Factors affecting 'invasion efficiency' of steinernematid entomopathog
enic nematodes into hosts were elucidated. The phenomenon that only pa
rt (10-40%) of the nematode population invades the target host has bee
n recorded in many studies. It has been mainly ascribed to differences
in the ability of individual nematodes to infect. In the present stud
y the effect of an infected host, the wax moth Galleria mellonella, on
subsequent infection of the entomopathogenic nematodes Steinernema ca
rpocapsae Mexican, S. riobravus and S. feltiae was evaluated. The inva
sion rate of the 3 nematode species to a non-infected host was reduced
by 40-60% after pre-exposure to infected hosts. These nematodes regai
ned their full invasion potential after they were rinsed with water. I
nvasion 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 speci
es specific. This phenomenon was also observed with other lepidopteran
pests (Helicoverpa armigera and Spodoptera littoralis). The data indi
cate 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 currentl
y under investigation.