Seasonal windows of opportunity are intervals within a year that provide improved prospects for growth, survival, or reproduction. However, few studies have sufficient temporal resolution to examine how multiple factors combine to constrain the seasonal timing and extent of developmental opportunities. Here, we document seasonal changes in milkweed (Ascelpias fascicularis) – monarch (Danaus plexippus) interactions with high-resolution throughout the last three breeding seasons prior to a precipitous single-year decline in the western monarch population. Our results show early- and late-season windows of opportunity for monarch recruitment that were constrained by different combinations of factors. Early-season windows of opportunity were characterized by high egg densities and low survival on a select subset of host plants, consistent with the hypothesis that early-spring migrant female monarchs select earlier-emerging plants to balance a seasonal trade-off between increasing host plant quantity and decreasing host plant quality. Late-season windows of opportunity were coincident with the initiation of host plant senescence and caterpillar success was negatively correlated with heatwave exposure, consistent with the hypothesis that late-season windows were constrained by plant defense traits and thermal stress. Throughout this study, climatic and microclimatic variation played a foundational role in the timing and success of monarch developmental windows by affecting bottom-up, top-down, and abiotic limitations. More exposed microclimates were associated with higher developmental success during cooler conditions and more shaded microclimates were associated with higher developmental success during warmer conditions, suggesting that habitat heterogeneity could buffer the effects of climatic variation. Together, these findings show an important dimension of seasonal change in milkweed-monarch interactions and illustrate how different biotic and abiotic factors can limit the developmental success of monarchs across the breeding season. These results also suggest the potential for seasonal sequences of favorable or unfavorable conditions across the breeding range to strongly affect monarch population dynamics.

We collected data at approximately weekly intervals throughout each growing season in 2015, 2016, and 2017. For each observation on each milkweed, we used a standardized protocol that included assessments of plant status (presence of emergent stems, percentage of non-senescent tissue, percentage of leaf area removed by herbivores), measurements of plant size (number of stems >5 cm, mean stem length, mean stem diameter), counts of milkweed reproduction (number of open floral umbels, number of non-senescent seed pods longer than 1 cm), and measurements of any monarch eggs or caterpillars present (number of monarchs eggs, number of monarchs of each larval instar, larval length). Percentages of non-senescent tissue and leaf area removed were estimated visually, measurements of stem length were taken with meter sticks to the nearest cm, and measurements of stem diameter and larval length were measured using dial calipers to the nearest 0.1 mm. Finally, participants collected additional notes, including observations of the surrounding predator and herbivore community. 

Please see the associated publication for method details. Please see the included Yang_et_al_MMMILC_README.txt for meta-data.