Insecticidal activities of the local entomopathogenic nematodes and cell-free supernatants from their symbiotic bacteria against the larvae of fall webworm, Hyphantria cunea
Introduction
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.
Section snippets
Production of entomopathogenic nematode isolates for pathogenicity bioassays
Three different EPN species (Heterorhabditis bacteriophora AVB-15, Steinernema bicornotum MGZ-4S, and Steinernema feltiae KCS-4S) originating from Central Anatolia and the Mediterranean region of Turkey were used for the experiments (Canhilal et al., 2017). The last instar larvae of Galleria mellonella (Linnaeus) (Lepidoptera: Pyralidae) were used to establish a laboratory culture after obtaining EPN isolates as described by Woodring and Kaya (1988). IJs collected with modified White's traps
Virulence of entomopathogenic nematodes
The differences in the mortality rates of FWW larvae were significantly affected by all main factors [Nematode species (N), exposure time (T), and bioassays (B)] except for concentrations of IJs (C) (Table 1). The interactions of all the main factors (4-way interaction; NxTxCxB) had also a significant effect on the mortality rates of FWW larvae.
All nematode species were able to penetrate and kill the larvae of FWW with varying levels of virulence and the mortality rates of FWW larvae increased
Discussion
This study shows the potential of entomopathogenic nematodes and cell-free supernatants of their symbiotic bacteria to control the larvae of FWW. Although several studies were conducted to assess the usage potential of EPNs on different pests, there is a limited number of studies examining the effects of EPNs on the larvae of FWW. However, to our knowledge, we evaluated, for the first time, the insecticidal activity of Xenorhabdus and Photorhabdus cell-free supernatants against FWW.
The previous
Conclusion
The results of this study show that the entomopathogenic nematode isolates and cell-free supernatants of their symbiotic bacteria might have potential to control FWW. These results will have also importance to the arrangements of further semi-field and field experiments which might help to determine the EPN/CFS potential to control FWW. Considering that FWW eggs are laid in leaves, additional studies could be conducted at the beginning of oviposition. Foliar application of IJs in this study
Author statement
EY and EÖ conceived, designed and conducted the experiments. RAD and EÖ analysed the data. EÖ and EY wrote the manuscript. RC helped in the interpretation and integration of the results. EÖ revised the manuscript. All authors read and approved the manuscript.
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