Influence of production and bioassay methods on infectivity of two ambush foragers (Nematoda: Steinernematidae)

The influence of production and bioassay methods on infectivity of two ambush foragers, Steinernema carpocapsae and Steinernema scapterisci, was evaluated. Both species were mass-produced in vitro in liquid culture and in vivo using live insects: S. carpocapsae in last instar wax moth Galleria mellonella larvae and S. scapterisci in adult house crickets Acheta domesticus. Infectivity was assessed at 28 degrees C against last-instar G. mellonella larvae using a filter-paper bioassay and a newly developed "sand-well" bioassay. The infectivity of S. carpocapsae was not influenced by the method of production or bioassay, whereas the infectivity of S. scapterisci was influenced by both factors. Both in vitro and in vitro produced S. carpocapsae caused >60% larval mortality at one nematode per larva (1:1) in the filter-paper bioassay, but S. scapterisci elicited less than 10% mortality. At 50 nematodes per larva, in vitro S. scapterisci caused 41.7% mean larval mortality in the filter-paper bioassay, whereas in vivo S. scapterisci elicited only 28.5% mortality. The replacement of filter paper with a 2.5-mm-deep layer of sand (termed sand well) resulted in 2.5-fold increase in infectivity of S. scapterisci. In the new sand-well procedure, 15 S. scapterisci per larva (15:1) caused an overall mean larval mortality of 47.5% and the pattern of mortality showed a normal distribution. The infectivity of S. carpocapsae was not different in the 1:1 filter paper or 1:1 sand-well bioassay. These results demonstrate that nematode infectivity could be strongly influenced by both the production and bioassay methods, and there are no universal assays even when nematodes have similar foraging strategies. (C) 1999 Academic Press.