Captive rearing of monarch butterflies is a commercial and personal pursuit enjoyed by many different groups and individuals. However, the practice remains controversial, especially after new evidence showed that both a group of commercially derived monarchs reared outdoors and a group of wild-derived but indoor-reared monarchs failed to orient south, unlike wild-derived monarchs reared outdoors. To more fully characterize the mechanisms responsible for the loss of orientation in both commercial and indoor-reared monarchs, we performed flight simulator experiments to determine (i) whether any fraction of commercial monarchs maintains a southern heading over multiple tests, and (ii) whether indoor conditions with the addition of sunlight can induce southern flight in wild-derived monarchs. Commercial monarchs changed their flight direction more often over the course of multiple tests than wild-derived monarchs. While as a group the commercial monarchs did not fly south on average, a subset of individuals did orient south over multiple tests, potentially explaining the discordance between flight simulator assays and the recovery of tagged commercial monarchs at overwintering locations. We also show that even when raised indoors with sunlight, wild-derived monarchs did not consistently orient south in the flight simulator, though wild-derived monarchs reared outdoors did orient south.
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Migratory monarchs that encounter resident monarchs show life-history differences and higher rates of parasite infection
- Award ID(s):
- 1754392
- PAR ID:
- 10073610
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- Ecology Letters
- Volume:
- 21
- Issue:
- 11
- ISSN:
- 1461-023X
- Page Range / eLocation ID:
- 1670 to 1680
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Insect–pathogen dynamics can show seasonal and inter‐annual variations that covary with fluctuations in insect abundance and climate. Long‐term analyses are especially needed to track parasite dynamics in migratory insects, in part because their vast habitat ranges and high mobility might dampen local effects of density and climate on infection prevalence.
Monarch butterflies
Danaus plexippus are 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 on
OE infection 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 of
OE , 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.
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For highly specialized insect herbivores, plant chemical defenses are often co-opted as cues for oviposition and sequestration. In such interactions, can plants evolve novel defenses, pushing herbivores to trade off benefits of specialization with costs of coping with toxins? We tested how variation in milkweed toxins (cardenolides) impacted monarch butterfly (
Danaus plexippus ) growth, sequestration, and oviposition when consuming tropical milkweed (Asclepias curassavica ), one of two critical host plants worldwide. The most abundant leaf toxin, highly apolar and thiazolidine ring–containing voruscharin, accounted for 40% of leaf cardenolides, negatively predicted caterpillar growth, and was not sequestered. Using whole plants and purified voruscharin, we show that monarch caterpillars convert voruscharin to calotropin and calactin in vivo, imposing a burden on growth. As shown by in vitro experiments, this conversion is facilitated by temperature and alkaline pH. We next employed toxin-target site experiments with isolated cardenolides and the monarch’s neural Na+/K+-ATPase, revealing that voruscharin is highly inhibitory compared with several standards and sequestered cardenolides. The monarch’s typical >50-fold enhanced resistance to cardenolides compared with sensitive animals was absent for voruscharin, suggesting highly specific plant defense. Finally, oviposition was greatest on intermediate cardenolide plants, supporting the notion of a trade-off between benefits and costs of sequestration for this highly specialized herbivore. There is apparently ample opportunity for continued coevolution between monarchs and milkweeds, although the diffuse nature of the interaction, due to migration and interaction with multiple milkweeds, may limit the ability of monarchs to counteradapt.
