Warmer temperatures increase disease transmission and outbreak intensity in a host-pathogen system.

While rising global temperatures are increasingly affecting both species and their biotic interactions, the debate about whether global warming will increase or decrease disease transmission between individuals remains far from resolved. This may stem from the lack of empirical data. Using a tractable and easily manipulated insect host-pathogen system, we conducted a series of field and laboratory experiments to examine how increased temperatures affect disease transmission using the crop-defoliating pest, the fall armyworm (Spodoptera frugiperda) and its species-specific baculovirus, which causes a fatal infection. To examine the effects of temperature on disease transmission in the field, we manipulated baculovirus density and temperature. As infection occurs when a host consumes leaf tissue on which the pathogen resides, baculovirus density was controlled by placing varying numbers of infected neonate larvae on experimental plants. Temperature was manipulated by using open-top chambers (OTCs). The laboratory experiments examined how increased temperatures affect fall armyworm feeding and development rates, which provide insight into how host feeding behaviour and physiology may affect transmission. Disease transmission and outbreak intensity, measured as the cumulative fraction infected during an epizootic, increased at higher temperatures. However, there was no appreciable change in the mean transmission rate of the disease, which is often the focus of empirical and theoretical research. Instead, the coefficient of variation (CV) associated with the transmission rate shrunk. As the CV decreased, heterogeneity in disease risk across individuals declined, which resulted in an increase in outbreak intensity. In the laboratory, increased temperatures increased feeding rates and decreased developmental times. As the host consumes the virus along with the leaf tissue on which it resides, increased feeding rate is likely to increase the probability of an individual consuming virus-infected leaf tissue. On the other hand, decreased developmental time increases the sloughing of midgut cells, which is predicted to hinder viral infection. Increases in outbreak intensity or epizootic severity, as the climate warms, may lead to changes in the long-term dynamics of pests whose populations are strongly affected by host-pathogen interactions. Overall, this work demonstrates that the usual assumptions governing these effects, via changes in the mean transmission rate alone, may not be correct.