Introduction

Aphids are sap-feeding insects that tap into the phloem of plants. Phloem sap is low in the nitrogen that bugs need. As a result, aphids are high throughout feeders and much of the sap that goes into an aphid gets excreted as honeydew. This honeydew is a critical currency in the mutualism with ants: ants protect aphids from predators while consuming honeydew.

Since 2011, our team has been monitoring colonies of aphids (Aphis asclepiadis) on the host plant Ligusticum porteri. This aphid forms dense colonies on host plant flowering stalks. Each summer we visit ten host plant populations and count insects on ten focal plants. We also record the flowering stage of the host plant during our weekly visits. Our monitoring effort expanded to an additional ten populations along a wider elevation gradient in 2017. These simple counts have revealed key patterns for how plants, insects, and all manner of interactions are changing in response to earlier springs and hotter summers.

What We’ve Found Out

Years with early loss of snow cover are associated with low numbers of aphid colonies. Through several experiments, we found that this is due to changes in both predators and host plants. When snowmelt comes later, soils stay moist and host plants become drought stressed. This lowers their quality for aphid colony growth. Aphid predators known as lygus bugs also surge in abundance when spring arrives early. Predation by adult lygus bugs can drive aphid colonies to extinction. Lygus feeding also changes the volatiles that aphids use to choose host plants.

What’s Next?

There is a whole community of interactions happening on the flowering stalks of Ligusticum porteri: aphids are sucking sap, ants are catching honeydew droplets, and flies are buzzing flowers. Many of these flies are so-called flower flies (Syrphidae). In addition to being pollinators, flower flies are also important predators of aphids. Their larvae are exclusively aphidophagous, i.e. they only eat aphids! Interestingly, rates of predation by flower fly larvae are closely correlated with counts of adult flies on the flowers. Year-to-year variation in counts of adult flies on flowers are associated with the flowering phenology of Ligusticum. We will keep up monitoring to see how predation by flower flies and other insects interacts with changing climate cues.

Acknowledgments

Land Acknowledgment: We acknowledge that this research takes place in the traditional territories of Núu-agha-tʉvʉ-pʉ̱ (Ute) .
Students: James Den Uyl, Maria Mullins, Eva Medina, Cheryl Sandrow, Elsa Godtfredsen, Emily Cruz, Ben Davidson, and Ayla Robinson.
Funding: This project has been supported through funds from the National Science Foundation (Award ID), the UCCS CRCW fund, and by fellowships to individual students from Colorado College and RMBL.