Introduction

Spodoptera exigua Hubner (Lepidoptera: Noctuidae) is a polyphagous species, considered as one of the most important insect pests of agricultural and industrial crops, especially sugar beet (Ruberson et al. 1994; Capinera 2014). Larvae feed on leaves and reduce photosynthesis. In addition, the presence of larvae, their feeding symptoms, and residues on the leaves reduce the value of vegetables and ornamental plants (Capinera 2001).

High damage level and pesticide resistance have led to an overuse of synthetic insecticides thereby increasing pest control costs and side effects (Benz 1975). Therefore, many efforts are being made to reduce these side effects and use effective and safe control agents, including different microbial insecticides (Darabian and Yarahmadi 2017). Various strains of Bacillus thuringienais (Bt) are one of the most widely used biological control agents (Nazarpour et al. 2016). Botanical insecticides also play an effective role in pest control, without harming the environment (Mugnai 2009; Irigaray et al. 2010). Matrin is a contact-stomach botanical insecticide and has a high effect on insect pests. Its toxicity is low for non-target organisms and its residues do not accumulate in the environment (Sheikhi Garjan and Lashgari 2015).

In Iran, the current method to control S. exigua in the field is application of chemical insecticides like carbaryl, phosalone, and chlorpyrifos (Sheikhzadeh et al. 2014). The aim of this study was searching for some alternatives for chemical control of S. exigua in black-eyed pea.

Material and methods

Experimental site

Experiments were conducted in the summer of 2016 and 2017 in an area of 2000 m2, located in the Elhaee region (31.652935° N, 48.595880° E) in the north of Khuzestan province, Iran. The climate of the Elhaee region is subtropical hot desert with long, extremely hot summers and mild, short winters. The maximum summer temperature is at least 45 °C, and in winters, the minimum temperature can fall to around 5 °C. The average annual rainfall is around 213.4 mm. The minimum of average relative humidity is 22% in June and the maximum is 71% in January.

Material

The seeds of black-eyed pea var. Parastoo were used in the trials and experiments were set up in the last decade of July. Six insecticides were evaluated for their efficiency in control of S. exigua on the plants and totally seven treatments were applied. The field was divided into 28 plots each measuring 56 m2. All the treatments were replicated four times. One meter buffer zone was considered for each treatment and replication. The row spacing was 50 cm and the distance of plants on a row was 10 cm. The insecticides used in the experiment are shown in Table 1. Water was used in the control group.

Table 1 information about the insecticides used in experiments
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Methods

In mid-September, when the first insects were observed in the field, plants were sprayed by insecticides in a quiet and sunny weather. The sprayer used was the motorized backpack one, from Shakhes industrial company, the 423 model with an air speed of 100 m/s and a spraying rate of one litre per minute.

Samplings for estimation of S. exigua population were conducted 1 day before treatment (DAT) and 1, 3, 5, 7, and 10 days after treatment (DAT). At each sampling date, 10 plants were selected randomly with an X-shaped movement in each plot and the number of larvae and egg clusters were recorded on each plant.

At the end of the growing season, harvesting was carried out by hand. In each plot, a plant was randomly selected at intervals of six meters and at a distance of two meters from the beginning and the end of the planting lines, and the number of pods and seeds were counted in each plant. After collecting and mixing the seeds of each plot, 100 seeds were randomly selected and their weight was calculated and recorded by electronic scale.

Data analysis

The experiment was carried out in a complete randomized block design with seven treatments and four replications. Analysis of variance (ANOVA) was performed to compare egg clusters, larvae population, and plant growth factors including the number of pods in plant, number of seeds per pods, and 100-seed weight in different treatments. Means were compared with Duncan’s Multiple Test Range using SPSS software (version 20).

Results and discussion

Effects of different insecticides tested on Spodoptera exigua eggs

Table 2 gives the results on the effects of the different pesticides applied on S. exigua eggs. According to these results, indoxacarb in 2016 at 3, 5, 7, and 10 DAT and in 2017 at 3, 7 and 10 DAT significantly reduced the number of S. exigua eggs compared to the control. The other pesticides tested were not significantly different from the control.

Table 2 Mean ± SE of Spodoptera exigua eggs in different insecticidal treatments at 1, 3, 5, 7 and 10 days after treatment
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The results indicate that indoxacarb may have a negative effect on the oviposition of S. exigua. In other laboratory studies, the effect of sub-lethal dose of indoxacarb on Plutella xylostella L. caused a decrease in the fertility of the pest (Mahmoudvand et al. 2011). Investigations on the inhibitory effect of some insecticide on Spodoptera litura (Fabricius) oviposition showed that indoxacarb was one of the insecticides that can better prevent the oviposition of the pest (Natikar and Balikai 2015). On the other hand, lack of effect of Bt on S. exigua oviposition in the current study was confirmed by some previous studies (Donovan et al. 2002; Darabian and Yarahmadi 2017). However, in some other studies conducted on the other moths species like tobacco bud worm, Heliothis virescens (F.), (Abdul-Sattar and Watson 1982) and Spodoptera frugiperda (J.E. Smith) (Polanczyk and Alves 2005) Bt has been able to influence the quality and quantity of eggs. Differences in the results of those studies with the present study may be due to the different susceptibility of the Lepidoptera species to the Bt insecticide.

Effects of different insecticides tested on Spodoptera exigua larvae

The highest reduction in larvae population was observed in treated plots 1 DAT in 2016 and 2017 (Table 3) due to the use of indoxacarb and chlorpyrifos. In both years, at 5 DAT, similar effects on S. exigua larvae were observed for matrin, deltamethrin, indoxacarb and chlorpyrifos but with significantly lower larval number compared to the other pesticides tested and the control. The results of sampling 10 DAT showed that the population of larvae significantly decreased in all treatments compared to the control. However, no significant differences were observed between the tested insecticides (Tables 3).

Table 3 Mean ± SE of Spodoptera exigua larvae in different insecticidal treatments at 1, 3, 5, 7 and 10 days after treatment
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The short-term effects of indoxacarb and chlorpyrifos is due to the rapid activity in closing the sodium channels in the neural axons (Derbalah et al. 2012) and deactivation of Acetylcholinesterase in nerve connections (Barron and Woodburn 1995), respectively. In some previous studies by other researchers, the positive effects of indoxacarb on the control of larvae of butterflies have been demonstrated. For example, investigating the digestive toxicity effect of indoxacarb on the larvae of Trichoplusia ni (Hubner) in the cabbage field caused 100% mortality in the second instar larvae after 2 days (Liu et al. 2002). Also, in studies conducted on chlorpyrifos, significant reduction of S. exigua larvae were observed in the field at all sampling periods (3, 5, 7 and 10 DAT), especially on the third day (Mascarenhas et al. 1998).

The effect of the botanical insecticide (matrin) and the biological insecticide (Bt) 10 DAT reached its highest level. In other studies, the positive effects of botanical insecticides such as Azadirachtin and biologic insecticide (Bt) on S. exigua have been observed later than chemical insecticides (Saleh et al. 1990; Darabian and Yarahmadi 2017). Insecticidal activity of B. thuringiensis is due to the production of delta-endotoxin poison as Cry proteins (Hofte and Whiteley 1989). Previous studies have proved the effect of Bt on larvae of some moths, such as Tuta absoluta Mayrick in tomato fields (Gonzales-Cabera et al. 2011; Nazarpour et al. 2016), and S. exigua in sugar beet fields (Darabian and Yarahmadi 2017), which is similar to the results of current research.

Effects of different insecticides tested on plant growth

In 2016, the number of bean pods in plots treated with hexaflumuron and matrin did not show significant difference with control, but this difference was significant in other insecticidal treatments. Also, the highest number of seeds per pod was observed in deltamethrin, chlorpyrifos, indoxacarb, and matrin treatments which had a significant difference with control. The 100-seeds weight in all treatments was significantly higher than control in this year (Table 4). In 2017, in all insecticidal treatments, the number of pods per plant and the number of seeds per pod were significantly higher than that of the control. The highest number of pods per plant was recorded in indoxacarb and hexaflumuron treatments and the highest number of seeds per pod was seen in indoxacarb treatment. No significant difference was observed in 100-seeds weight among insecticidal treatments, and it was significantly higher than control in all treatments (Table 4). In sugar beet fields, treatments that were sprayed with chlorfenapyr, azadirachtin, and Bt to control S. exigua had a significantly higher yield compared to the control (Darabian and Yarahmadi 2017).

Table 4 plant growth characteristics under the different experimental treatment
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Conclusion

All insecticides tested in the current study were effective in controlling S. exigua larvae after 10 DAT and increased the product yield. Although the effect of Bt on pest larvae was gradual, it ultimately led to successful pest control. The botanical insecticide, matrin, also was able to reduce larval population significantly but in a longer period than chemical insecticides (10 DAT) in both years. Considering that biological and botanical insecticides may have less harmful effects on the environment, beneficial organisms, and natural enemies, they can be recommended as a suitable alternative to common chemical pesticides against S. exigua larvae. However, further studies on their probable side-effects on non-target fauna especially beneficial insects is recommended.