Case study: heat tolerance of three cereal aphid species (Li 2023)

This article compares the heat tolerance of three cereal aphid species — Metopolophium dirhodum, Rhopalosiphum padi, and Sitobion avenae — from the thermal-tolerance assays of Li et al. (2023). It asks the comparative-physiology question directly: do the species differ in their critical temperature CTmax and thermal-sensitivity slope z? It mirrors the cereal-aphid case study in bayesTLS, fit here by maximum likelihood instead of MCMC, with one CTmax and one z per species.

library(freqTLS)

The dataset and the question

aphid_tdt is vendored from bayesTLS (Li et al. 2023) under CC0; cite the source and citation("freqTLS") when you use it (see ?aphid_tdt). It records survival counts in a temperature × duration grid for the three species, crossed with a cold and a heat exposure branch and three nymph ages. We take the heat branch at a single focal age (6-day-old nymphs) so the cross-species comparison is made at one consistent developmental stage, and let CTmax and z depend on species. (The other ages and the cold branch are available in aphid_tdt for the same workflow.)

data(aphid_tdt)
aphids <- subset(aphid_tdt, branch == "heat" & age == "6")
table(aphids$species)
#> 
#> M_dirhodum   S_avenae     R_padi 
#>        169        165        165
std <- standardize_data(
  aphids,
  temp = "temp", duration = "duration_min",
  n_total = "n_total", n_surv = "n_surv",
  duration_unit = "minutes"
)

The design spans 7 assay temperatures and exposures up to 2000 minutes across the three species (499 rows). species is the covariate of interest, and CTmax is reported at a one-hour reference exposure (t_ref = 60 minutes).

The grouped fit (live)

A single fit_4pl() call fits all three species at once, each with its own CTmax and z and a shared constant 4PL shape. The fit runs by maximum likelihood (TMB, no Stan); for this well-identified three-group fit we report the fast Wald confidence intervals (the profile-likelihood intervals that are the freqTLS default are showcased on the single-fit studies, e.g. vignette("case-study-shrimp")).

fit <- suppressWarnings(fit_4pl(
  std,
  ctmax = ~ 0 + species,
  z     = ~ 0 + species,
  t_ref = 60,
  family = "beta_binomial"
))
ax <- tls(fit, by = "species", method = "wald")
ax$summary
#> # A tibble: 6 × 5
#>   species    quantity median lower upper
#>   <chr>      <chr>     <dbl> <dbl> <dbl>
#> 1 M_dirhodum CTmax     35.4  35.2  35.5 
#> 2 S_avenae   CTmax     36.6  36.5  36.7 
#> 3 R_padi     CTmax     37.2  37.1  37.3 
#> 4 M_dirhodum z          4.55  4.34  4.77
#> 5 S_avenae   z          3.46  3.33  3.60
#> 6 R_padi     z          3.62  3.42  3.83

The species order from least to most heat-tolerant (one-hour CTmax) as M_dirhodum (35.4 °C) < S_avenae (36.6 °C) < R_padi (37.2 °C). Where two species’ CTmax intervals are disjoint the data reject equal tolerance at the 95% level; the table above shows which separations the data support.

A Confidence Eye per species

The freqTLS uncertainty visual is the Confidence Eye — a confidence lens with a hollow point estimate, carrying no prior. One lens per species makes the CTmax ranking legible; the parameter names are read off the fit so they always match the fitted labels.

plot_confidence_eye(fit, parm = get_ctmax(fit)$parameter, method = "wald")

Confidence Eyes for cereal-aphid CTmax by species: three pale confidence lenses with hollow point estimates, one per species, ordered along the temperature axis.

The same display for the thermal-sensitivity slope z:

plot_confidence_eye(fit, parm = get_z(fit)$parameter, method = "wald")

Confidence Eyes for cereal-aphid z (thermal sensitivity) by species: three pale confidence lenses with hollow point estimates.

Seeing the fit

The fitted survival surface, one curve per species and assay temperature, with the observed proportions overlaid:

plot_survival_curves(fit)

Fitted cereal-aphid survival curves by species: probability of survival declining with exposure duration, one curve per assay temperature, the three species overlaid.

Boundary: what this case study does not cover

We fit the heat branch at a single nymph age (6 days), letting only CTmax and z vary by species (the constant-shape configuration that matches the bayesTLS analysis). The cold branch, the other ages, and species-specific curve shapes are extensions: the shape covariate (letting low, up, k vary by group) is demonstrated in vignette("comparing-to-bayesTLS"). Formal species contrasts (a difference in CTmax or z with its own confidence interval and a likelihood-ratio test) are available via the contrast interface described in vignette("frequentist-and-bayesian").

Where to next: vignette("random-effects") for many-group hierarchical fits; vignette("comparing-to-bayesTLS") for the three-way method comparison.