Phytotoxic effect and efficacy of chemical insecticides used for management of mulberry thrips on natural enemy (Coccinellid) population

Introduction

Sericulture is an agro based industry and an integral part of rural economy in India. The production of good quality leaves in required quantity is influenced by pests, diseases, weeds and others (Govindaiah et al. 2005) [2] . Earlier good number of methods were adopted for the management of mulberry thrips like cultural, physical, rising of pest resistant/tolerant variety, biological and chemical methods (Reddy and Narayanaswamy 1999) [6] and through IPM approach (Singh and Sarathchandra 2011) [9] . As traditional methods of pest management were tedious and time consuming and it is the need of hour to use chemical insecticide for the better management of mulberry thrips with production good quality and quantity of mulberry leaves and also without effecting natural enemies of thrips

Materials and Methods

The investigation on "Management of mulberry thrips through chemical insecticides and their impact on silkworm rearing performance" was conducted during 2020-21, at the Department of Sericulture, University of Agricultural Sciences, GKVK, Bengaluru. The experimental site is located in the Eastern Dry Zone (zone-5) of Karnataka at 12°58′ N latitude and 77°35′ E longitude and at an altitude of 930 m above mean sea level. 3).

Safety to natural enemies

The most predominantly found natural enemy of mulberry thrips found in the mulberry ecosystem is coccinellid beetle whose population was recorded pre and post application of chemicals, in order to understand the impact of spraying insecticides on their population. Pre-treatment count was recorded prior to spraying and post treatment count were recorded on 7 th and 15 th day after the spraying of insecticides. Coccinellid population was counted with the help of magnifying hand lens on fifteen plants in each treatment.

Results and Discussion

Effect of chemical insecticides on natural enemy (coccinellid) population

The coccinellid count per plant taken before spraying of chemical insecticides showed non -significant difference among different treatments and it ranged from 3 -4.33 / plant. On 7 DAS of chemical insecticides, the control (Water spray) recorded highest coccinellid population of 4 per plant and the lowest coccinellid population was observed in T2 (acetamiprid 20 SP @ 0.2 g / l) (0.33 coccinellid / plant). Among the insecticides treated on mulberry, T6 (dichlorvos 76 EC @ 2.63 ml / l) recorded highest coccinellid population (3.33 / plant) followed by T4 (fipronil 5 SC @ 2 ml / l) (2.67 / plant), T1 (acephate 75 SP @ 1 g / l) (2.33 / plant), T5 (imidacloprid 17.8 SC @ 0.3 ml / l) (2.33 / plant) and T3 (dimethoate 30 EC @ 2 ml / l) showed the coccinellid population of 2 per plant (Table 5). After 15 DAS control (Water spray) recorded highest coccinellid population of 5 per plant and 0 per plant was the least coccinellid population count recorded from the treatment T2 (acetamiprid 20 SP @ 0.2 g / l), followed by T5 (imidacloprid 17.8 SC @ 0.3 ml / l) (3 / plant), T6 (dichlorvos 76 EC @ 2.63 ml / l) (3 / plant) and T4 (Fipronil 5 SC @ 2 ml / l) (3 / plant). T3 (dimethoate 30 EC @ 2 ml / l) showed the population of 2.67 per plant followed by T1 (Acephate 75 SP @ 1 g / l) (2 / plant). Among the treatments T2 (Acetamiprid 20 SP @ 0.2 g / l) showed lowest coccinellid population i.e., 0.33 and 0.00 per plant both in 7 and 15 DAS respectively. In the control (water spray) the coccinellid population was found to be the highest in both 7 th (4 / plant) and 15 th (5 / plant) day after spraying (Table 5). Similarly, Mukhopadhyay and Santha Kumar (2010) [4] reported that mortality of natural enemies of thrips of mulberry ecosystem was least in dichlorvos. Roychoudhury and Mishra (2015) [7] reported that spraying of dichlorvos 76 EC is most preferred due to the fact that it is harmless to the natural enemies. Sakthivel et al. (2011) [8] compared efficacy of water jetting and chemical measures against major sucking pests of mulberry and their safety to natural enemies. Result indicated that water jetting in mulberry garden showed slight or no deleterious effect on predatory coccinellids. Whereas, there was drastic reduction in their population with chemical control measures. In contrary, Ghosh et al. (2014) [1] reported that dinotefuran at 15, 20, 25 and 30 g a.i., per ha, along with imidacloprid at 25 g a.i., per ha, acephate at 400 g a.i., per ha against brown plant hopper of rice suggested that, dinotefuran 25 g a.i., per ha molecule was also found to be very safe to natural enemies.

Selected insecticides were studied for their efficacy against mulberry thrips in the current study. As thrips feed primarily on mulberry leaves, any reduction in pest population as a consequence of insecticide spraying could contribute for increase in mulberry leaf yield. Hence, the mulberry leaf yield (V-1 variety) was estimated in all the insecticide treated blocks at 45 days after pruning. Mulberry leaf yield was recorded initially per plant and leaf yield recorded on 45 th day after pruning showed highest in the treatment T6 (dichlorvos 76 EC @ 2.63 ml / l) (0.37 kg / plant) followed by T3 (dimethoate 30 EC @ 2 ml / l) (0.35 kg / plant), T5 (imidacloprid 17.8 SC @ 0.3 ml / l) (0.34 kg / plant), T4 (fipronil 5 SC @ 2 ml / l) (0.33 kg / plant) and acephate 75 SP @ 1 g / l recorded the leaf yield of 0.30 kg per plant. The least leaf yield per plant was recorded in acetamiprid 20 SP @ 0.2 g / l (0.29 kg / plant). Further the mulberry leaf yield was calculated per hectare per crop and was found that the highest leaf yield per hectare per crop was recorded in T6 (Dichlorvos 76 EC @ 2.63 ml / l) (5069.12 kg / ha / crop) followed by T3 (Dimethoate 30 EC @ 2 ml / l) (4851.54 kg / ha / crop), T5 (Imidacloprid 17.8 SC @ 0.3 ml / l) (4758.95 kg / ha / crop), T4 (Fipronil 5 SC @ 2 ml / l) (4587.67 kg / ha / crop) and acephate 75 SP @ 1 g / l recorded the leaf yield of 4175.66 kg per hectare per crop. The least leaf yield was recorded in the treatment T2 (Acetamiprid 20 SP @ 0.2 g / l) (3971.97 kg / ha / crop). The control (Water spray) also recorded lowest yield of 0.31 kg / plant and 4245.10 kg / ha / crop along with insecticide, acetamiprid 20 SP @ 0.2 g / l. When no spray was given in control, the thrips population increased over a period of time. Due to severe infestation of thrips mulberry leaf yield in control was observed to be lowest. Spraying of dichlorvos 76 EC @ 2.63 ml / l was effective in controlling thrips population resulting in maximum yield in this treatment and did not show any phytotoxic symptoms. Therefore, there was no wastage of leaves due to phytotoxic symptoms viz., epinasty, hyponasty, necrosis etc. in this treatment (Table 4). Though, acephate and acetamiprid were effective against thrips, they exhibited phytotoxic effect on mulberry plants. Severe phytotoxic symptoms viz., epinasty, hyponasty and necrosis on leaves were noticed after spraying of this insecticide molecule. The phytotoxic symptoms were more conspicuous and prominent at 15 DAS coinciding with the mulberry leaf harvest. Leaves with these phytotoxic symptoms were poor in quality and hence deemed unfit for consumption by silkworm. Owing to these reasons, the mulberry leaf yield was lowest in the blocks sprayed with acetamiprid 20 SP @ 0.2 g / l. Similar to the current findings, reduction in yield due to thrips infestation has been reported from other studies on mulberry. In contrary Khaliq et al. (2014) [3] reported that the chemical treatments resulted in a higher yield compared to the untreated control. Poornima (2015) [5] reported that the mulberry yield was enhanced by spraying spinosad 45 SC @ 0.25 ml / l.

Conclusion

Among the insecticides assessed, fipronil 5 SC @ 2 ml / l was observed to be more effective in controlling thrips population, non-phytotoxic to mulberry though acephate and acetamiprid recorded higher reduction in population of thrips, mulberry leaves exhibited phytotoxic symptoms as well as reduced population of natural enemy (coccinellid).

References

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