Resource limitation has a limited impact on the outcome of virus-fungus co-infection in an insect host

Abstract

Infection by pathogens is strongly affected by the diet or condition of the prospective host. Studies that examine the impact of diet have mainly focused on single pathogens; however, co-infections within a single host are thought to be common. Different pathogen groups might respond differently to resource availability and diverse infections could increase the costs of host defense, meaning the outcome of mixed infections under varying dietary regimes is likely to be hard to predict. We used the generalist cabbage looper, Trichoplusia ni and two of its pathogens, the DNA virus T. ni nucleopolyhedrovirus (TniSNPV) and the entomopathogenic fungus, Beauveria bassiana to examine how nutrient reduction affected the outcome of mixed pathogen infection. We challenged insects with a low or high effective dose of virus, alone or combined with a single dose of fungus. We manipulated food availability after pathogen challenge by diluting artificial diet with cellulose, a non-nutritious bulking agent, and examined its impact on host and pathogen fitness. Reducing diet quantity did not alter overall or pathogen-specific mortality. In all cases, TniSNPV-induced mortality was negatively affected by fungus challenge. Similarly, B. bassiana-induced mortality was negatively affected by TniSNPV challenge, but only at the higher virus dose. Dietary dilution mainly affected B. bassiana speed of kill when mixed with a high dose of TniSNPV, with an increase in the duration of fungal infection when cellulose was low (high quantity). One pathogen dominated the production of transmission stages in the cadavers and co-infection did not affect the yield of either pathogen. There was no evidence that co-infections were more costly to the survivors of pathogen challenge. In conclusion, dietary dilution did not determine the outcome of mixed pathogen infection, but it had more subtle effects, that differed between the two pathogens and could potentially alter pathogen recycling and host-pathogen dynamics.

Keywords: Beauveria bassiana; baculovirus; host nutrition; mixed infection; pathogen replication; trade‐offs.

FIGURE 1

Fungus‐induced mortality of 4th instar T. ni larvae challenged either with B. bassiana alone (No virus) or co‐infected with TniSNPV at a low (100 OBs) or high (1000 OBs) dose. Letters indicate significant differences at p < .05 and numbers show the sample size

FIGURE 2

Overall speed of kill of 4th instar T. ni larvae challenged with TniSNPV (a) at a low (100 OBs) or a high (1000 OBs) dose, alone (Single) or co‐infected with B. bassiana (Mixed) (b) either alone or co‐infected with B. bassiana, on diets varying in levels of cellulose, and (c) at a low or a high dose, on diets varying in levels of cellulose. Letters indicate significant differences at p < .05 (Tukey's HSD)

FIGURE 3

Fungal speed of kill of 4th instar T. ni larvae challenged with B. bassiana alone (No virus) or co‐infected with TniSNPV at low (100 OBs) or high (1000 OBs) dose, on diets varying in the amount of cellulose. Letters indicate significant differences at p < .05 (Tukey's HSD)

FIGURE 4

Virus yield and speed of kill trade‐off. Lines show fitted statistical models with 95% confidence intervals and raw data points (n = 140)

FIGURE 5

Effect of diet quantity on T. ni pupal weight (ns, non‐significant; *.05 < p < .01; **.01 < p < .001; ***p < .001)