Virus infection significantly decreases insect fitness: a meta-analysis

Organisms are constantly at risk of being infected by pathogens such as viruses. Adaptations against viral infection include immune defenses encoded within the host genome and associations with defensive symbionts such as Wolbachia. Wolbachia is widely spread among insects, and its success in nature may be due to its antiviral effects, which can benefit hosts if viruses significantly reduce host fitness. Here, we conducted a meta-analysis to assess the degree to which viral infection affects the fitness of insect hosts, and which factors may influence the impact of viral infection on hosts such as if the insect host is a vector or not of viruses that cause diseases in humans and plants, and if the insect is a new or a natural host of a specific virus. We gathered 1,040 effect sizes from 150 studies. Our results show that viruses significantly reduce host fitness, especially their survival. The decrease in host fitness is higher in non-vector than in vector insects, and we found no difference in fitness decrease caused by viral infection between new and natural hosts. Moreover, we found that fitness effects caused by viruses vary between host order and fitness components. In conclusion, our results show that viruses exert severe harmful effects on hosts by decreasing their fitness. In this context, harboring symbionts that confer antiviral protection, such as Wolbachia, can be highly advantageous for hosts, enhancing their fitness. Conversely, Wolbachia can benefit from the presence of viruses, facilitating its spread in insect populations by offering antiviral protection.