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Monarch caterpillars from the eastern North American and Puerto Rican populations were fed a diet of eitherAsclepias incarnataL. (native to the eastern North American population) orAsclepias curassavicaL. (native to the Puerto Rican population and uncommon in eastern North America). We flew the monarchs on a tethered flight mill to acquire flight performance metrics including velocity, distance, duration, power, and oxygen consumption rate.

Monarchs reared on theA. incarnata L. milkweed showed slower, shorter, and less powerful flights than those fed onA. curassavicaL. However, eastern North American and Puerto Rican monarchs, which were reared under summer conditions, did not differ in flight metrics or post‐flight metabolic rates.

The results suggest that flight in eastern North American and Puerto Rican monarchs is similar during the breeding season, yet the milkweed the caterpillars consume has important implications for flight performance.

Monarch butterfliesDanaus plexippusare commonly infected with the protozoanOphryocystis elektroscirrha(OE). Because this parasite lowers monarch survival and flight performance, and because migratory monarchs have experienced declines in recent decades, it is important to understand the patterns and drivers of infection.

Here we compiled data onOEinfection spanning 50 years, from wild monarchs sampled in the United States, Canada and Mexico during summer breeding, fall migrating and overwintering periods. We examined eastern versus western North American monarchs separately, to ask how abundance estimates, resource availability, climate and breeding season length impact infection trends. We further assessed the intensity of migratory culling, which occurs when infected individuals are removed from the population during migration.

Average infection prevalence was four times higher in western compared to eastern subpopulations. In eastern North America, the proportion of infected monarchs increased threefold since the mid‐2000s. In the western region, the proportion of infected monarchs declined sharply from 2000 to 2015, and increased thereafter. For both eastern and western subpopulations, years with greater summer adult abundance predicted greater infection prevalence, indicating that transmission increases with host breeding density. Environmental variables (temperature and NDVI) were not associated with changes in the proportion of infected adults. We found evidence for migratory culling of infected butterflies, based on declines in parasitism during fall migration. We estimated that tens of millions fewer monarchs reach overwintering sites in Mexico as a result ofOE, highlighting the need to consider the parasite as a potential threat to the monarch population.

Increases in infection among eastern North American monarchs post‐2002 suggest that changes to the host’s ecology or environment have intensified parasite transmission. Further work is needed to examine the degree to which human practices, such as mass caterpillar rearing and the widespread planting of exotic milkweed, have contributed to this trend.