Quantifying Infection-Feeding Interactions

To examine the relationship between infection‐mediated feeding behaviors and host susceptibility, we measured the proportion of hosts that became infected and the survivorship of infected hosts relative to control hosts (0 sp/mL treatment). Then, to determine how changes in feeding behaviors affect the realized fitness of both hosts and pathogens, we quantified host growth, fecundity, and transmission potential (spore yield). We maintained individuals at 22°C in vials of 15 mL of COMBO for 14 days post‐exposure (or death, whichever came first). Individuals were moved to fresh media, fed (1 mgC/L for the first 7 days and 2 mgC/L for the rest), and checked for offspring every other day. To keep results consistent across individuals, we ended the experiment 14 days after exposure, when the first individuals began to die; our central goal here was to quantify changes in feeding rate and transmission potential (spore yield) and not longevity.
We visually diagnosed terminal infections with a dissecting microscope, measured final body size (at 5× magnification), and collected individuals to quantify spore loads using flow cytometry. We used a DxP10 flow cytometer (Cytek) equipped with a BD FACSort system (Becton Dickinson Biosciences). To isolate mature transmission‐ready spores from algae, animal debris, or immature spores (Stewart Merrill & Caceres, 2018 (link)), we used custom gates based on fluorescence forward scatter (FSC) and side scatter (SSC) with 488 and 561 nm lasers and fluorescent beads as standards (SPHERO; AccuCount Fluorescent Particles, 7.0–8.0 μm) at a ratio of 12:1 for each individual's spore solution (1 animal in 300 μL of COMBO). We then verified flow cytometry spore counts by randomly selecting five individuals from each genotype and spore exposure level and manually counting spores using a hemocytometer (for “Standard,” R² = .91; patterns for all other genotypes were comparable—see Appendix S1).