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.

Compatibility of the fungus Beauveria bassiana and Trichoplusia ni SNPV against the cabbage looper Trichoplusia ni: crop plant matters

“…bassiana on viral infection in T . ni larvae shown previously on diet 33 . Under laboratory‐controlled conditions when spongy moth larvae, Lymantria dispar , were infected by both LdMNPV and the fungus Entomophaga maimaiga synchronously, larvae generally died of fungal infection due to its more rapid speed of kill 46 .…”

“…21 Our present study using crop plants supports the negative impact of B. bassiana on viral infection in T. ni larvae shown previously on diet. 33 Under laboratory-controlled conditions when spongy moth larvae, Lymantria dispar, were infected by both LdMNPV and the fungus Entomophaga maimaiga synchronously, larvae generally died of fungal infection due to its more rapid speed of kill. 46 Similarly, the rapid speed of kill of E. maimaiga in L. dispar field populations was shown to limit opportunities for LdMNPV within-host replication.…”

“…bassiana acted faster and had a negative effect on TniSNPV in T . ni , 33 when applied synchronously, we added two asynchronous treatments. In these, TniSNPV was applied 24 h before the fungus to reduce potential competition and increase spray efficacy on the host plant compared to single and synchronous pathogen applications.…”

“…Using the application rates described above, a total of nine treatments were used: four single applications, four mixed applications and an untreated control (see Table 1). Following a previous laboratory study showing that B. bassiana acted faster and had a negative effect on TniSNPV in T. ni, 33 when applied synchronously, we added two asynchronous treatments. In these, TniSNPV was applied 24 h before the fungus to reduce potential competition and increase spray efficacy on the host plant compared to single and synchronous pathogen applications.…”

“…We first focused on (i) whether mixed pathogen applications altered overall larval mortality then (ii) we examined varying the timing of application. Based on early laboratory studies on artificial diet, 33 we predicted that there would be a negative interaction between the pathogens when they were applied together. Thus, we also included asynchronous applications in our experimental design, where the slower‐killing virus was applied before the fungus.…”