Impact Of Environmental Variation On Host Performance Differs With Pathogen Identity: Implications For Host-Pathogen Interactions In A Changing Climate

Abstract

Specialist and generalist pathogens may exert different costs on their hosts; thereby altering the way hosts cope with environmental variation. We examined how pathogen-challenge alters the environmental conditions that maximize host performance by simultaneously varying temperature and nutrition (protein to carbohydrate ratio; P:C) after exposure to two baculoviruses; one that is specific to the cabbage looper, Trichoplusia ni (TnSNPV) and another that has a broad host range (AcMNPV). Virus-challenged larvae performed better on more protein-biased diets, primarily due to higher survival, whereas unchallenged larvae performed best on a balanced diet. The environmental conditions that maximized host performance differed with virus identity because TnSNPV-challenge inflicted fitness costs (reduced pupal weight and prolonged development) whereas AcMNPV-challenge did not. The performance of TnSNPV-challenged larvae rose with increasing P:C across all temperatures, whereas temperature modulated the optimal P:C in AcMNPV-challenged larvae (slightly protein-biased at 16 °C to increasingly higher P:C as temperature increased). Increasing temperature reduced pupal size, but only at more balanced P:C ratios, indicating that nutrition moderates the temperature-size rule. Our findings highlight the complex environmental interactions that can alter host performance after exposure to pathogens, which could impact the role of entomopathogens as regulators of insect populations in a changing climate.

Figure 1. Impact of temperature and dietary P:C ratio on the survival, development time (log₁₀ mean days to pupation ± SE) and dry pupal weight (mg ± SE ) of unchallenged control (a,d,g), TnSNPV-challenged (b,e,h) or AcMNPV-challenged (c,f,i) Trichoplusia ni in the fifth instar.

Lines represent fitted minimal models (Table 2).

Figure 2. Fitness costs associated with virus challenge in fifth instar Trichoplusia ni measured by (a) the proportion of larvae survived, (b) development time (least squares mean log₁₀ days to pupation ± SE) and (c) least squares mean dry pupal weight (mg).

Different letters indicate significant differences (P < 0.05) identified by the main effect of virus treatment (no virus, TnSNPV challenge or AcMNPV challenge) from the GLM for survival (pairwise contrast; Table 1) and ANCOVA for development time and pupal weight (Tukey HSD comparison; Table 1).

Figure 3. Response surface illustrating the bivariate effects of temperature and dietary P:C ratio on Trichoplusia ni performance after (a) no virus challenge, (b) TnSNPV challenge and (c) AcMNPV challenge.

Performance was calculated by multiplying the rescaled values (1 to 2) for the individual probability of survival, dry pupal weight and the inverse of development time. Warmer colours and higher values on contour lines indicate higher performance (deep red, highest performance; deep blue, poorest performance).