Wet years have more caterpillars: interacting roles of plant litter and predation by ants

Abstract

Abstract. Climate is widely recognized as an important factor that affects temporal and spatial patterns of occurrence and abundance of herbivorous insects, although the ecological mechanisms responsible are poorly understood. We found that precipitation and standing water were positively correlated with locations and years of high abundance of caterpillars of the ranchman's tiger moth, Platyprepia virginalis. We analyzed 30 years of survey data and found that the number of large rainfall events was a better predictor of caterpillar abundance than total annual accumulation. We considered three ecological mechanisms that could drive this relationship and conducted observations and manipulative experiments to evaluate these mechanisms.(1) Rainfall facilitates more plant growth, although we found no evidence that increased food quality or quantity was causing the positive association between precipitation and caterpillar abundance. (2) Large rainfall events cause predatory ground-nesting ants to be less abundant and we found that the number of ants that recruited to local sites was negatively associated with survival and abundance of caterpillars. (3) We found that litter from wet sites provided a refuge from ant predation; litter from wet sites was not beneficial to caterpillars in the absence of ants. Both abiotic factors (precipitation) and biotic factors (predatory ants) affected the temporal and spatial abundance of caterpillars directly and interactively. Climate models predict that rainfall will become more variable, suggesting that populations of this caterpillar may also become more variable in the future.

As temperature increases, predator attack rate is more important to survival than a smaller window of prey vulnerability

“…Previous work that focused on expectations of lower future precipitation predicted higher predation by ants due to higher ant densities and fewer refuges for caterpillars (Karban et al. ).…”

“…Previous long‐term work on A. virginalis at Bodega Marine Reserve, Sonoma, California, has shown large annual fluctuations in caterpillar densities (Karban and de Valpine ), and suggested that precipitation and ant predation may be important drivers of observed dynamics (Karban et al. ). Ant predation has been shown in the laboratory to be affected by temperature (Karban et al.…”

“…In this study, we examined the effect of experimental warming on the interaction between the ant Formica lasioides and its prey, caterpillars of the tiger moth, Arctia virginalis (formerly Platyprepia virginalis). Previous longterm work on A. virginalis at Bodega Marine Reserve, Sonoma, California, has shown large annual fluctuations in caterpillar densities (Karban and de Valpine 2010), and suggested that precipitation and ant predation may be important drivers of observed dynamics (Karban et al 2017). Ant predation has been shown in the laboratory to be affected by temperature , and by the depth and quality of litter substrate (Karban et al 2013).…”

Rain downpours affect survival and development of insect herbivores: the specter of climate change?

“…), which, in turn, can affect the behavior and development of insect herbivores (Huberty and Denno ), as well as the interaction with their natural enemies (Karban et al. ). In extreme cases, prolonged drought can lead to early plant estivation or even death (McDowell et al.…”

“…Prolonged lack of water and exposure to extreme heat can inflict huge physiological stresses both on plants and higher trophic level organisms. For instance, drought can lead to changes in primary and secondary metabolism and the overall physiological status of the plant (Jamieson et al 2012), which, in turn, can affect the behavior and development of insect herbivores (Huberty and Denno 2004), as well as the interaction with their natural enemies (Karban et al 2017). In extreme cases, prolonged drought can lead to early plant estivation or even death (McDowell et al 2011), and this, in turn, will clearly affect and entire community of organisms associated with the plant.…”