Plant water limitation and its impact on the oviposition preferences of the monarch butterfly, Danaus plexippus (Lepidoptera: Nymphalidae)

Intensifying drought conditions across the western United States due to global climate change are altering plant-insect interactions. Specialist herbivores must find their host plants within a matrix of nonhosts, and thus often rely upon specific plant secondary chemistry for host location and oviposition cues. Climate-induced alterations to plant chemistry could thus affect female selection of larval food-plants. Here, we investigated whether host-plant water limitation influenced oviposition preference in a threatened invertebrate: the monarch butterfly (Danaus plexippus). We found that females deposited more eggs on reduced-water than on well-watered narrowleaf milkweed plants (Asclepias fascicularis), but we could not attribute this change to any specific change in plant chemistry. Specialist herbivores, such as the monarch butterfly, which are tightly linked to specific plant cues, may experience a shift in preferences under global-change conditions. Understanding oviposition prefe..., Seeds of A. fascicularis were collected from Reno, NV (39.49361, -119.85459) in 2018 and 2019 and germinated in May 2020. Plants were grown in 164 mL treepots with 50% peat moss: 34% vermiculite: 16% perlite. To manipulate water availability, soil saturation was maintained at 70% field capacity in control (well-watered) plants and at 30% in reduced-water plants for 1–3 weeks using a gravitational dry-down method following Diethelm et al. (2022). The variation in dry-down time is due to initially high dieback in the reduced-water group, which led us to add more plants to that treatment group, with the treatment maintained for 1 week. The 70% control level reflects what plants typically experience at agricultural field edges, whereas 30% represents a dry treatment that does not induce wilting (Diethelm et al. 2022). We avoided wilting because females may discriminate against wilted plants, independent of plant chemistry (Aikins et al. 2023).
To allow female monarchs to select a mate, each..., Instrument- or software-specific information needed to interpret the data: -R Core Team (2023). R: A language and environment for statistical computing. R Foundation for Statistical Computing, Vienna, Austria. URL https://www.R-project.org/.-RStudio (2023) URL http://www.rstudio.com/-Rstudio packages: tidyvers, ggplot, ggforce (for jitter plot), Hmisc, Rmisc (for SE summary), lmtest (for model comparisions), glmmTMB (for mixed models), MuMin, DHARMa, BBmisc, hillR, ggpubr