines also occasionally host monarch larvae including Cyanchum leave (honeyvine milkweed), Matelea retiuclata (green milkweed vine), and Funastrum crispum (wavy leaf milkweed vine) (see Texas Monarch Watch, The population of D. p. plexippus in western North America utilizes multiple species of milkweeds to reproduce, including the broadly distributed A. fascicularis and A. speciosa, along with other locally common species such as A. eriocarpa (woollypod milkweed), A. cordifolia (heartleaf milkweed), and A. vestita (woolly milkweed) (see The distribution of milkweeds in the landscape influences monarch productivity. The amount of time a female monarch spends searching for host plants, the number of eggs laid in a given area, and the degree of parasitism and predation of immature stages can be affected by the density and size of milkweed patches in different habitats (Zalucki and Lammers 2010, Pleasants and Oberhauser 2012). Monarchs lay more eggs per plant on milkweeds that occur in smaller milkweed patches (Oberhauser et al. 2001, Zalucki and Lammers 2010). In studies of the distribution of common milkweed (A. syriaca) in Iowa, Maryland, Minnesota, Wisconsin, and Ontario, researchers found that milkweed density was higher and patch size was larger in nonagricultural habitats (such as road right of ways, pastures, and abandoned fields) than in cornfields (Oberhauser et al. 2001, Pleasants and Oberhauser 2012), meaning that monarchs are more likely to lay higher numbers of eggs per milkweed in the smaller milkweed patches found within agricultural fields (Zalucki and Lammers 2010). In Iowa, Minnesota, and Wisconsin, egg densities were higher on milkweeds within fields of corn and soybeans than on milkweeds at field edges or in non-agricultural habitats (Oberhauser et al. 2001). Further assessment over four years in Iowa revealed that milkweed growing in cropland harbored on average 3.89 times more eggs per plant versus that growing in other habitats (Pleasants and Oberhauser 2012). Females may prefer agricultural milkweeds because of their higher nitrogen content, because they can locate milkweed plants more readily within a corn or soybean monoculture because milkweed chemical cues stand out more, or because larval success rate may be higher within smaller patches (Ibid.). Monarch ESA Petition 29

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