Effect of host irradiation on bio-infectivity and proliferation capacity of Steinernema glaseri as entomopathogenic nematodes on a serious tropical pest, Spodoptera litura

Bioefficacy of entomopathogenic nematodes (EPNs). Steinernema glaseri on irradiated lepidopteran pest, Spodoptera litura (Fabr.) was studied to understand the impact of host irradiation on behaviour of EPNs in order to establish these nematodes as potential biocontrol agents by using nuclear techniques. Initially, radiation impact was ascertained on growth and reproductive behaviour of host, S. litura when treated in different stages, viz. 5th instar larva (L5), 6th instar larva (L6) and pre-pupa, to select the range of gamma sterilizing doses for these insect stages. ED₅₀ and ED₉₀ gamma doses resulting into 50% and 90% check, respectively, in pupa formation, adult emergence and egg viability of host with respect to control, were determined. Then, based on these results, the efficacy of the selected doses, (40-70Gy), was evaluated on the metamorphosis, degree of malformation and reproductive behaviour of S. litura before studying the EPNs' bio-infectivity on irradiated host. 70 Gy was determined as an overall sterilizing dose for L5, L6 and pre-pupa stages. The data indicated that radiosusceptibility decreased with age. Host irradiation of L5 and L6 stages did not influence significantly the parasitization by infective juveniles (IJs) of EPNs. Irradiation decrease the pre-pupal parasitization by EPNs, but in case pre-pupa escaped as being target for parasitization, impact of host irradiation was not apparent on the parasitization of the ensuing pupal stage, although total parasitization by EPNs in pre-pupal and pupal stages (subsequent to pre-pupal irradiation and pre-pupal exposure to IJs) was decreased due to radiation. The impact of host irradiation with 40 Gy was not significant on the bio-infective behaviour of EPNs except harvest of IJs from host stage-pupa. Host irradiation dose of 70 Gy reduced the IJs harvest per mg host weight by 9-11.7% on L5 and by 9-19.8% on L6 in comparison to control. The effect of host radiation on harvest of IJs was more severe in pupa than pre-pupa. Therefore, based on the radiation mediated effects on host's survival, metamorphosis and reproduction, 40-70 Gy dose range could be further evaluated for assessing the performance of EPNs reared in irradiated hosts. In view of the range of gamma doses evaluated on EPNs infective behaviour on different treated host stages, L6 could be considered an ideal stage for in-vivo mass rearing and transport of EPNs. (author)