Effects of Phenological Shifts in Timing of Monarch Larval Herbivory on Milkweed Host Plants

Over the past few decades, there has been a significant decline in western monarch butterfly populations. Phenological mismatches between the butterflies and their milkweed host plants mediated by climate change have been hypothesized as a possible cause of this decline. The timing of phenological events, such as bird migration, varies among years. More specifically, variation in migration and breeding phenology of monarchs appears to be driven by complex patterns of precipitation across time and space. Understanding the consequences of this variation is important to understanding the temporal dynamics of monarch communities and how they are affected by climate change. Phenological shifts driven by such variation have already been documented in nearly all major taxonomic groups in habitats across the world. These shifts have the potential to change interspecific (between species) interactions by altering which ontogenetic (developmental) stages of species occur simultaneously. 

Statista graph illustrating the decline in monarch populations

Many traits change over ontogeny, such as size and behavior. The combination of traits that organisms exhibit at the time they encounter one another could affect both the strength of the interaction and the probability of the interaction occurring in the first place. If climate change reduces or eliminates the overlaps between organisms in their life stages, it could cause significant changes to demographic rates and community structure.

To better understand the potential mismatches in monarch and milkweed phenology, this study determined how shifts in the phenological timing of monarch larval herbivory affect milkweed host plant performance. This was tested by manipulating the timing of larvae herbivory to determine the consequences for growth and reproduction of milkweed.

A monarch caterpillar feeding on milkweed, courtesy of Scientific American

Monarch larvae can inflict intense herbivory damage, sometimes completely defoliating host plants. The intensity of monarch damage appears to depend on host plant species and age, with particularly strong effects on small or young hosts with relatively weak defensive traits. This experiment utilized 4 treatments to determine the effects of herbivory timing shifts on milkweed plant performance as they vary in their ontogenetic stages. These treatments included an herbivory-free control group, along with 3 groups that varied in timing of monarch herbivory, including early, mid, and late herbivory treatments. The effects of this variation were determined by measuring a variety of traits related to plant growth and reproduction, including dry mass, shoot dry mass, mean stem length, total stem length, and maximum stem length. The study also evaluated subsequent effects on flowering traits, including first flowering age, mean flowering age, flowering duration, and within-plant flowering synchrony. Effects on fruit production were determined by counting the number of pods matured per plant and documenting various fruiting phenology traits for each plant. Finally, annual reproductive success was measured by examining the viability of the seeds in each of the collected pods, which was determined by the proportion of seeds that germinated. 

The study found that variations in the timing of herbivory affected both plant growth and reproduction. The percentage of foliage removed by the monarchs declined with later larval arrival, and plants in the control and late treatments were significantly larger than those subjected to early and mid treatments. This is likely due to the fact that late herbivory plants were significantly larger and harbored more defensive traits than those in the early herbivory treatments. 

On the other hand, effects of herbivory timing on plant reproductive life stages were more variable. Though late herbivory did not significantly affect final plant size, it strongly reduced the number of umbels (a reproductive plant part), the likelihood of fruiting, and the number of germinated seeds. And while late herbivory plants appeared to prioritize defense and compensatory growth of biomass, early herbivory plants appeared to prioritize reproduction. This illustrates the tradeoffs inherent to host responses in the case of exploitative interactions. Indeed, the mid herbivory plants exhibited the fewest umbels, the shortest flowering duration, the lowest proportion of plants producing pods, and the shortest fruiting duration, demonstrating the difficulties of balancing regrowth and reproduction with the timing of herbivory.

These findings suggest that phenological shifts in herbivory can indeed have different effects on host plants as they vary in their life stages across the season. Even the relatively small changes in the timing of milkweed–monarch interactions incurred by the experiment demonstrate how such changes may affect both species in complex and significant ways. This study highlights the importance of measuring responses to various perturbations across multiple life stages to more fully capture their subsequent effects. This is especially important as scientists begin to examine phenological changes and consequences over longer seasonal trajectories in order to better understand climate change’s effects on natural phenomena in the coming decades.     

 

References:

    Rasmussen, Nick L., and Yang, Louie H.. 2022. “ Timing of a Plant–Herbivore Interaction Alters Plant Growth and Reproduction.” Ecology e3854. https://doi.org/10.1002/ecy.3854