Background

The variegated cutworm (VCW), Peridroma saucia (Hubner, 1808) (Lepidoptera: Noctuidae), is a common polyphagous pest of many vegetable and field crops and found in many areas of the world (Rings et al., 1976 and Klein Koch and Waterhouse, 2000). VCW was first recorded in Europe in 1790 and then caused serious outbreaks in many countries throughout the Americas in 1841 (Capinera et al., 1988). The adults of these cutworms were discovered in 1967 in Turkey. At the present time, VCW is widely distributed in Turkey’s agricultural areas and it is one of the most abundant and damaging cutworm species of Turkey (Akdagcik and Ulusoy, 2007). VCW has a wide range of host plants which includes economic crops such as potato, tomato, corn, lettuce, carrot and sugar beet. VCW larvae do much damage to crops and cause considerable mortality to seedlings in the early growing season by cutting off the plant at the soil surface and feeding on the foliage of these crops (Capinera et al., 1988). Due to the high tolerance of VCW, repeated applications of conventional insecticides are widely used. Development of resistance and concerns over the destructive effects of chemicals to environmental and human safety have accelerated the development of alternative control methods for this pest (Yoshida, 2010).

Entomopathogenic nematodes (EPNs) are highly effective biological control agents against many agricultural pests particularly soil-inhabiting lepidopterous larvae because of their presence in larval stages below ground (Vashisth et al., 2013). EPNs have searching ability on hosts and the potential to survive in the soil environment. They possess free-living third-stage infective juvenile (IJ) that can survive a long time without feeding (Koppenhöfer et al., 2000). The IJs invade their hosts via natural body openings, such as the mouth, the anus and the spiracles. Once they enter to haemocoel, the mutualistic bacteria Xenorhabdus in Steinernema and Photorhabdus in Heterorhabditis are released to kill the host within 2 days (Gaugler, 2002; Griffin, et al., 2005; Kaya et al., 2006).

EPNs that infect insects have received considerable attention by scientists for their potential in the biological management of many agricultural pests (Gaugler, 1981; Gaugler and Kaya, 1990; Georgis et al., 2006; Koppenhöfer et al., 2000; Smart Jr, 1995). Many studies have been started for testing the pathogenicity of these indigenous EPN species all over the world (Ozer et al., 1995; Kepenekci, 2002; Hazir et al., 2003; Unlu et al., 2007 and Erbas et al., 2013). There are some differences known in terms of survival, pathogenicity and host range between indigenous and non-indigenous EPN species (Lacey and Georgis, 2012). Indigenous species of EPNs may be more successful in biocontrol as a consequence of compatibility to native habitats (Goudarzi et al., 2015).

The aim of this study is to evaluate the effect of four Turkish species of EPNs for the biological control of the variegated cutworm (VCW), P. saucia, under laboratory conditions.

Materıals and methods

Insect culture

Healthy VCW larvae were collected on a regular basis from different vegetable fields throughout the vicinity of Central Anatolia and Mediterranean Region, Turkey. For the experiment, larvae were reared on lettuce. P. saucia was established in a growth chamber at a temperature of 25 ± 1 °C, relative humidity of 60% and a photoperiod of 16:8 (L:D) (Scott-Dupree et al., 2008). Healthy last instar larvae were selected to be used for testing the virulent effects of the nematodes in a dose-response experiment.

Entomopathogenic nematodes

Four indigenous nematode isolates of Heterorhabditis bacteriophora FLH-3-H, H. indica 216-H, Steinernema feltiae Y29-S and S. carpocapsae E76-S were collected during the surveys conducted in Adana and Kahramanmaras provinces in the Mediterranean region of Turkey.

Pathogenicity test

Nematode pathogenicity was tested against last instar larvae of P. saucia in a Petri dish arena. In order to avoid cannibalism, only one last instar larva was placed on two moist filter papers in each 100 × 15 mm Petri dish inoculated with 1 ml of water containing different concentrations of IJs (10, 50, 100 and 200 IJs/larva). Each nematode concentration was tested against ten P. saucia larvae and replicated for three times. Control plates were treated with distilled water only. Petri dishes were kept at 25 ± 1 °C. The number of dead larvae was recorded at two different exposure times (48 and 96 h) and dead insects were dissected to determine whether nematodes were present or not.

Statistical analysis

Data was evaluated without being regulated by the Abbott formula because there was no mortality in control plates (Abbott, 1925). Statistical analyses were carried out by SAS software (Version 9.1.3; SAS Institute, Cary, NC (1990)). The experiment was established in a completely randomized design with a factorial treatment arrangement consisting of four nematode species and four application rates. Mean values were separated using Tukey Multiple Range Test (P < 0.05).

Results and discussion

The virulence of four indigenous nematode species against last larval instar of P. saucia was evaluated in a laboratory experiment. Results revealed that all EPN species had the ability to kill and reproduce in P. saucia larvae by sickening them. All EPN species tested and application concentrations significantly affected the mortality rates (F: 27.85, df: 3, P < 0.001 for EPN species and F: 18.12, df: 3, P < 0.001 for application of the tested concentrations after 48 h; F: 5.14, df: 3, P < 0.05 for EPN species and F: 1.30, df: 3, P < 0.05 for application of concentrations after 96 h). No significant differences were noted statistically in mortality rates caused by the concentrations-nematode interaction.

Mortality rates have increased generally with increasing concentrations. H. bacteriophora and H. indica strains showed higher effects at all application concentrations than S. feltiae and S. carpocapsae strains at the first exposure time (48 h) except H. indica and S. carpocapsae at 10 IJs/larva concentrations. The highest larval mortality was achieved when EPNs were applied at the concentrations of 200 IJs/larva after 48 h (Table 1). However, after the second exposure time (96 h), generally no differences were found among strains except 10 IJs/larva concentrations.

Table 1 The effect of different concentrations of EPNs on the mortality of last instar larvae of P. saucia for 48 and 96 h post application at 25 ± 1 °C
Full size table

All EPN species tested showed a great mortality at the lowest concentration of 10 IJs/larva after 96 h of exposure time and at least 80% of P. saucia were killed by S. feltiae Y29-S strain. The difference in mortality between H. bacteriophora FLH-3-H (100%) and S. feltiae Y29-S (80%) was statistically significant. At the exposure time of 48 h, the highest mortality rate was induced by H. indica isolate, while the lowest one was determined by S. carpocapsae isolate at the concentrations of 10 and 50 IJs/larva. Only the mortality produced by S. carpocapsae E76-S was different than the one by H. indica 216-H significantly at the concentration of 10 IJs/larva, but at 50 IJs/larva concentrations, they were divided into two groups which statistically differed when compared to each other. The interaction effect of the different entomopathogenic species and the concentrations on larval infection were not significant after exposure for 96 h at 50, 100 and 200 IJs/larva. The most virulent species was H. bacteriophora at the lowest IJ concentration of 100% mortality after 96 h of exposure time followed by S. carpocapsae and H. indica with 93.3% mortality (Table 1).

S. carpocapsae and S. feltiae caused equal mortality in the last instar larvae which was less virulent than H. bacteriophora and H. indica, at a concentration of 200 IJs/larva over the exposure time of 48 h (Table 1).

In a laboratory experiment, Heterorhabditis spp. proved to be more efficient in suppressing P. saucia. Increase in the time of exposure caused more mortality to last instar P. saucia larvae by all the tested nematode species (Table 1). Similar studies have been carried out to evaluate the effectiveness of EPNs against P. saucia larvae where it was revealed that EPNs have great potential for the management of P. saucia larvae (Yoshida, 2010; Morris and Converse, 2012). The pathogenicity of steirnematid and heterorhabditid nematode species against P. saucia larvae was variable in previous studies. Morris and Converse (2012) exposed P. saucia larvae to six strains of steirnematid and two species of heterorhabditid nematodes in the soil surface. S. feltiae was found as the most virulent nematodes while the heterorhabditid nematodes were the best performing species in our study. The pathogenicity of 17 Japanese isolates of EPNs was tested against the middle instar larvae of P. saucia in a laboratory experiment at different temperatures. Three isolates belonging to S. feltiae caused 70% average mortality at 25 °C which is similar to our study (Yoshida, 2010).

Conclusions

Under laboratory conditions in the petri dish experiment, the heterorhabditid nematodes proved a more effective control of P. saucia larvae and increase in time of exposure led to more mortality of the tested nematode species. More studies are needed to determine the real potential of these four indigenous EPNs both in the open field and greenhouse environments to be included in biological control programs of P. saucia.