The Invasion and Encapsulation of the Entomopathogenic Nematode, Steinernema abbasi, in Aedes albopictus (Diptera: Culicidae) Larvae

SIMPLE SUMMARY: The Asian tiger mosquito, Aedes albopictus, is widely considered to be one of the most dangerous vectors transmitting human diseases. Meanwhile, entomopathogenic nematodes (EPNs) have been applied for controlling insect pests of agricultural and public health importance for years. However, infection of mosquitoes with the infective juveniles (IJs) of terrestrial-living EPNs when released to aquatic habitats needs further investigated. In this study, we observed that a Taiwanese isolate of EPN, Steinernema abbasi, could invade through oral route and then puncture the wall of gastric caecum to enter the body cavity of Ae. albopictus larvae. The nematode could complete infection of mosquitoes by inserting directly into trumpet, the intersegmental membrane of the cuticle, and the basement of the paddle of pupae. After inoculation of mosquito larvae with nematode suspensions, the invading IJs in the hemocoel were melanized and encapsulated and only a few larvae were able to survive to adult emergence. The mosquitocidal effect of S. abbasi could be due to its overload or destruction of the mosquito defense systems. Eventually, the mosquito larvae failed to recover and died a few days after infection. Our results suggest that the terrestrial EPN, S. abbasi, is effective against the aquatic living mosquitoes. ABSTRACT: The Asian tiger mosquito, Aedes albopictus, is of crucial concern to the public and veterinary health because of its vector role in transmission of several mosquito-borne diseases. Over the past decades, entomopathogenic nematodes (EPNs) have been used to control important agricultural insect pests and are considered to be effective against mosquitoes as well. The objectives of this study were to investigate the mosquitocidal effects of Steinernema abbasi to Ae. albopictus and the encapsulation processes of invading nematodes in the mosquito host. In this study, we found that S. abbasi was pathogenic to 3rd and 4th instar larvae of Ae. albopictus by entering the hemocoel of the 3rd and 4th instar larvae mainly through mouth and gastric caecum or by penetrating pupae through the intersegmental membrane or trumpet. The mosquito larvae infected with a single nematode caused a high mortality. Although EPNs in the hemocoel of mosquitoes were melanized and encapsulated, most Ae. albopictus larvae failed to survive after infection with S. abbasi. Overall, we demonstrated that S. abbasi is pathogenic to Ae. albopictus larvae, suggesting that this S. abbasi isolate has potential as a biocontrol agent for managing this vector mosquito.