3 years ago



pathogens infect

pathogens infect monarchs including viruses, bacteria, and protozoan parasites. Common monarch infectious agents include Pseudomonas bacteria, a nuclear polyhedrosis virus, the protozoan parasite Ophryocystis elektroscirrha (OE), and a microsporidian Nosema species (McLaughlin and Myers 2007). The protozoan parasite O. elektroscirrha has been relatively well studied and has significant lethal and sub-lethal effects on monarch populations. Monarchs that are infected with this parasite have reduced flight ability and reduced longevity (Altizer and de Roode 2010, p. 23). Female butterflies appear to be more susceptible to OE infection than males. In general, female butterflies exhibit higher infection intensities (de Roode et al. 2008) and greater reductions in body size due to infection than males (de Roode et al. 2007) (Davis and Rendon-Salinas 2010, p. 47), though on the Hawaiian Islands, Pierce et al. (2014) found that 49 percent of males were infected, but only 44 percent of females were infected (p. 7). The OE parasite has become so prevalent that it may be responsible for the increasingly skewed sex ratio of monarchs with declining proportions of females. An analysis of 30 years of monarch population data reveals that between 1976 and 1985, 53 percent of overwintering monarchs in Mexico were female, but since the year 2000, the proportion of females has declined to 43 percent (Davis and Rendon-Salinas 2010). The proportion of females in the fall migration has also declined (Ibid., p. 45). Declining proportion of females is of conservation concern and could have serious ramifications for population growth and recovery. The recent drastic reduction in the availability of milkweed in agricultural fields exacerbates the threat posed to monarchs by OE infection. OE spores can persist for years and accumulate in the environment as they are spread in milkweed patches by male and female adult butterflies (Zalucki 1993, de Roode et al. 2009). Ingestion of a single OE spore can cause heavy infections in adult butterflies (de Roode et al. 2007). Because of OE’s environmental persistence, its high capacity to be spread by adult butterflies, and the low exposure rate needed for infection, there is high potential for rapid increases in infection among monarchs that use the same milkweed patches in multiple overlapping generations (Bartel et al. 2011, p. 345). Reduced availability of milkweed will push monarchs into smaller habitat patches and thus increase their infection risk. Non-migrating monarchs can suffer especially high rates of infection. Along the Gulf and southern Atlantic coasts, monarchs are subject to very high rates of disease prevalence and reductions in overall population health due to their dependence on patches of tropical milkweeds that produce vegetation year-round (Bartel et al. 2011, p. 349). On the Hawaiian archipelago, Pierce et al. (2014) found that on average, 35.5 percent of monarchs across islands were heavily infected with OE across all study sites and years. They found high variation in prevalence both within and among islands, with the average proportion of heavily infected monarchs per site per year ranging from as low as zero to as high as 88 percent (Pierce et al. 2014, p. 7). Human activities are influencing parasite dynamics in monarch populations due to several factors including the loss of breeding and overwintering habitat, the release of captive-bred butterflies, and factors related to global climate change including the spread of tropical milkweed (A. currasavica) and increased stress due to drought and severe temperatures (Bartel et al. 2011, p. 349). Where tropical milkweed has been widely planted, especially in the southern United States Monarch ESA Petition 76

and California, monarchs are able to breed through the winter. These year round patches of tropical milkweed facilitate increased transmission of OE (Monarch Joint Venture 2014, see: Overall, climate change will have serious ramifications for disease in monarchs. Global climate change will influence butterfly diseases by affecting pathogen development, survival rates of parasites and hosts, processes of disease transmission, and stress and host susceptibility. Increasingly warm winters in North America will prevent the die-off of pathogens that would otherwise be killed by cold weather. Warmer temperatures and reduced seasonality will likely lead to increased pathogen survival and transmission (Altizer and de Roode 2010, p. 25). Modification and curtailment of habitat and range will crowd monarchs into smaller habitat patches, increasing the risk of disease transmission, and also increasing competition and exposure to pesticides and other environmental stressors that will heighten the susceptibility of monarchs to infection (Altizer and de Roode 2010, p. 25). In sum, increasingly small population size, less habitat availability, and high magnitude ongoing threats to monarch habitat make disease a very real threat to the persistence of monarch butterflies, and one that could increase rapidly in synergy with other threat factors. Predation Though monarchs are important in the food web and predation occurs naturally, monarchs are increasingly threatened by predation due to declining populations and reduced habitat. The protective chemicals monarchs obtain from milkweeds provide some defense against predation, but monarchs have many natural predators, some of whom are capable of consuming large numbers of eggs, caterpillars, and butterflies. Predators exhibit differing levels of sensitivity to monarch toxins. Avian predation of monarch adults at overwintering sites has been reported in Mexico and in California (Tuskes and Brower 1978, Sakai 1994) and can result in very high levels of mortality. At overwintering sites in Mexico, birds including black-backed orioles (Icterus abeillei) and black-headed grosbeaks (Pheucticus melanocephalus) consume very large numbers of monarchs (Fink and Brower 1981). These two species in particular are capable of circumventing the monarch’s chemical defense by avoiding eating the cuticle and/or by taking a recovery period after accumulating large amounts of cardenolides (Arellano et al. 1993, p. 315). Grosbeaks detach and consume the monarch’s abdomen, and orioles strip out the abdominal contents and thoracic muscles (Arellano et al. 1993, p. 316). Brower and Calvert (1985) reported that orioles and grosbeaks consumed more than 2 million monarchs over the course of the winter at a 2.25 hectare colony in Sierra Chincua, Mexico. Estimates of bird mortality at winter colonies range from 9 to 44 percent (Arellano et al. 1993, p. 315). Also, Calvert et al. (1979) found that the smaller the colonies, the greater was the percent bird predation. During especially cold winters, birds consume even more butterflies than in moderate years (Arellano et al. 1993). While predation is a natural phenomenon, high levels of predation such as those reported in overwintering colonies are of increasing concern given recent dramatic population declines and shrinking availability of forest habitat due to illegal logging, climate change, and forest diseases. Monarch ESA Petition 77

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