Injection but not ingestion of a mixed virus suspension in Queensland fruit fly (Bactrocera tryoni) shifts persistent covert infection to acute pathology

BACKGROUND: Pest fruit flies commonly carry diverse RNA viruses with unknown host effects that may affect pest management strategies. We investigated effects of horizontally transmitted cripavirus and vertically transmitted iflavirus in Queensland fruit fly that also carried orbivirus, toti-like virus and xinmovirus as persistent covert infections. RESULTS: Individuals persistently infected with these five viruses had slower egg-to-pupa development, lower emergence and lower adult survival under stress than individuals without cripavirus and iflavirus, but persistently infected with the other three viruses. Pupal weight and flight ability did not differ. Cripavirus and iflavirus loads were stable, even under stress, except for the iflavirus load which was higher in flies that died late and lower in flies under stress after cold treatment. Injection into flies of a suspension containing all five viruses caused high mortality within 20 days irrespective of a persistent iflavirus infection. This resulted in an increased iflavirus load 2 days after injection, whereas the load of the other four viruses did not change, suggesting that iflavirus may have caused the mortality. By contrast, feeding the mixed virus suspension did not reduce survival. Although iflavirus was detected immediately after ingestion, its load decreased over 14 days as expected for a maternally transmitted virus. The load of the other four viruses remained unchanged. CONCLUSION: RNA viruses can affect control strategies involving mass-reared flies such as the sterile insect technique in different ways. Persistent virus infections may moderately influence fly fitness, whereas newly acquired infections may result in increased virus replication and fly mortality.