Entomopathogenic nematodes (EPNs) in the families' Steinernematidae and Heterorhabditidae are obligate insect parasites. Their easy multiplication, broad host range, compatibility with chemical pesticides, and ease in application has grabbed interest among research practitioners to work on these beneficial microorganisms. Till date, around 100 valid species of Steinernema and 21 species of Heterorhabditis have been identified from different countries of the world. Extensive surveys have been conducted across the globe to isolate locally adapted EPN species and exploit them to suppress soil-dwelling and foliar insect pests in agricultural fields. Most of the new species have been described from Asia, whereas research in some Asian countries are still at infancy. Some new species have been recorded from Australia but very few surveys have been conducted in New Zealand. Likewise, less information about these tiny creatures is from Central America; however, in North America many new species have been described, some of which have been commercialized for insect pest control, whereas in South America, several native nematode species have been described and exploited as biological control agents. European countries have also been explored for EPN diversity and new species have been reported, exploited under field condition, and commercialized. Many new species and other previously described species have been reported from Africa. Despite frequent surveys in different continents of the world, number of sites touched are low and, therefore, further surveys are still needed to explore untouched geographic areas and climatic conditions, both in plantations and indigenous forests with an aim to identify and exploit additional EPN species.

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During a survey in agricultural fields of the sub-humid region of Meerut district, India, two strains of entomopathogenic nematodes, labelled CS31 and CS32, were isolated using the Galleria baiting technique. Based on morphological and morphometric studies, and molecular data, the nematodes were identified as Steinernema pakistanense, making this finding the first report of this species from India. For the first time, we performed a molecular and biochemical characterization of the bacterial symbiont of S. pakistanense. Furthermore, a co-phylogenetic analysis of the bacteria from the monophyletic clade containing a symbiont of S. pakistanense, together with their nematode hosts, was conducted, to test the degree of nematode-bacteria co-speciation. Both isolates were also tested in a laboratory assay for pathogenicity against two major pests, Helicoverpa armigera and Spodoptera litura. The morphology of the Indian isolates corresponds mainly to the original description, with the only difference being the absence of a mucron in first-generation females and missing epiptygmata in the second generation. The sequences of bacterial recA and gyrB genes have shown that the symbiont of S. pakistanense is closely related to Xenorhabdus indica, which is associated with some other nematodes from the 'bicornutum' group. Co-phylogenetic analysis has shown a remarkable congruence between the nematode and bacterial phylogenies, suggesting that, in some lineages within the Steinernema / Xenorhabdus complex, the nematodes and bacteria have undergone co-speciation. In the virulence assay, both strains caused a 100% mortality of both tested insects after 48 h, even at the lowest doses of 25 infective juveniles per insect, suggesting that S. pakistanense could be considered for use in the biocontrol of these organisms in India.

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