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1. Impacts of larval host plant species on dispersal traits and free-flight energetics of adult butterflies

   https://doi.org/10.1038/s42003-022-03396-8  

   Pocius, Victoria M.; Cibotti, Staci; Ray, Swayamjit; Ankoma-Darko, Obenewa; McCartney, Nathaniel B.; Schilder, Rudolf J.; Ali, Jared G. (December 2022, Communications Biology)

   Abstract Animals derive resources from their diet and allocate them to organismal functions such as growth, maintenance, reproduction, and dispersal. How variation in diet quality can affect resource allocation to life-history traits, in particular those important to locomotion and dispersal, is poorly understood. We hypothesize that, particularly for specialist herbivore insects that are in co-evolutionary arms races with host plants, changes in host plant will impact performance. From their coevolutionary arms-race with plants, to a complex migratory life history, Monarch butterflies are among the most iconic insect species worldwide. Population declines initiated international conservation efforts involving the replanting of a variety of milkweed species. However, this practice was implemented with little regard for how diverse defensive chemistry of milkweeds experienced by monarch larvae may affect adult fitness traits. We report that adult flight muscle investment, flight energetics, and maintenance costs depend on the host plant species of larvae, and correlate with concentration of milkweed-derived cardenolides sequestered by adults. Our findings indicate host plant species can impact monarchs by affecting fuel requirements for flight. 

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2. Exposure to Non-Native Tropical Milkweed Promotes Reproductive Development in Migratory Monarch Butterflies

   https://doi.org/10.3390/insects10080253  

   Majewska, Ania A.; Altizer, Sonia (August 2019, Insects)

   Background: North American monarchs (Danaus plexippus) are well-known for their long-distance migrations; however, some monarchs within the migratory range have adopted a resident lifestyle and breed year-round at sites where tropical milkweed (Asclepias curassavica) is planted in the southern coastal United States. An important question is whether exposure to exotic milkweed alters monarch migratory physiology, particularly the ability to enter and remain in the hormonally-induced state of reproductive diapause, whereby adults delay reproductive maturity. Cued by cooler temperatures and shorter photoperiods, diapause is a component of the monarch’s migratory syndrome that includes directional flight behavior, lipid accumulation, and the exceptional longevity of the migratory generation. Methods: Here, we experimentally test how exposure to tropical milkweed during the larval and adult stages influences monarch reproductive status during fall migration. Caterpillars reared under fall-like conditions were fed tropical versus native milkweed diets, and wild adult migrants were placed in outdoor flight cages with tropical milkweed, native milkweed, or no milkweed. Results: We found that monarchs exposed to tropical milkweed as larvae were more likely to be reproductively active (exhibit mating behavior in males and develop mature eggs in females) compared to monarchs exposed to native milkweed. Among wild-caught fall migrants, females exposed to tropical milkweed showed greater egg development than females exposed to native or no milkweed, although a similar response was not observed for males. Conclusions: Our study provides evidence that exposure to tropical milkweed can increase monarch reproductive activity, which could promote continued residency at year-round breeding sites and decrease monarch migratory propensity. 

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3. Host plant specificity of the monarch butterfly Danaus plexippus: A systematic review and meta-analysis

   https://doi.org/10.1371/journal.pone.0269701  

   Greenstein, Lewis; Steele, Christen; Taylor, Caz M. (June 2022, PLOS ONE)

   Mansour, Ramzi (Ed.)

   The preference-performance hypothesis explains host specificity in phytophagous insects, positing that host plants chosen by adults confer the greatest larval fitness. However, adults sometimes oviposit on plants supporting low larval success because the components of host specificity (adult preference, plant palatability, and larval survival) are non-binary and not necessarily correlated. Palatability (willingness to eat) is governed by chemical cues and physical barriers such as trichomes, while survival (ability to complete development) depends upon nutrition and toxicity. Absence of a correlation between the components of host specificity results in low-performance hosts supporting limited larval development. Most studies of specificity focus on oviposition behavior leaving the importance and basis of palatability and survival under-explored. We conducted a comprehensive review of 127 plant species that have been claimed or tested to be hosts for the monarch butterfly Danaus plexippus to classify them as non-hosts, low performance, or high performance. We performed a meta-analysis to test if performance status could be explained by properties of neurotoxic cardenolides or trichome density. We also conducted a no-choice larval feeding experiment to identify causes of low performance. We identified 34 high performance, 42 low performance, 33 non-hosts, and 18 species with unsubstantiated claims. Mean cardenolide concentration was greater in high- than low-performance hosts and a significant predictor of host status, suggesting possible evolutionary trade-offs in monarch specialization. Other cardenolide properties and trichome density were not significant predictors of host status. In the experiment, we found, of the 62% of larvae that attempted to eat low-performance hosts, only 3.5% survived to adult compared to 85% of those on the high-performance host, demonstrating that multiple factors affect larval host plant specificity. Our study is the first to classify all known host plants for monarchs and has conservation implications for this threatened species. 

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4. Aphid abundance and monarch butterfly oviposition in urban gardens

   https://doi.org/10.1007/s10841-025-00720-5  

   Quiroz, Edwin; Ashley, Mary V; Zaya, David N (October 2025, Journal of Insect Conservation)

   Abstract Monarch butterflies (Danaus plexippus) have suffered dramatic declines and, as of 2025, are being considered for listing as a threatened species under the U.S. Endangered Species Act. Monarch declines have been explained, at least in part, by the loss of milkweeds (Asclepiasspp.), their larval host plant. Increasing milkweeds in urban gardens may aid in monarch conservation efforts but concerns have been raised regarding negative interactions between monarchs and aphids co-occurring on milkweeds in these gardens. We made 934 observations on approximately 150Asclepias syriaca(common milkweed) stems in garden beds in a highly urbanized landscape in Chicago, IL, USA over two summers. Using generalized linear mixed-effects models, we found that the number of monarch eggs and larvae was either not correlated or positively correlated with aphid abundance, indicating that aphid infestation did not deter oviposition by monarch butterflies on milkweed stems. Implications for Conservation: Planting milkweeds in urban gardens may aid in monarch population recovery, but aphid milkweed specialists often occur at high densities in urban gardens. We did not find that stems with high aphid abundance had fewer monarch eggs and larvae, suggesting that aphids may not reduce the value of native urban milkweeds for monarch oviposition as previously suggested. 

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5. Cardenolides, toxicity, and the costs of sequestration in the coevolutionary interaction between monarchs and milkweeds

   https://doi.org/10.1073/pnas.2024463118  

   Agrawal, Anurag A.; Böröczky, Katalin; Haribal, Meena; Hastings, Amy P.; White, Ronald A.; Jiang, Ren-Wang; Duplais, Christophe (April 2021, Proceedings of the National Academy of Sciences)

   null (Ed.)

   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. 

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