Tomato spotted wilt virus facilitates non-vector spider mite species (Tetranychus urticae and Tetranychus evansi) on whole tomato plants

Coinfections where hosts harbour more than one parasite species are common in nature. Facilitation among parasites enabling them to better exploit host resources is widespread with direct consequences on their life-history. Plant viruses can facilitate their vectors to increase their transmission, but equivalent studies in virus-non vectors are lacking. Here we study facilitation of two (non-vector) species of Tetranychus spider mites, Tetranychus urticae and T. evansi, by a plant virus, tomato spotted wilt virus (TSWV), in coinfections on a host tomato plant, Solanum lycopersicum. We compared the effect of different strains of TSWV on life-history traits of Tetranychus mites on cut leaves and whole plants of different tomato varieties. TSWV facilitated both species of spider mite on two different tomato varieties on whole plants, with more offspring of both species becoming adults. In contrast, on cut leaves, facilitation of T. urticae was much more variable depending on the experiment, viral strain and tomato variety. We attribute this to the non-homogeneous spread of virus throughout the host plant. Virus is first transported to the top leaves, and not middle leaves which we had used for the experiment. Indeed, if facilitation is indirect via the host immune system or resource based via the release of free-amino acids then this may be less efficient on cut plant parts. These results highlight that facilitation among parasites may increase parasite fitness and at the same time levels of virulence experienced by the host.