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).
Table 1.
Parameters of the model, including definitions, units, and values.
See the S2 File for derivation and data sources for parameter estimates.
Fig 2.
Experimental results on infectivity and pathogen load following spore exposure to sun or shade treatments for 0, 6, 11 or 16 days.
(a) Proportion of monarchs infected following inoculation with spores on milkweed leaves exposed for 0 days (n = 27); 6 days, in either sun (n = 15) or shade conditions (n = 15); 11 days in sun (n = 23) or shade (n = 24); or 16 days in sun (n = 25) or shade (n = 19). Error bars show Jeffrey’s 95% confidence intervals for binary data. (b) Pathogen load (log-transformed) of monarchs infected with spores exposed for 0 days (n = 22); 6 days, in either sun (n = 10) or shade conditions (n = 11); 11 days in sun (n = 14) or shade (n = 18); or 16 days in sun (n = 17) or shade (n = 16). Error bars show standard error of the mean.
Fig 3.
Results from the transmission model showing within-season dynamics.
Within-season dynamics in (a) infection prevalence and (b) adult population size within the milkweed patch for a range of pathogen persistence times (in days, 1/μw). Longer pathogen persistence produced higher infection prevalence and slightly lower adult population size. The dynamics shown assume an intermediate value for parasite shedding rate (λ = 150 leaves/day/adult); all other parameter values are listed in Table 1.
Fig 4.
Parasite prevalence and host population size at the end of the season based on the transmission model, for a range of pathogen persistence times (1/μW) and shedding rates (λ).
Results from the model shown as heat contours for (a) infection prevalence at the end of the breeding season, and (b) adult monarch population size (T = 100 days; scale bars), depending on a range of pathogen persistence times and shedding rates.