The fall webworm (FWW), Hyphantria cunea Drury (Lepidoptera: Erebidae), is considered one of the invasive pests of many plants. FWW has been recorded on over 600 plant species including forest, stone, and pome fruits (Yarmand et al., 2009). Although FWW is a native pest of North America, it has found its way onto four continents over 30 countries and become a major pest of many economically important crops (İren, 1977; Kim and Kil, 2012; Mouloudis et al., 1980; Japoshvili et al., 2006; Kaçar et al., 2019). The female of this pest can lay up to 2000 eggs on the undersurface of leaves of host plants under favorable conditions. Newly hatched larvae start feeding en masse on the leaves within a protective nest-web which they constructed the following hatching. They continue feeding inside the web by enlarging it through larval stages (Yarmand et al., 2009). The larval stages of FWW damage the foliage of host plants by defoliating and in high population levels, the larvae cause serious losses leaving the host vulnerable to secondary pests and pathogens and thus leading the plant to complete destruction (Zhao, 2005; Ge et al., 2019). The fall webworm, which has been in Turkey for nearly half a century, has settled widely into the black sea region that accounts for 70% of the world's hazelnut production and becomes a major destructive pest of hazelnuts and mulberry in the region (Işık and Yanılmaz, 1992; Kaçar et al., 2019; Avcı and Öztemiz, 2020). In addition to its damage to the plants, the last instar larvae also cause a nuisance to people in late September by invading surrounding living spaces due to their overwintering behaviors (Wagner, 2005; Akkuzu and Torul, 2006; Yarmand et al., 2009).

The control of FWW is challenging due to their feeding habits, wide host range, adaptation capability, and reproduction rate (Yarmand et al., 2009; Saruhan et al., 2014; Kaçar et al., 2019). The chemical insecticides have been the most preferred control methods by farmers in severe infestations although the treatments do not give satisfactory results (Yarmand et al., 2009). It is also a well-known fact that there are many unwanted and irreversible effects of chemicals on both biotic and abiotic environments. Therefore, numerous researchers all around the world are currently looking for alternative control strategies to control FWW (Chkhubianishvili et al., 2007; Aker and Tuncer, 2016; Albayrak İskender et al., 2017; Gokturk et al., 2017; Saruhan et al., 2017; Gözel, 2019).

Entomopathogenic nematodes (EPNs), which are one of the successful groups of entomopathogens, have been getting the attention of many researchers around the world due to some features such as high speed of kill, their capability to reach pests in cryptic habitats by active host-seeking behavior of infective juveniles (IJs), and compatibility with many insecticides (Kaya and Gaugler, 1993; Grewal et al., 1998; Lacey, 2017; Kwizera and Susurluk, 2017; Özdemir et al., 2020). Although EPNs are soil-dwelling organisms, foliar applications of EPNs which are applied directly against the target pests yielded promising results thanks to enhanced application technology and formulations (Shapiro-Ilan et al., 2006; Laznik et al., 2012; Beck et al., 2013; Acar, 2019).

Symbiotic bacteria of EPNs from the genera Xenorhabdus and Photorhabdus play a key role in the virulence of EPNs by releasing toxic substances into the hemolymph and multiplying in there (Forst et al., 1997; Boemare, 2002; Snyder et al., 2007). Previous studies have shown that cell suspension of in vitro culture of Xenorhabdus spp. And Photorhabdus spp. have lethal properties against some economically important agricultural pests (Mohan et al., 2003; Sergeant et al., 2006; da Silva et al., 2013, Shawer et al., 2018; Eroglu et al., 2019). However, the efficacy of both native EPNs and their symbionts on FWW has not been studied in Turkey. It is essential to reveal the separate efficacy of EPNs and their symbiotic bacteria on the target host due to the varying pathogenicity of both EPNs and their symbiotic bacteria depending on species and strains. Therefore, a laboratory study was conducted to evaluate the contact and oral efficacy of both local isolates of IJs and cell-free supernatant of in vitro cultured symbiotic bacteria on the larvae of FWW in the Petri dish.