Insect migrations are diverse, pervasive, and increasingly disrupted by human activities [1
]. Physical obstacles and roads can reduce movement, alter migration routes, and directly cause mortality [5
]. Climate warming is affecting resource phenology and the timing of insect departure and arrival [7
]. Some migratory insects are responding to anthropogenic activities by traveling shorter distances, while others are forming subpopulations that no longer migrate [10
]. Declining migratory insect populations and shifts in migratory behavior are especially concerning given the importance of insect migrants in the transport of nutrients and biomass over vast distances and across ecosystems [3
], as well as the benefits of migration in allowing some insects to escape from natural enemies [11
Physiological changes associated with migratory behavior in insects commonly occur in response to changes in environmental cues [13
]. Shorter photoperiods, cooler temperatures, reduced food quality, and changes in moisture, pH, and secondary plant compounds are known to induce insects to prepare for and initiate migration or diapause-related dormancy [15
]. Milkweed bugs (Oncopeltus fasciatus
), for example, exhibit a facultative reproductive diapause associated with migratory behavior, triggered by the exposure of immature stages to short photoperiods and cooler temperatures [17
]. Disruptions to the timing, frequency, or intensity of the cues has the potential to impact insect life history and migration. For instance, higher temperatures led to increased residency of migratory Lepidoptera
in Europe [19
], with some species capable of surviving mild winters in England [20
]. Because the mechanisms that promote migration in insects can be complex, identifying which human activities are threatening their migrations requires detailed study.
One of the best-studied insect migrations is that of the North American monarch (Danaus plexippus
), a species that is increasingly impacted by anthropogenic activities. In eastern North America, monarchs emerge at the end of the summer and early autumn in a state of reproductive diapause [1
], and migrate south to wintering sites in central Mexico. Delayed reproduction in monarchs is thought to facilitate the energy storage and greater longevity needed to complete this successful two-way migration [1
]. After spending up to five months at the wintering sites, the same monarchs that flew south then mate, and fly north in the spring to recolonize the southern part of their breeding range [24
]. Resident (non-migratory) populations have been reported since at least the early 2000s along parts of the southern coastal United States [26
] in response to the human planting of exotic tropical milkweed (Asclepias curassavica
). Unlike the estimated 100 milkweed species native to North America [27
] that senesce during late summer and fall, tropical milkweed remains in leaf and flowers year-round in mild climates [10
]. For this reason, tropical milkweed provides monarch adults with nectar and foliage for oviposition and caterpillar feeding during the times of year when native host plants are largely absent [28
]. Resident monarchs show mating and egg laying activity on tropical milkweed throughout the autumn and winter months [26
], during a time of year when migratory monarchs in Mexico are non-reproductive. This year-round breeding activity is associated with high local densities and greater transmission of the debilitating protozoan parasite Ophryocystis elektroscirrha
Factors that facilitate the formation persistence of resident behavior in the southern coastal United States require further study. To date, nearly all resident monarch populations in the United States are found at sites with non-native A
]. Also known as bloodflower or scarlet milkweed, this species native to central and South America was introduced into the United States, as well other countries across the globe, by the horticulture business as an easy-to-grow ornamental plant that attracts butterflies and other beneficial insects [10
]. Tropical milkweed is not naturally present in the United States, but has been planted throughout gardens and parks (Figure S1
], and is available for purchase via popular garden centers (e.g., Lowes) and online stores (e.g., Amazon). In addition to remaining in leaf and flower year-round in areas protected from winter frosts, tropical milkweed is highly attractive to monarchs, yielding higher egg and caterpillar densities compared to native milkweeds [35
]. This is likely due to tropical milkweed’s high concentration of cardenolide secondary compounds, which monarchs sequester to aid in anti-predator defense through early adulthood [37
]. Moreover, tropical milkweed is attractive to ovipositing females infected by O. elektroscirrha
], and the concentrated cardenolide compounds are known to lower parasite spore loads for monarchs relative to those that feed on less toxic milkweed species as larvae [40
Two underlying physiological mechanisms could explain the formation of resident year-round breeding populations in the presence of tropical milkweed in the southern United States. First, tropical milkweed might limit diapause induction during larval development, increasing the probability of the emergence of breeding adults. Previous work on reproductive diapause in monarchs shows that the induction of diapause depends on multiple environmental cues experienced during pre-adult development, including milkweed quality and plant age [13
]. Specifically, milkweed that is aged and of poor quality tends to result in a higher proportion of monarchs that emerge in diapause [13
]. Yet, whether diapause induction in monarchs differs between native versus non-native milkweed diets remains unclear. It is possible that the high concentration and diversity of cardenolide compounds in tropical milkweed [42
] could affect the hormonal mediation of reproductive diapause in monarch caterpillars. Further, as proposed by Batalden and Oberhauser [28
], tropical milkweed might initiate the development of the reproductive system in migratory monarchs (i.e., adults in reproductive diapause). Tropical milkweed encountered by fall migrant monarchs is likely to be an attractive reproductive resource [28
]. Past studies suggest that migratory monarchs captured at sites in Texas with tropical milkweed showed reproductive activity and remained at these sites for weeks [28
], in contrast to migrants captured at sites without tropical milkweed. It is also possible that exposure to any milkweed species as a reproductive resource (i.e., milkweed with good quality foliage), rather than tropical milkweed per se, could cause adult monarchs in diapause to become reproductively active. Since native milkweeds during fall migration tend to be nearing dormancy and poor quality, testing the effect of milkweed species identity versus milkweed presence on monarch reproductive diapause requires controlled experiments.
Here we examine how tropical milkweed exposure during larval and adult stages influences the induction and maintenance of reproductive diapause. If tropical milkweed as a larval diet limits the induction of reproductive diapause, then monarch larvae fed tropical (versus native) milkweed will be more likely to emerge as adults in a reproductively active state. Second, if tropical milkweed exposure increases the chances of reproductive development in fall migrating monarchs (presumably already in a state of diapause), then a higher proportion of migratory monarchs exposed to tropical milkweed will become reproductively active, relative to those exposed to native milkweed or no milkweed. We evaluate these hypotheses via laboratory and field experiments, whereby monarch larvae were fed different milkweed diets in controlled fall-like conditions previously shown to induce reproductive diapause; and wild-caught migratory monarchs were exposed to tropical milkweed, native milkweed, and no milkweed in field cages during the autumn months. We compared reproductive activity by observing mating events and via dissections to evaluate oocyte development in females, and reproductive tract mass in males.
The results here show that monarchs exposed to tropical milkweed as larvae are more likely to be reproductively active, compared to monarchs exposed to native milkweed or no milkweed. When caterpillars were reared on tropical milkweed under autumn-like conditions, previously shown to induce reproductive diapause, both males and females were more likely to show evidence for reproductive development as adults, in comparison with monarchs reared on native milkweed. However, this effect was not apparent in sex-specific analyses of finer-scale reproductive development measures. Similarly, for fall migrant adults exposed to milkweed in flight cages, females showed evidence of reproductive development when exposed to tropical milkweed, relative to native or no milkweed. In contrast, male migrants exposed to native milkweed showed the greatest degree of reproductive activity, relative to tropical or no milkweed.
Previous work on diet and reproduction in other migratory insects showed that plant quality and secondary plant compounds can influence the maturation of reproductive organs in insect herbivores. For example, desert locusts (Schistocerca gregaria
) fed old senescent leaves showed delayed sexual maturation, which was attributed to declining levels of plant hormones (auxins, gibberellins, and kinins) [63
]. Work on tortricid moth larvae has shown that plant quality (protein concentrations) affects diapause induction [64
], such that larvae that fed on higher quality plant species were less likely to undergo diapause induction at the larval stage. Work on swallowtail butterflies fed on high- versus low-quality host plants (of the same species) has similarly shown that low plant quality (tough leaves, older plants) was more likely to induce diapause in the pupal stage [41
]. However, the degree to which host plant quality (including protein content and digestibility) versus the presence of specific chemicals (such as plant secondary compounds) are important for inducing or terminating diapause in insects remains unclear, and is likely species-specific [66
In monarchs, milkweed plant age can also influence reproductive status, with larval diets of older milkweed leaves inducing higher rates of diapause compared to a diet of new milkweed growth [13
]. We did not find this effect, possibly because the field-collected milkweed was chemically similar to the greenhouse-grown native milkweed diet. In particular, the native milkweed chosen here, Asclepias incarnata
, has low concentrations of cardenolides and smooth leaves [40
], and both young and older plants might be relatively nutritious and easy to digest [42
]. How chemical composition changes as milkweeds age and enter dormancy is largely unknown, but could play a key role in induction of diapause in monarchs, potentially influencing juvenile hormone production. Future studies that include chemical analysis of the milkweed diets, particularly flavonoids, are needed to elucidate whether tropical milkweed lacks compounds that induce diapause, or contains components that influence levels of juvenile hormone and increase the probability of reproductive maturation.
In agreement with previous studies, we found that a fraction of wild southbound monarchs during the fall migration period showed evidence of reproductive activity [30
], possibly due to warm conditions monarchs can experience in the fall. For example, Satterfield et al. [30
] found that 10 to 35% of wild fall migrants sampled in Texas showed evidence of reproductive activity, and Borland et al. [46
] found that 14% of females sampled migrating through Texas had previously mated, as evidenced by spermatophore presence. Our experiment further showed that exposure to tropical milkweed was associated with a higher proportion of reproductive females that had fully developed reproductive organs and mature eggs present, compared to females exposed to native milkweed, or to no milkweed. Past studies of the effects of tropical milkweed on reproductive status of wild migratory monarchs found similar results: Batalden and Oberhauser [28
] found that a small proportion of wild monarchs collected at an overnight roost during fall migration showed reproductive activity after being held in cages with tropical milkweed for 11 days. Satterfield et al. [30
] found that wild migratory monarchs visiting tropical milkweed gardens in Texas had three times higher rates of reproductive development compared to migratory monarchs at stopover sites with no tropical milkweed. It appears likely that tropical milkweed advances the termination of reproductive diapause in migrating adults, resulting in the maturation of ovaries and production of eggs [41
]. Tropical milkweed might have particularly high nutritional value or high concentrations of volatile compounds, such as flavonoids, that have been shown to stimulate female monarchs to produce mature eggs [42
]. Although more detailed work is needed to determine the exact chemical stimulus responsible for sexual maturation in monarchs, our work adds to the growing body of evidence that planting tropical milkweed in eastern United States impacts southbound migration by influencing the number of monarchs with the propensity to breed.
Reproductive activity early in fall migration might be a bet-hedging strategy for migratory species. The timing and location of mating and egg laying are important in determining fitness, and if environmental conditions are favorable, engaging in reproduction rather than undertaking risky and energetically costly migration might be an option that yields higher fitness [1
]. Partial migration, where both migratory and resident individuals are found in a population, is common in wildlife, with some fitness advantages associated with either strategy [71
]. Warm fall temperatures, along with flowering and foliage-full tropical milkweed, likely signal to monarch butterflies the presence of a habitat that can support successful reproduction, ultimately contributing to the formation of resident subpopulations.
Wild-caught females in the adult exposure experiment showed a lower propensity for reproductive activity than wild males, regardless of the treatment, possibly owing to the costs of reproduction and migration between the sexes [73
]. Specifically, females might experience higher energetic costs of egg development, locating suitable host plants, and oviposition [73
]. Therefore, females likely require more resources and time to mature reproductively than males, even when cues to initiate reproduction are present. This idea is consistent with the observation of wild monarchs near the end of the overwintering period in Mexico, with males ending reproductive diapause several weeks before females [75
]. Further evidence from monarchs and other insects suggests that males might be able to continue migration after attaining reproductive maturity [1
]. Interestingly, the females in our larval diet experiment showed the opposite trend of the adult exposure experiment: regardless of the diet, females were more likely to be classified as reproductive compared to males. In addition, we found that the tropical milkweed diet produced more reproductive females than males. These results might provide an explanation as to why the sex ratio at the overwintering sites has become male-biased over than last 30 years [77
]; if monarchs are increasingly raised on tropical milkweed as caterpillars, then fewer females emerge in reproductive diapause in the fall. Yet, it is important to note that we determined male reproductive activity in the diet experiment based on the observation of successful matings, and the actual number of males with mature reproductive organs may have been higher. Furthermore, the methods we used to assess reproductive activity were inherently different between males and females.
We found several interesting associations between physical characteristics of migratory monarchs and their reproductive activity. Specifically, females with more developed reproductive organs tended to have higher wing scores, which reflect wear of the wings as well as age. The simplest explanation for this pattern is that older females had more time to develop their reproductive system. Alternatively, older and more damaged females were less likely to complete the journey, and therefore initiation of reproduction was the better strategy. A similar explanation might apply to migratory males, which showed a significant negative relationship between the number of mating events and wing length. Previous work suggests that monarchs overwintering in Mexico have larger wings [67
] and engage in mating events later than those with shorter wings [68
Numerous questions remain regarding the impacts of tropical milkweed on the migratory monarch butterfly of eastern North America. While our work suggests that milkweed species can influence reproduction in wild migratory monarchs, we do not know the fate of the adults that end reproductive diapause and engage in reproductive activity. Previous work indicates that reproductively active monarchs, compared to those in diapause, are less efficient fliers and incur higher energy costs with flight [51
]. Thus, we can speculate that successful migration to overwintering sites after breaking diapause is unlikely. A recent study examining the induction of reproductive diapause and the ability to orient towards Mexico in the fall showed that the two processes are not necessarily coupled [4
]. Thus, what proportion of these monarchs attempt to continue their journey south to Mexico, or remain to reproduce at the sites with tropical milkweed, as well as how these activities might depend on other environmental conditions, are key questions that need further study (however, see Satterfield et al. [30
]). Finally, tropical milkweed is naturally found in the lowlands of Mexico [32
], yet sedentary monarch populations in these areas have not been reported, which suggests that additional factors, such as climate and the availability of foraging resources, might play a role in the successful formation of non-migratory monarch populations.
Further work is needed to explore how the underlying properties of different milkweed species, including plant chemistry and nutritional quality, influence monarch reproductive activity. Moreover, studies are needed to test how environmentally induced variation, such as plant age and herbivore activity, influence how monarch reproductive development responds to host plants. In particular, our experiments tested only greenhouse-grown tropical milkweed, rather than field-collected tropical milkweed grown under autumn conditions. Although tropical milkweed appears visually similar in autumn relative to plants during summer months (e.g., Figure S1b–d
), changes in chemical properties, leaf traits, and growth rate in response to changes in photoperiod and temperature could occur.
Particularly interesting is the possibility of high fitness costs for monarchs that halt the migration to Mexico and instead remain in the southern United States to reproduce. Monarchs that remain in the United States face the risk of winter frosts and other unexpected weather events that can severely damage tropical milkweed plants, resulting in the starvation of caterpillars as well as mortality of the adults. Furthermore, recent work suggests that monarchs breeding during autumn and winter at tropical milkweed sites experience high risk of infection (prevalence reaching 100%) with a protozoan parasite, due to crowding [10
]. This in turn poses an increased risk of infection to migratory monarchs in the spring as the migrants recolonize areas resided by the highly infected sedentary populations [30
]. The sharp declines of monarch population size that have been documented at the overwintering sites in Mexico since the mid-90s [78
] leads us to ask what the overall impact of planting of tropical milkweed might be on monarchs. The availability of tropical milkweed across the United States, particularly along their autumn migratory flyways (Figure S1a
), might contribute to the loss of migratory monarchs (i.e., through migratory dropout) and lower the numbers of monarchs that ultimately reach the wintering sites in Mexico.