Impact of abamectin 1.9% EC and diafenthiuron 50% WP on natural enemies and plant health while managing mites and thrips in mulberry

Introduction

Mulberry (Morus spp.) is a versatile, fast-growing deciduous tree that is believed to have originated in the Himalayan foothills of India and China (Khan et al., 2013; Yuan and Zhao, 2017; Rohela et al., 2020) [10,24,13] . Mulberry is a unique plant that can adopt to a wide range of climates, soil conditions, and altitudes ranging from 0 to 4000 meters above sea level (Sarkar et al., 2017) [16] and has over 1000 cultivated varieties, mainly in Asia (Sanchez, 2001) [15] . Southern Indian states like Karnataka, Andhra Pradesh, Tamil Nadu and Kerala have embraced sericulture, dedicating approximately 2,42,244 hectares to mulberry cultivation. However, the industry faces significant threats from sucking pests in mulberry cultivation especially; thrips are causing 42.55% infestation followed by mealy bugs 20.88%. Globally, 35 species of thrips pose a substantial threat and 21 reported from India. Mites, once considered as minor pests are now causing 5-10% of damage, worsened by climate change, creating a severe mite and thrips complex in sericulture regions. Climate change exacerbates this problem, impacting cocoon production and industry sustainability, demanding urgent measures to safeguard mulberry cultivation and the silk industry's future. The pesticide use pattern in the present day situation has led to pesticide residues in the harvestable produce, resistance build-up by pests and eradication of non-target organisms, which demands newer and safer pesticides with different modes of action. Abamectin (Avermectin B1) and diafenthiuron (1-tert-butyl-3-(2,6-diisopropyl thio-urea)) are two such newer molecules having novel insecticide and acaricidal activity against insect pests and mites either as neurotoxins (Abamectin) or by inhibiting or enhancing the metabolic processes such as respiration (diafenthiuron). Abamectin 1.9% EC is reported to be effective against phytophagous mites and insect pests on a variety of agricultural and horticultural crops worldwide (Lasota and Dybas, 1990) [11] and diafenthiuron 50% WP is reported to be effective against whiteflies and leaf hoppers in brinjal, whiteflies, aphids and thrips in tomato, thrips and mites in chillies, tea and all sucking pests in cotton, diamond back moth (DBM), shoot and capsule borer and thrips in cardamom (Stanley et al., 2016) [18] and thrips and mites in mulberry (Kalpana et al., 2022) [8] . In light of this context, a comprehensive research endeavour has been formulated with the following primary goals to identify promising chemical compounds that can effectively replace DDVP in countering the infestation of thrips and mites in mulberry. This study aims to evaluate chemicals exhibiting dual insecticidal and acaricidal properties, with a specific focus on their compatibility and indigenous natural predators within the mulberry ecosystem.

Materials and Methods

Experiment and site description

The experiment was conducted during 2022-23 in a wellestablished V-1 mulberry garden ((90+150)*60 cm (Paired row system)) following Randomized Complete Block Design (RCBD) with 7 treatments (Table 1) and four replications. The population of mites and thrips were recorded one day before and 1, 3, 8, 15, and 21 days after spraying (DAS) of the chemicals by visual scoring method. A total of five plants were selected and tagged in each replication for recording the observations.

Statistical analysis

The data collected from the experimental field and silkworm rearing were analysed statistically by using one-way RCBD and CRD for testing of significance by Fisher's method of analysis of variance as outlined by Sundaraaj et al. (1972) [19] . The level of significance used in the F-test was P = 0.05. The critical difference (CD) values were computed to compare significance of the treatments.

Phytotoxicity

Phytotoxicity symptoms like leaf injury on tips or surface of leaves, wilting of the plants, vein clearing or discolouration on the leaves, necrosis (Figure 1), epinasty and hyponasty was taken into consideration and observations were made on these symptoms on 1 st , 3 rd , 8 th ,15 th a n d 2 1 st DAS of insecticides. These symptoms were recorded based on the standard scoring method with 0-10 scale (Mazarura, 2001) [12] (Table 2).

Safety on natural enemies

The population of natural enemies was recorded before and after application of chemicals in the experimental site in order to study the safety of chemicals on their population. Pre-treatment count was recorded one day prior to spraying (DBS) and post treatment count was recorded on 1 st , 3 rd , 8 th , 15 th and 21 st DAS.

Table 2: Standard score of phytotoxicity (Mazarura, 2001) [12]

Scale

Results and Discussions 3.1 Phytotoxicity effect of chemical pesticides on mulberry under field condition

The chemicals not only act on the pests but also may cause injury to the plant if not used with right dosage and appropriate time. Further, if left to nature, the natural enemies like predators and parasitoids would kill different stages of pests without affecting the plant. An ideal chemical should therefore be safe for the plant and the natural enemies when used in the mulberry ecosystem. The chemicals abamectin 1.9% EC and diafenthiuron 50% WP were evaluated for their phytotoxicity and safety to natural enemies and the observation are discussed in this section. 3, Figure 2). .291 T1= Diafenthiuron 50% WP (@ 1 g/l); T2= Abamectin 1.9% EC(@ 0.75ml/l); T3=Diamethoate 30% EC @ 2 ml/l; T4=Cyanopyrafen 30% EC @ 0.5ml/l; T5=Azadirachtin 1% EC @ 1ml/l; T6= Control (water spray); T7= Absolute control. *Significant at 0.05; DAS: Days after spray of the chemical; NS: Non significant; Numbers superscript with same alphabets are statistically on pa Irrespective of the season of spray, diamethoate 30% EC (@ 2 ml/l) and cyenopyrafen 30% EC (@ 0.5ml/l) showed necrosis (0.00, 0.25 on 1 st , 3 rd DAS; 1.25 on 8,15 and 21 st DAS of dimethoate 30% EC and 0.00 on 1, 3 DAS; 0.13 on 8 DAS and 0.88 on 15, 21 st DAS of cyenopyrafen 30% EC) (Table 3). An ideal pesticide is the one which produces no or negligible phytotoxicity symptoms on the plant when sprayed at a specific concentration but should effectively control the pest against which it is used. Kalpana (2022) [8] reported that abamectin 1.9% EC (@ 0.75 ml/l) and diafenthiuron 50% WP (@ 1 g/l) were did not showing any phototoxicity at 15 and 21 DAS, which was used in current study. Aravind et al. (2017) [2] did not observe any phytotoxic symptoms (Necrosis, epinasty and hyponasty) on cardamom at 1, 3, 7, 10, 14 and 20 DAS of diafenthiuron 50% WP. Yeshika et al. (2020) [22] reported that among different chemicals used to control mealy bugs in mulberry, flonicamid 50% WG (0.15 and 0.3 g/l) exhibited phytotoxicity symptoms at 7 and 15 DAS.

Population of coccinellid predators/plant at different days after spray of selected chemicals in mulberry ecosystem

Coccinellids are very effective as predators on thrips and mite pests. Different species of coccinellids were observed on the mulberry plant (Figure 4). The population count of different coccinellids was recorded separately per plant counted before (Pre) and after (Post) spray of chemicals and the observations are compared across the treatments. The pre-treatment count of coccinellid population was recorded at a day before spray of the chemicals (4.07 and 5.18 coccinellid predators/plant during Jan-Feb, 2023 and March-April, 2023, respectively). The number of coccinellid predators observed per plant varied significantly across the treatments. Highest number of coccinellids were observed on absolute control 4, Figure 3). Among different chemicals used in the form of spray solution, azadirachtin 10000 ppm (@ 1 ml/l) recorded least population of coccinellid predators (0.75,1.50, 1.25, 1.75 and 0.75 predators/plant, respectively on 1, 3, 8, 15 and 21 st DAS during Jan-Feb, 2023 and 1.25, 1.50, 1.25, 1.50 and 1.25 predators/plant, respectively on 1, 3, 8, 15 and 21 st DAS during Mar-April, 2023) and diamethoate 30% EC sprayed @ 2 ml/l (2.00, 1.25, 0.75 1.00 and 1.75 predators/plant, respectively on 1, 3, 8, 15 and 21 st DAS during Jan-Feb, 2023 and 0.75, 1.25, 1.50, 1.25 and 1.50 predators/plant, respectively on 1, 3, 8, 15 and 21 st DAS during Mar-April, 2023) followed by cyenopyrafen 30% EC sprayed @ 0.5 ml/l (1.00, 1.75, 2.50, 2.00 and 1.50 predators/plant, respectively on 1, 3, 8, 15 and 21 st DAS during Jan -Feb, 2023 and 1.00, 0.75, 3.25, 3.00 and 3.25 predators/plant, respectively on 1, 3, 8, 15 and 21 st DAS during Mar-April, 2023). On contrary to this, the natural enemy population was significantly higher upon spraying abamectin 1.9% EC @ 0. 4). The chemicals present in pesticides and acaricides possess repellent and anti-feeding property that might influence the insects including natural enemies and other beneficial organisms along with the targeted pests (Khan et al., 2014) [9] . The repellent action of chemical molecules might affect the population of coccinellids (Yeshika et al., 2019) [21] . Hence spraying of plain water like rain, irrigation and water jetting might conserve the natural enemies like coccinellid predators while reduce the population of minute sucking pests like thrips and mites, which is evidenced in the present study. Sakthivel et al. (2011) [41] reported that the predatory coccinellids were also killed significantly along with the major sucking pests when chemical control was followed in mulberry while water jetting recorded no deleterious effect. Abamectin and diafenthiuran were found safer to natural enemies in various crops (Singh et al., 2017) [17] . Kalpana (2022) [8] studied that among six selected chemical molecule abamectin 1.9% EC (@ 0.75 ml/l) and diafenthiuron 50% WP (@ 1 g/l) found safer to natural enemies of mulberry ecosystem. .501 T1= Diafenthiuron 50% WP (@ 1 g/l); T2= Abamectin 1.9% EC (@ 0.75ml/l); T3=Diamethoate 30% EC @ 2 ml/l; T4=Cyanopyrafen 30% EC @ 0.5ml/l; T5=Azadirachtin 1% EC @ 1ml/l; T6= Control (Water spray); T7= Absolute control. DBS = Day before Spray, DAS: Days after Spray of the chemical; *Significant at 0.05; Numbers superscript with same alphabets are statistically on par.

The mulberry leaf yield estimated on 60 th DAP recorded significant difference among different chemicals sprayed on mulberry for management of thrips and mites (Table 5). The estimated leaf yield (kg/ha/year) was significantly highest in abamectin 1.9% EC (@ 0.75 ml/l) followed by diafenthiuron 50% WP (@ 1 g/l) and cyenopyrafen 30% EC (@ 0.5 ml/l) (58244.11, 56757.62 and 56616.24 kg/ha/year, respectively) while the least was reported in azadirachtin (52242.12 kg/ha/year) (Table 5, Figure 5). The pesticides can produce negative effects on physiological processes, especially the photosynthesis of crop plants leading to substantial reduction in both growth and productivity (Gimenez-Moolhuyzen et al., 2020) [4] . The negative effects of various pesticides on photosynthesis have been reported in different crops viz., lettuce (Toscano et al., 1982; Johnson et al., 1983 and Haile et al., 2000) [20,6,5] , alfalfa (Haile et al., 2000) [5] , cotton (Youngman et al., 1990) [23] , citrus (Jones et al., 1983) [7] and soybean (Abdel-Reheem et al., 1991) [1] . However, the right time and dosage might be advantageous as reported by Chung et al. 2000 [3] who obtained high yields with low damage to the egg fruit by the use of molecule, abamectin 1.8% EC as a spray in the eggplant. .191 T1= Diafenthiuron 50% WP (@ 1 g/l); T2= Abamectin 1.9% EC(@ 0.75ml/l); T3=Diamethoate 30% EC @ 2 ml/l; T4=Cyanopyrafen 30% EC @ 0.5ml/l; T5=Azadirachtin 1% EC @ 1ml/l; T6= Control (water spray); T7= Absolute control. *Significant at 0.05; DAS: Days after Spray of the chemical; Numbers superscript with same alphabets are statistically on par.

Summary

Safety of chemical pesticides to the natural enemies on mulberry was assessed during both Jan-Feb, 2023 and Mar-April 2023 in mulberry. Among the pesticides sprayed on mulberry, and abamectin 1.9% EC (@ 0.75 ml/l) and diafenthiuron 50% WP (@ 1 g/l) recorded highest flourishing coccinellid population (4.00, 3.25, 4.25, 4.63, 4.38 and 3.38, 3.63, 5.00, 4.00, 3.63) on par with the control (4.63, 3.75,5.25,5.62, 5.50) on 1, 3, 8, 15 and 21 DAS, respectively. The leaf yield recorded maximum of 58244.11 kg/ha/year in abamectin 1.9% EC (@ 0.75 ml/l) and 56757.62 kg/ha/year in diafenthiuron 50% WP (@ 1 g/l) reflecting that the use of these two molecules has yield advantage in mulberry apart from effective management of thrips and mites.

Acknowledgement

AcknowledgementsThe authors would like to thank Head, Department of Sericulture, UAS, GKVK Bangalore for providing all the facilities to conduct the experiment and this research did not receive any specific grants from funding agencies.

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