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 ambu sh foragers, Steinernema carpocapsae and Steinernema scapterisci, was evalu ated. Both species were mass-produced in vitro in liquid culture and in viv o using live insects: S. carpocapsae in last instar wax moth Galleria mello nella 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" bioa ssay. The infectivity of S. carpocapsae was not influenced by the method of production or bioassay, whereas the infectivity of S. scapterisci was infl uenced 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 morta lity 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 la yer 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 l arva (15:1) caused an overall mean larval mortality of 47.5% and the patter n of mortality showed a normal distribution. The infectivity of S. carpocap sae was not different in the 1:1 filter paper or 1:1 sand-well bioassay. Th ese results demonstrate that nematode infectivity could be strongly influen ced by both the production and bioassay methods, and there are no universal assays even when nematodes have similar foraging strategies. (C) 1999 Acad emic Press.