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.

Sub-lethal viral exposure and growth on drought stressed host plants changes resource allocation patterns and life history costs in the Speckled Wood butterfly, Pararge aegeria

“…With global climate change emergent infectious diseases have risen significantly and climate warming has resulted in changes in the severity and prevalence of some infectious diseases, pests and pathogens (Daszak et al, 2000;Jones et al, 2008;Bebber, 2015;Shikano and Cory, 2015;Kolb et al, 2016;Ogden and Lindsay, 2016). In ectotherms, like herbivorous insects, climate change can impact host-pathogen dynamics both directly and indirectly.…”

“…For example, rising temperatures can directly increase the likelihood of an insect herbivore encountering pathogens via increased feeding rates (Eldred and Reilly, 2014). Increasing temperature and carbon dioxide levels may also reduce insect immune functioning and thereby directly increase susceptibility to disease by decreasing host plant quality (increasing plant defensive traits and decreasing nitrogen content) thereby decreasing immune functioning (Shikano and Cory, 2015). Another, as yet, underexplored mechanism by which climate change may impact host-pathogen dynamics is via an increase in the rate and magnitude of climatic extremes such as drought (e.g.…”

“…Effective resistance to pathogens is often dependent on resource availability and resource quality, because resource availability and quality improves body condition and also immune system deployment (Shikano and Cory, 2015;McKay et al, 2016). From ecoimmunological studies and life history theory, there is good evidence that investment in pathogen defences comes at the expense of other physiological processes such as growth and reproduction (Stearns, 1992;Ricklefs and Wikelski, 2002;Schmid-Hempel, 2005;McKay et al, 2016).…”

“…From ecoimmunological studies and life history theory, there is good evidence that investment in pathogen defences comes at the expense of other physiological processes such as growth and reproduction (Stearns, 1992;Ricklefs and Wikelski, 2002;Schmid-Hempel, 2005;McKay et al, 2016). These life history trade-offs may be exacerbated by development on low quality host plants, as immune response and compensatory growth in response to low resource quality both use energy reserves from the same resource pool (Shikano and Cory 2015;McKay et al, 2016). Food restriction during development can alter fitness and immunity across both juvenile and adult life stages (McKay et al, 2016).…”

“…Mangel and Munch (2005) identified a range of possible compensatory responses such as; i) faster than normal growth immediately following the end of the deprivation period, ii) faster than normal growth at some time later (long term compensation) and iii) faster than normal growth and over-taking control individuals that continue to grow normally. As such there is unlikely to be a simple easily predictable response, and just knowing that immune functioning increases with resource quality may not be sufficient to determine the response of different species, or even different individuals within a species, to the interacting effects of food quality and pathogen exposure (Shikano and Cory, 2015).…”