Influence of cell density and phase variants of bacterial symbionts (Xenorhabdus spp.) on dauer juvenile recovery and development of biocontrol nematodes Steinernema carpocapsae and S. feltiae (Nematoda: Rhabditida)

The rhabditid nematodes Steinernema carpocapsae and Steinernema feltiae are used in biological control of insect pests. Mass production is done in liquid culture media pre-incubated with their bacterial symbionts Xenorhabdus nematophila and Xenorhabdus bovienii, respectively, before nematode dauer juveniles (DJs) are inoculated. As a response to food signals produced by the bacterial symbionts, the DJs exit from the developmentally arrested dauer stage (they recover development) and grow to adults, which produce DJ offspring. Variable DJ recovery after inoculation often causes process failure due to non-synchronous population development and low numbers of adult nematodes. This contribution investigated the influence of the bacterial cell density on DJ recovery and development to adults. At higher density of 10¹⁰ bacterial cells ml⁻¹, a higher percentage of DJ recovery was induced, and adults occurred earlier in both Steinernema spp. than at lower density of 10⁹ and 10⁸ cells ml⁻¹. Xenorhabdus symbionts produce phase variants. Recovery in bacteria-free supernatants was lower than in supernatants containing bacterial cells for both primary and secondary phase Xenorhabdus spp. and lower in secondary than in primary phase supernatants or cell suspensions. In general, recovery was lower for Steinernema feltiae and the time at which 50% of the population had recovered after exposure to the food signal was longer (RT₅₀ = 17.1 h) than for Steinernema carpocapsae (RT₅₀ = 6.6 h). Whereas >90% S. carpocapsae DJs recovered in hemolymph serum of the lepidopteran insect Galleria mellonella, recovery of S. feltiae only reached 31%. Penetration into a host insect prior to exposure to the insect’s food signal did not enhance DJ recovery. Consequences for liquid culture mass production of the nematodes and differences between species of the genera Steinernema and Heterorhabditis are discussed.