Diversity in pathogenicity of Steinernema carpocapsae and its symbiotic bacterium for Spodoptera spp.

Pathogenicity of five isolates of Steinernema carpocapsae (GER: Germany, BUW: Becker Underwood, KOP: Koppert, CZ: Czech Republic, BEL: Belgium) were tested against the fourth instar larvae of the cotton leaf worm, Spodoptera littoralis and the beet armyworm, Spodoptera exigua (Lepidoptera: Noctuidae) at different concentrations (0, 5, 10, 25, and 50 infective juveniles/cm2) in Petri dishes (9 cm dia.) lined with filter paper. Results obtained after 48 h exposure indicated that the pathogenicity varied in time, dose of application and host species. On S. littoralis, KOP, BUW, CZ and GER were more virulent than BEL, whereas all isolates showed 100% control of S. exigua after 72 h. The LC50 values of all five isolates were lower for S. exigua than for S. littoralis. Among the isolates tested, KOP showed the lowest LC50 value in both S. exigua (3.84) and S. littoralis (4.44 infective juveniles). To further check the pathogenicity, the symbiotic bacterium of S. carpocapsae, Xenorhabdus nematophilus, was isolated from Galleria mellonella 30 h after its infection with the nematode. Ten microlitres of 50 mM phosphate buffer containing 0, 10, 100, 1000, or 10,000 cfu were injected into the haemocel of fifth instar larvae of S. exigua. Significant differences in mortality of S. exigua larvae were observed 96 h post-injection with different bacterial doses. Cell-free filtrates of X. nematophilus were isolated from a bacterial suspension; 10 microlitres of filtrates were injected into fifth instars of S. exigua. All larvae had died 72 h post-injection. To check the insecticidal capability of X. nematophilus via oral uptake, suspensions of bacteria at concentrations of 0, 1010, 109, and 108 cfu/ml were sprayed onto tomato leaves cv. Moneymaker infected with fourth instar larvae of S. exigua. None of the larvae were killed after 24 h. Finally, we tested the virulence of selected isolates of S. carpocapsae (KOP, BUW, and BEL) in the glass-house on tomato plants infested with S. exigua or S. littoralis larvae. Nematodes were sprayed at a concentration of 2.5 billion/ha. After 48 h isolate KOP caused higher mortality (99% and 85% on S. exigua and S. littoralis, respectively), than other tested isolates. S. carpocapsae isolates BUW and BEL caused 61-65% mortality on S. exigua and S. littoralis, respectively. As most of the isolates had the same origin, viz. the ALL strain, these results demonstrate differences in pathogenicity of the production.