Introduction
Insect populations are declining in abundance around the world (Wagner et al. 2021). Even abundant and well-known species such as the monarch butterfly are disappearing (Thogmartin et al. 2017). This ‘insect apocalypse’ is a concern given that insects provide many ecosystem services that ensure human well being (Noriega et al. 2018). Our project aims to understand the the factors that govern the abundance of the hops azure butterfly (Celastrina humulus), a G2 imperiled species in Colorado (NatureServe, 2021). Since 2020, we have monitored the abundance of caterpillars and the interactions they have with host plants and other insects.
Natural History
In Colorado’s Front Range, caterpillars of the rare hops azure butterfly (Celastrina humulus, Family Lycaenidae) feed on the male cones of American hops (Humulus lupulus var. neomexicanus, Family Cannabaceae). The caterpillars in turn interact with ants, which walk over the caterpillars and drink nectar from specialized organs on the caterpillar’s back. Interactions between ants and lycaenid larvae are notably variable, ranging from obligate to facultative and from mutualism to parasitism. Kubik and Schorr (2018) found hops azure larvae associated with ants in 10 of 15 sites along Monument Creek. This on again, off again association suggests a facultative rather than an obligate mutualism.
Large patches of the host plant tend to be particularly attractive to these butterflies. These large patches are often comprised of many flowering vines known as bines. Hops azure females lay eggs on immature male inflorescences in late June through early July. The larvae develop while feeding on pollen inside of the male flowers. Pollen is rich in nitrogen-containing protein. To access the pollen, larvae chew through the sepals surrounding the anthers. This exposes larvae to the resin glands that give hops its characteristic scent and flavor. Ants show a variety of behaviors towards larvae. Ants alternate between resting on or near larvae and actively touching their antennae to the larvae’s dorsal nectary organ. This collection of ant behaviors is known as tending.
Field Monitoring
The UCCS Plant Ecology laboratory began work in this system in 2020. We sought to determine which factors or combination of factors are associated with ant tending. To this end, we monitored larvae on bines across replicate dates in early July. We recorded which larvae were tended by ants and those that were untended. We also recorded the phenological stage of the flowers where larvae fed. Larvae develop very quickly on their protein rich diets! Most larvae had moved off bines (presumably to pupate) after about a week. We then measured a variety of factors that could be associated with ant tending. Our goal is to repeat these measures each year to get a sense of year-to-year variation in this system.
What We’ve Found Out
We used a model selection approach to sort out which of the abiotic and biotic factors were associated with ant tending. To fit these models, we combined monitoring data from 2020 through 2021. The model that was the closest match to the data included the explanatory variables of larvae per bine, patch area, and phenological stage of the hops flowers. We found that the likelihood of ant tending increased with the number of larvae per bine. Similarly, the likelihood of ant tending increased with the area of the hops patch. This may be simple density dependence whereby ants concentrate their foraging where larvae are most abundant. Lastly, larvae on later stage flowers were more likely to be tended by ants. We explored this last result more with several laboratory assays. We presented our results as a poster at the 2021 Entomological Society of America Meeting.
Acknowledgments
Land Acknowledgment: We acknowledge that this research takes place in the traditional territories of Núu-agha-tʉvʉ-pʉ̱ (Ute) .
Students: Abbey Swift, Clint Hamilton, Hailee Nolan, Eryn Callanan, and Ryan Callanan.
Funding: This project has been supported through funds from the John Marr Grant of the Colorado Native Plants Society and the UCCS Undergraduate Research Academy.
UCCS Plant Ecology 