Effects of infected insects on secondary invasion of steinernematid entomopathogenic nematodes

Factors affecting 'invasion efficiency' of steinernematid entomopathogenic nematodes into hosts were elucidated. The phenomenon that only part (10-40%) of the nematode population invades the target host has been recorded in many studies. It has been mainly ascribed to differences in the ability of individual nematodes to infect. In the present study the effect of an infected host, the wax moth Galleria mellonella, on subsequent infection of the entomopathogenic nematodes Steinernema carpocapsae Mexican, S. riobravus and S. feltiae was evaluated. The invasion rate of the 3 nematode species to a non-infected host was reduced by 40-60% after pre-exposure to infected hosts. These nematodes regained their full invasion potential after they were rinsed with water. Invasion into insects which were previously injected with nematodes was significantly reduced by 60-80% 6-9 h after injection. The reduction in subsequent invasion due to the initial infection was nematode species specific. This phenomenon was also observed with other lepidopteran pests (Helicoverpa armigera and Spodoptera littoralis). The data indicate that the initial infection by entomopathogenic nematodes induced the release of a substance which reduced the subsequent invasion. The chemical and biological characteristics of this substance are currently under investigation.