Incorporating host traits into analysis of parasite life history reveals facilitation in coinfection experiments.

Coinfection, when hosts are infected with more than one parasite, can modify life-history traits of both parasites and their host. We investigated reciprocal interactions between tomato spotted wilt virus (TSWV) and a non-vector spider mite, Tetranychus urticae, in coinfections on tomato plants. We compared the number of T. urticae daughters and adults and the viral load for two TSWV isolates (France81, which is more virulent, and LYE1137vir) in single and coinfections, and measured host traits (height, fresh weight, chlorophyll content, and number of flowers). TSWV infection reduced plant height, weight, flowers and chlorophyll content, especially when infected with isolate France81. T. urticae only reduced plant height. The more virulent TSWV isolate facilitated spider mites by increasing the number of daughters per unit of plant height, thus potentially enabling equivalent levels of mite transmission as from virus-free plants. In contrast, there was no effect of T. urticae on the load of either TSWV isolate. Nevertheless, the additive reduction in plant height in coinfection may increase host mortality and reduce the time available for transmission of both parasites. This work highlights that it can be important to incorporate measures of host and parasite life-history traits across an entire infection when investigating the consequences of coinfection for parasite fitness.