Fig 1. Schematic of model representing protozoan pathogen transmission in monarch hosts.
Monarchs are represented as uninfected or infected immature stages (SL or IL) or adults (SA or IA). Milkweed leaves with spores (W) arise when infectious parasite doses are deposited by infected adult monarchs (at rate λ), and are lost through larval consumption (c) or through decay in dose viability (at rate μW). The model assumes that all larvae produced by infected adult females (IA) are born with infections (IL; perfect vertical transmission). Uninfected larvae (SL) can become infected (IL) by consuming leaves with spores (environmental transmission, dashed line).