Comparative Assessment of Toxicity, Persistence, and Latent Bio-Effects Post Semi-Field Treatment of Some Conventional and Non-Conventional Insecticides upon the Cotton Leafworm, Spodoptera littoralis (Boisd.) (Lepidoptera: Noctuidae).

INTRODUCTION

The Egyptian cotton leafworm, Spodoptera littoralis (Boisd.) (Lepidoptera: Noctuidae), is one of the most severe widespread pests destroying the national cotton industry. It is one of the pests under quarantine in the EPPO zone and continuously infests cotton as well as more than 87 host species of cultivated crops (El-Sheikh, 2015, Hamama and Fergani, 2019). The control programs against S. littoralis primarily rely on the intensive application of conventional pesticides, which cause undesirable effects on the environment, non-target organisms, beneficial insects, and also the progressive development of resistance and multiple resistance problems (Aydin and Gürkan, 2006, Fergani et al., 2022). Therefore, the application of less toxic, environmentally safe, and biodegradable insecticides was important.

The mechanisms of conventional insecticides and organophosphates such as Prophenofos mainly affect acetylcholinesterase activity (AChE) followed by accumulation in peripheral and central nervous systems (Smallman and Mansingh, 1969). Pyridalyl has strong insecticidal properties against lepidopterous insects with a specific form of toxicity on the epidermal cell preventing the growth of the insects (Sakamoto et al., 2004). Emamectin benzoate, a bio-insecticide produced by the modified soil bacterium, Streptomyces avermitilis affects the insect's neurological systems by an increase in chloride ion flux at the neuromuscular junction, which causes feeding to stop, inhibits muscle contraction and causes irreversible paralysis. Additionally, it acts as an agonist for GABA and glutamate-gated chloride channels. The blockage of voltage-dependent sodium channels is another mode of action induced by Indoxacarb (Fanigliulo and Sacchetti, 2008; Fergani, 2019). The bio-rational insecticides, insect growth regulators such as triflumuron interfere with the physiology of the pest through the stimulation of specific deterrent receptors or disrupt the chitin synthesis and development of the target pest (El-Sabrout and Zahran 2016).

The current semi-field study aims to evaluate the extent of initial and residual toxicity of the non-conventional insecticides (Pyridalyl, Emamectin benzoate, Indoxacarb, and Triflumuron) compared to the conventional one (Profenofos) on the fourth instar larvae of a field strain of S. littoralis. In addition, the persistence of these insecticides was examined in terms of the determination of the median lethal time (LT₅₀) and their latent effects on some biological parameters during the insect life cycle.

MATERIALS AND METHODS

Tested Insecticides:

Five technical-grade pesticides were evaluated:

1-    Pyridalyl (Pleo® 50% EC) (2, 6-dichloro-4-4 (3, 3- dichloroallyloxy) phenyl-3-[5-(trifluoromethyl) - 2- pyridyloxy] propyl ether), supplied by Sumitomo Chemical Co. Ltd, applied at the rate of 100ml/Feddan.

2-    Emamectin-benzoate (Amazon® 5.7% SG), (Avermectin) (4"-epi-methylamino-4"-deoxyavermectin Blb benzoate), supplied by Syngenta Co., was applied at the rate of 80 g /Feddan.

3-    Indoxacarb (Eso plus® 30% WG), (Carbamate: oxadiazine),  (S-Methyl-7-chloro-2[methoxycarbonyl-[4-(trifluoromethoxy)phenyl]carbamoyl]-3,5-dihydroindeno[1,2-e][1,3,4]oxadiazine-4a-carboxylate )( supplied by Du Pont Co., applied at 60 g /Feddan.

4-    Prophenofos (Celian® 72% EC) (Organophosphate) supplied by Adwia Co., was applied at the rate of 750 ml/Feddan.

5-    Triflumuron insecticides (Elsystin® 48% SC) (CSI), supplied by Syngenta Co., was applied at the rate of 100 ml/Feddan.

Rearing technique:

To establish a culture of the S. littoralis field strain, egg patches were collected from untreated cotton plants at Sakha Agricultural Research Farm, Kafr El-Sheikh Governorate; and transported to the lab at 27°C and 70°RH. On caster bean leaves (Ricinus communis), neonate larvae were reared until they reached the fourth larval instar, at which point they were treated according to   El-Defarwi et al. (1964).      

Semi-field applications:

On the farm of the Sakha Agricultural Research Station, the experiments were conducted by the end of May in the 2022 growing season. The local cotton plant variety (Giza 94) was planted in an experimental area of around 2000 m² in a randomized complete block design that was untreated with pesticides, and the recommended agricultural practices were implemented. The experimental area was divided into six equal plots. Four plots (each about 95m²) were assigned to each treatment in both the treated and untreated areas. Between plots, two rows of plants were left unsprayed. The spraying was carried out once on July 18 on cotton plants during the 2022 season with 100 liters of water for Feddan. Treatments were performed at sunset using a knapsack motor sprayer (CP3) of 20-liter capacity with one nozzle. The recommended field rate of each tested insecticide was used in conjunction with standard agricultural practices. The tested insecticides were diluted using irrigation water. Sprays completely cover the plants. Meanwhile, control plots were sprayed with water only.

Laboratory trials:

Toxicity studies:

To evaluate the toxicity of conventional (Profenofos) and non-conventional (Pyridalyl, Emamectin benzoate, Indoxacarb, and Triflumuron) insecticides against the field strain of S. littoralis, some egg masses were brought into the lab for hatching and reared in the laboratory at Sakha Agricultural Research Station, Plant Protection Research Institute Branch, Cotton Pesticides Evaluation Department under constant conditions of 27 ± 2°C, 70 ± 5 RH. Ten newly moulted 4^th instar larvae of S. littoralis were transferred to plastic pots (500 ml) covered with a clean piece of muslin cloth. The treated cotton leaves (0, 1, 3, 7, and 10 days) were randomly selected from each plot in the field and offered daily for feeding. Seven replicates were performed for each treatment and the control. Mortality percentages were recorded 48 hrs post-treatment. Mortality percentages were corrected according to Abbott’s equation (Abbott, 1925). The LT₅₀ and slope values of the tested insecticides were calculated using probit analysis at a 95% (p≤0.05) confidence level (Finney’s equation, 1971).

Biological studies:

To evaluate the effect of the insecticides under investigation on some biological parameters, seventy newly moulted fourth instar larvae distributed in seven replicates (10 larvae\replicate) for each treatment were starved for six hrs. Treated cotton leaves were introduced to larvae and allowed to feed for 24 hrs on the treated cotton leaves. Alive larvae were transferred to jars containing fresh untreated cotton leaves and observed daily. The same technique was used for control treatment except those larvae were allowed to feed on untreated cotton leaves. Daily inspections were implemented until adult emergence. Larval mortality, larval and pupal duration, deformed pupa percentage, and adult emergence percentages were recorded compared to untreated control.  Adult fecundity was determined by placing one female and one male in a glass jar (500 ml) provided with a piece of cotton soaked in 10% sugar solution as a source of food and covered with a clean piece of muslin cloth. Four jars were prepared with one male and one female for each treatment and the control. Clean sheets of twisted paper were put in each jar for oviposition. The jars were checked up daily for counting the number of laid eggs.  Egg hatchability was determined by transferring four egg patches (more than 100eggs/patch) into clean plastic jars during the first three days of oviposition and incubating under laboratory conditions until hatching.  The number of eggs /female, eggs’ incubation period and hatchability percentages were recorded. Statistically significant differences were determined by one-way analysis of variance (ANOVA) (SPSS, 2004). Fecundity percentages were calculated according to Crystal and Lachance (1963) as follows:

     Fecundity % = ×100.

RESULTS

Toxicity of the tested insecticides against the fourth instar larvae of S. littoralis:

          The toxicity of conventional (Profenofos) and non-conventional (Pyridalyl, Emamectin benzoate, Indoxacarb, and Triflumuron) insecticides with different insecticidal groups against fourth instar larvae of S. littoralis under laboratory conditions during the 2022 cotton growing seasonis shown in (Table 1). The obtained data showed that all the tested insecticides showed high initial toxicity (initial kill) on the fourth-instar larvae of S. littoralis. In terms of larval mortality, in a-descending order Pyridalyl, Emamectin benzoate, profenofos, Trprofenofosand Indoxacar,bcaused mortality of 93.25, 90.31, 86.52, 75.91, and 65.24%, respectively.

Among the five tested insecticides, Pyridalyl was significantly the most toxic one after the tenth day against the 4^th instar larvae of field strain causing (78.21%) followed by Emamectin benzoate causing (72.11%). Meanwhile, Indoxacarb was the least potent of other tested insecticides causing (45.62%). The obtained results showed that the residual effects decrease with increasing the exposure time in all treatments.

Insecticide Field Persistence:

To evaluate the persistence (the residual effect) of the tested insecticides through this investigation, the median lethal time (LT₅₀), the time needed to kill 50% of fourth-instar larvae of S. littoralis was determined (Table 2). Data showed the maximum persistence period (LT₅₀) value at the recommended rate of Pyridalyl was 5.87 days. Emamectin benzoate and Profenofos came in the second order with LT₅₀ values of 4.62 and 2.37 days, respectively.  On the other hand, Indoxacarb and Triflumuron showed minimal persistence periods with LT₅₀ values of 1.62 and 1.37 days, respectively.

Table1. Initial and residual mortality of S. littoralis fourth-instar larvae upon treatment with conventional and non-conventional insecticides under semi-field conditions

-In a column, means followed by the same letters are non-significantly different, P≥0.05.

Table 2. Median lethal time (LT50) of the tested insecticides against the fourth instar larvae of S. littoralis under semi-field conditions

Latent effects of the tested insecticides on some biological aspects of S. littoralis:

 The extended effects of the tested insecticides after semi-field treatments on larval mortality and some biological parameters of S. littoralis were assessed compared to the control (Tables 3 and 4).

Pyridalyl and Triflumuron significantly prolonged the larval duration (19.51±1.52 and 18.81±1.21 days), respectively, followed by Emamectin benzoate and Profenofos (17.61±1.11 and 16.51±0.89 days). No significant effect on the larval duration was recorded in the case of Indoxacarb treatment compared to the control (14.50±1.60 days). However, Triflumuron, Profenofos, and Pyridalyl significantly prolonged the pupal duration by 11.78±1.23,10.45±1.15,and 9.67±1.21 days, respectively, whereas Emamectin benzoate and Indoxacarb were less effective, 8.11±0.89 and 7.89±0.45 days, respectively compared to the normal untreated pupal duration (4.24 days).

Regarding the latent effects of the tested insecticides on the percentage of pupation and deformation in pupae, The data in Table 3 showed that the treatments with Pyridalyl and Emamectin benzoate significantly increased pupal deformation (14.76±1.62 and 13.23±1.98 %) respectively, followed by Indoxacarb treatment (8.81±1.33 %), Profenofos and Triflumuron treatment (5.41±1.21 and 3.11±1.24 %), compared to the control  (2.21%). Concerning their latent effects on the adult emergence percentage, Pyridalyl, Profenofos and, Emamectin benzoate significantly decreased the adult emergence percentages (73.11, 70.1 and, 70.1%), followed by Triflumuron and Indoxacarb treatments (85.12 and 81.11%) compared to the untreated, which recorded 98.11%.

The data in Table (4) clearly showed the latent effect of the tested insecticides on the treated S. littoralis fourth instar larvae with regard to calculated fecundity percentages, egg incubation periods and egg hatchability percentages. The obtained results showed that Profenofos reduced the mean number of eggs laid by adult females 7.58% (349.13 eggs) followed by Emamectin benzoate at 25.39% (504.13 eggs) and Pyridalyl at 48.31 %( 959.11eggs). while Triflumuron and Indoxacarb were slightly reduced the mean numbers of eggs laid by adult female that were 90.68 % (1800.14 eggs) and 83.12 %(1650.11 eggs) respectively, compared to the control's 100% (1985 eggs). On the other hand,  The treatment with Pyridalyl and Emamectin benzoate significantly prolonged the incubation period of egg (4.32 days) followed by Profenofos (4.0 days), while Indoxacarb and Triflumuron slightly decreased the incubation period of eggs (3.21 and 3.11 days) compared to the untreated (4.32days).  In connection with the extended effects of the tested insecticides on adult fertility termed by the percentage of egg hatchability recorded, Triflumuron and Indoxacarb slightly reduced the percentage of egg hatchability (84.11 and 80.23%) in the offspring generation, respectively. While Pyridalyl, Emamectin benzoate and Profenofos significantly reduced the percentage of egg hatchability (75.12, 72.32 and 67.21%), respectively, compared to the untreated (91.32%).

Table 3: The latent effects of the tested insecticides on some biological aspects of the surviving 4^th instar larvae of S. littoralis

-In a column, means followed by the same letters are non-significantly different, P≥0.05.

Table4: The latent effects of the tested insecticides on female fecundity, egg production, incubation period and hatchability of S. littoralis eggs

- In a column, means followed by the same letters are not significantly different, P≥0.05.