Migratory monarchs that encounter resident monarchs show life‐history differences and higher rates of parasite infection

Abstract

Environmental change induces some wildlife populations to shift from migratory to resident behaviours. Newly formed resident populations could influence the health and behaviour of remaining migrants. We investigated migrant-resident interactions among monarch butterflies and consequences for life history and parasitism. Eastern North American monarchs migrate annually to Mexico, but some now breed year-round on exotic milkweed in the southern US and experience high infection prevalence of protozoan parasites. Using stable isotopes (δ H, δ C) and cardenolide profiles to estimate natal origins, we show that migrant and resident monarchs overlap during fall and spring migration. Migrants at sites with residents were 13 times more likely to have infections and three times more likely to be reproductive (outside normal breeding season) compared to other migrants. Exotic milkweed might either attract migrants that are already infected or reproductive, or alternatively, induce these states. Increased migrant-resident interactions could affect monarch parasitism, migratory success and long-term conservation.

Mechanisms and Consequences of Partial Migration in Insects

“…However, the role of trophic interactions has received relatively little attention in the partial migration literature (Chapman et al, 2011) and has rarely been studied in migratory insects (Altizer et al, 2011;Chapman et al, 2015). Nonetheless, there is evidence that migration can reduce the prevalence of infection from the protozoan parasite, Ophryocystis elektroscirrha in monarchs (Bartel et al, 2011;Altizer et al, 2015;Flockhart et al, 2018), with resident populations having higher infection rates than migrant populations (Satterfield et al, 2015(Satterfield et al, , 2016(Satterfield et al, , 2018, providing evidence of "migratory escape" (Altizer et al, 2011) from contaminated environments.…”

Host Plants and Climate Structure Habitat Associations of the Western Monarch Butterfly

“…The breeding records necessarily involve observations on milkweed hosts, while the adult records include both observations of adults in association with host plants and in other settings (e.g., nectaring on other flowering plants). Finally, since tropical milkweed (Asclepias curassavica)-a nonnative species commonly planted in gardens, which persists yearround in areas with mild winters-has been implicated as a contributing factor for parasitism by the protozoan parasite, Ophryocystis elektroscirrha, and in disruption of reproductive behavior (Satterfield et al, 2016(Satterfield et al, , 2018Malcolm, 2018), we considered a fourth model of only breeding records that occur in areas >3.6 km from known occurrences of A. curassavica (based on a 3,600 m grid), which we refer to as our "non-tropical breeding" model.…”

Phytochemical changes in milkweed induced by elevated CO2 alter wing morphology but not toxin sequestration in monarch butterflies

“…Importantly, monarch wings are rounder and less dense when larvae are reared on A. curassavica, a plant exotic to N. America and increasing in prevalence in the southern United States (Satterfield, Maerz, & Altizer, 2015). This species of milkweed does not senesce in autumn and contributes to a loss of monarch migratory behaviour as butterflies encounter viable foliage during their late-season stopovers (Satterfield et al, 2015(Satterfield et al, , 2018. Our data suggest that the offspring of those sedentary monarchs fed A. curassavica will develop lower quality flight phenotypes, perhaps furthering the loss of migratory behaviour.…”