Management of mulberry thrips through chemical insecticides

Introduction

Sericulture is an integral part of the rural economy in India. Good quality and quantity mulberry leaves is hampered by pests (18%), diseases (24%), weeds (7%) and others (51%). Among the insect pests, mulberry is very much prone to sap feeders. They are commonly known as sap suckers which pierce the plant cell in leaves and suck the juice. Amongst the sap suckers infesting mulberry, infestation by thrips is highest with 42.55 percent which results in 40-50 per cent yield loss. In addition to decrease in yield and quality of mulberry leaves, thrips infestation lowers the moisture content, chlorophyll a/b ratio, protein content and carbohydrate content of mulberry leaves. Thrips infested mulberry leaves when fed to silkworms causes slower development, increased larval mortality and reduces cocoon yield and quality. Dichlorvos 76 EC is the most commonly recommended insecticide against insect pests in mulberry which has been banned from 31 st December 2020. Therefore, it is need of the hour to explore the efficacy of new molecules of insecticides introduced recently for management of thrips and to select the best insecticide among them as substitute for Dichlorvos 76 EC with minimum effect on silkworm performance and cocoon productivity.

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.

Experimental results

The number of thrips recorded for top 5 leaves at 1st, 3rd, 5th, 7th and 10th day after spraying of insecticides was significantly higher in control (Water spray) in both 1st and 2nd spray. Among the six insecticides evaluated, significantly lower number of thrips were recorded in acephate (3.80/5 leaves) followed by fipronil, acetamiprid, imidacloprid dichlorvos and dimethoate (

Population of thrips at 20 days after pruning

In this study efficacy of selected new insecticides was assessed against mulberry thrips Pseudodendrothrips mori in the field. Most of the insecticides assessed were found effective on mulberry thrips. The number of thrips for top five leaves recorded one day before spray revealed non-significant difference in thrips population indicating uniform distribution of thrips in all the treatments. The thrips population varied from 33.80 to 47.07 (Table 1; Fig 1). The number of thrips recorded for top five leaves on first day after spray was significantly higher in untreated control. Among the seven treatments, lowest number of thrips was recorded in T3 (Dimethoate 30 EC @ 2 ml/l) (0.60/top 5 leaves), followed by T6 (dichlorvos 76 EC @ 2.63 ml/l) (0.80/top 5 leaves), T1 (acephate 75 SP @ 1 g/l) (0.93/top 5 leaves), T4 (fipronil 5 SC @ 2 ml/l) (1.20/top 5 leaves), T2 (acetamiprid 20 SP @ 0.2 g/l) (1.27/top 5 leaves), T5 (Imidacloprid 17.8 SC @ 0.3 ml/l) (1.60/top 5 leaves), and control (water spray) recorded (31.87/top 5 leaves). However, there is no significant difference in the per cent reduction in the pest density over control among the treatments, but it was highest in case of T3 (dimethoate 30 EC @ 2 ml/l) (97.98%), followed by T1 (acephate 75 SP @ 1 g/l) (97.13%), T6 (Dichlorvos 76 EC @ 2.63 ml/l) (96.75%) and T4 (Fipronil 5 SC @ 2 ml/l) (95.75%), T2 (acetamiprid 20 SP @ 0.2 g/l) (95.73%) and the least was recorded in T5 (Imidacloprid 17.8 SC @ 0.3 ml/l) (95.05%). Dimethoate 30 EC is a widely used organophosphate insecticide, it is an acetylcholinesterase inhibitor which disables cholinesterase, an enzyme essential for central nervous system function. It acts as both contact and systemic insecticide On the third day after spray, significantly lowest number of thrips was recorded in T1 (Acephate 75 SP @ 1 g/l) (0.53/top 5 leaves), followed by T6 (Dichlorvos 76 EC @ 2.63 ml/l) (0.60/top 5 leaves), T3 (dimethoate 30 EC @ 2 ml/l) (0.73/top 5 leaves), T4 (fipronil 5 SC @ 2 ml/l) (1.07/top 5leaves), T2 (acetamiprid 20 SP @ 0.2 g/l) (1.53/top 5 leaves) and T5 (Imidacloprid 17.8 SC @ 0.3 ml/l) (1.87/top 5 leaves) and the highest population of thrips was recorded in control (Water spray) (31.40/top 5 leaves). However, the per cent reduction in the pest density over control was significantly higher in case of T1 (Acephate 75 SP @ 1 g/l) (98.03%), followed by T6 (Dichlorvos 76 EC @ 2.63 ml/l) (97.66%), T3 (dimethoate 30 EC @ 2 ml/l) (97.62%), T4 (fipronil 5 SC @ 2 ml/l) (96.41%), T2 (acetamiprid 20 SP @ 0.2 g/l) (94.94%) and T5 (imidacloprid 17.8 SC @ 0.3 ml/l) (94.29%). Acephate is an organophosphate systemic insecticide used to control sucking and biting insects by direct contact or ingestion, it also binds to and inhibit the acetylcholinesterase enzyme in nervous system tissues.

On fifth day after spray significantly lower number of thrips were recorded in T1 (Acephate 75 SP @ 1 g/l) (0.80/top 5 leaves) followed by T4 (Fipronil 5 SC @ 2 ml/l) (0.87/top 5 leaves), T3 (Dimethoate 30 EC @ 2 ml/l) (0.93/top 5 leaves), T6 (Dichlorvos 76 EC @ 2.63 ml/l) (0.93/top 5 leaves), T5 (imidacloprid 17.8 SC @ 0.3 ml/l) (1.07/top 5 leaves) and T2 (acetamiprid 20 SP @ 0.2 g/l) (1.67/top 5 leaves) and control (water spray) recorded the highest population of 33.93 for top 5 leaves. However, there was no significant difference in the per cent reduction in thrips density over control and it was higher in case of T4 (Fipronil 5 SC @ 2 ml/l) (97.31%) followed by T3 (Dimethoate 30 EC @ 2 ml/l) (97.10%), T5 (Imidacloprid 17.8 SC @ 0.3 ml/l) (96.96%), T6 (Dichlorvos 76 EC @ 2.63 ml/l) (96.91%), T1 (Acephate 75 SP @ 1g/l) (96.85%) and T2 (acetamiprid 20 SP @ 0.2 g/l) (95%).

On seventh day after spray significantly lowest number of thrips were recorded with fipronil 5 SC @ 2 ml/l (1.09/top 5 leaves) followed by T1 (acephate 75 SP @ 1 g/l) (1.93/top 5 leaves), (Imidacloprid 17.8 SC @ 0.3 ml/l) (1.93/top 5 leaves), Dichlorvos 76 EC @ 2.63 ml/l (2.07/top 5 leaves), T2 (Acetamiprid 20 SP @ 0.2 g/l) (2.33/top 5 leaves), T3 (Dimethoate 30 EC @ 2 ml/l) (6.53/top 5 leaves) and control (Water spray) (34.33/top 5 leaves). However, the per cent reduction in the pest density over control was significantly higher in case of T4 (fipronil 5 SC @ 2 ml/l) (95.63%) followed by T5 (imidacloprid 17.8 SC @ 0.3 ml/l) (94.68%), T6 (Dichlorvos 76 EC @ 2.63 ml/l) (93.71%), T1 (Acephate 75 SP @ 1 g/l) (93.42%), T2 (acetamiprid 20 SP @ 0.2 g/l) (92.48%), and T3 (dimethoate 30 EC @ 2 ml/l) (78.77%). Fipronil is a broad-spectrum insecticide that belongs to phenylpyrazole chemical family that disrupts the insect central nervous system by blocking (GABA) gamma -Aminobutyric acid -gated chloride channels and glutamate chloride channels (GluCl). However, on tenth day after spray the population of thrips gradually increased in all the treatments and it was significantly low in T1 (acephate 75 SP @ 1 g/l) (3.80/top 5 leaves), followed by T4 (fipronil 5 SC @ 2 ml/l) (3.93/top 5 leaves), T2 (acetamiprid 20 SP @ 0.2 g/l) (4.40/top 5 leaves), T5 (imidacloprid 17.8 SC @ 0.3 ml/l) (4.93/top 5 leaves), T6 (dichlorvos 76 EC @ 2.63 ml/l) (6.73/top 5 leaves) and T3 (dimethoate 30 EC @ 2 ml/l) (7.27/top 5 leaves). While control (water spray) recorded 34.80 for top 5 leaves. Further, the per cent reduction over control in the thrips density was significantly highest in case of T4 (fipronil 5 SC @ 2 ml/l) (88.44%) followed by T1 (acephate 75 SP @ 1 g/l) (88.37%), T2 (acetamiprid 20 SP @ 0.2 g/l) (86.04%), T5 (imidacloprid 17.8 SC @ 0.3 ml/l) (85.64%), T6 (Dichlorvos 76 EC @ 2.63 ml/l) (79.44%) and T3 (dimethoate 30 EC @ 2 ml/l) (77.19%). This might be due to reduced efficacy of insecticides. Population of thrips at 30 days after pruning Before taking up second spray at thirty days after pruning the thrips population recorded on tenth day after first spray was considered as pre-count. Significant difference was observed between the treatments which could be due to residual toxicity of the first spray given at 20 days after pruning (Table 2; Fig 2). The number of thrips recorded for top five leaves on first day after spray was significantly higher in control (Water spray) (33.87/top 5 leaves). Among the six treatments, significantly lowest number of thrips was recorded in T1 (Acephate 75 SP @ 1 g/l) (0.07/top 5 leaves) followed by T2 (Acetamiprid 20 SP @ 0.2 g/l) (0.47/top 5 leaves), T3 (Dimethoate 30 EC @ 2 ml/l) (0.53/top 5 leaves), T6 (Dichlorvos 76 EC @ 2.63 ml/l) (0.60/top 5 leaves), T4 (Fipronil 5 SC @ 2 ml/l) (1.07/top 5 leaves) and the least was recorded in T5 (Imidacloprid 17.8 SC @ 0.3 ml/l) (1.13/top 5 leaves). However, the per cent reduction in the pest density over control was highest in case of T1 (acephate 75 SP @ 1 g/l) (99.81%) followed by T2 (acetamiprid 20 SP @ 0.2 g/l) (98.39%), T6 (Dichlorvos 76 EC @ 2.63 ml/l) (98.25%), T3 (dimethoate 30 EC @ 2 ml/l) (98.19%), T4 (fipronil 5 SC @ 2 ml/l) (96.71%) and the least was recorded in T5 (imidacloprid 17.8 SC @ 0.3 ml/l) (96.35%).

On the third day after spray, 100 per cent mortality of thrips was recorded in T1 (acephate 75 SP @ 1 g/l) (0.00/top 5 leaves), followed by T2 (acetamiprid 20 SP @ 0.2 g/l) (0.27/top 5 leaves), T6 (dichlorvos 76 EC @ 2.63 ml/l) (0.27/top 5 leaves), T5 (imidacloprid 17.8 SC @ 0.3 ml/l) (0.27/top 5 leaves), T4 (fipronil 5 SC @ 2 ml/l) (0.40/top 5leaves), T3 (dimethoate 30 EC @ 2 ml/l) (0.73/top 5 leaves) and the least was recorded in control (Water spray) (33.80/top 5 leaves). However, the per cent reduction in the pest density over control was significantly higher in case of T1 (acephate 75 SP @ 1 g/l) (100%), followed by T6 (dichlorvos 76 EC @ 2.63 ml/l) (99.17%), T5 (imidacloprid 17.8 SC @ 0.3 ml/l) (99.09%), T2 (acetamiprid 20 SP @ 0.2 g/l) (99.06%) T4 (fipronil 5 SC @ 2 ml/l) (98.78%) and the least in T3 (Dimethoate 30 EC @ 2 ml/l) (97.35%).

On fifth day after spray also 100 per cent mortality of thrips was recorded in T1 (Acephate 75 SP @ 1 g/l) (0.00/top 5 leaves) followed by T2 (Acetamiprid 20 SP @ 0.2 g/l) (0.00/top 5 leaves), T4 (fipronil 5 SC @ 2 ml/l) (0.00/top 5 leaves) and T5 (Imidacloprid 17.8 SC @ 0.3 ml/l) (0.00/top 5 leaves). T6 (Dichlorvos 76 EC @ 2.63 ml/l) recorded 0.40/top 5 leaves followed by T3 (dimethoate 30 EC @ 2 ml/l) (1.13 per top 5 leaves) and the least was recorded in control (Water spray) (33.20/top 5 leaves). However, the per cent reduction in thrips density over control was 100 per cent in case of T1 (acephate 75 SP @ 1g/l) (100%) followed by T2 (acetamiprid 20 SP @ 0.2 g/l) (100%), T4 (fipronil 5 SC @ 2 ml/l) (100%) and T5 (imidacloprid 17.8 SC @ 0.3 ml/l) (100%). T6 (dichlorvos 76 EC @ 2.63 ml/l) recorded 98.78% and least was recorded in T3 (Dimethoate 30 EC @ 2 ml/l) (96.46%).

On seventh day after spray significantly lowest number of thrips were recorded in T2 (acetamiprid 20 SP @ 0.2 g/l) (0.73/top 5 leaves) followed by T1 (acephate 75 SP @ 1 g/l) (0.80/top 5 leaves), T4 (Fipronil 5 SC @ 2 ml/l) (0.87/top 5 leaves), T5 (Imidacloprid 17.8 SC @ 0.3 ml/l) (1.73/top 5 leaves), T6 (Dichlorvos 76 EC @ 2.63 ml/l) (1.93/top 5 leaves), T3 (Dimethoate 30 EC @ 2 ml/l) (2/top 5 leaves) and the least was recorded in control (water spray) (33.40/top 5 leaves). However, the per cent reduction in the thrips density over control was significantly higher in case of T1 (Acephate 75 SP @ 1 g/l) (97.58%), T2 (Acetamiprid 20 SP @ 0.2 g/l) (97.44%), T4 (fipronil 5 SC @ 2 ml/l) (97.05%), T5 (imidacloprid 17.8 SC @ 0.3 ml/l) (94.65%), T6 (dichlorvos 76 EC @ 2.63 ml/l) (94.29%) and it was found least in T3 (dimethoate 30 EC @ 2 ml/l) (93.40%).

Discussion

Subramanian et al. (2010) reported that dichlorvos (0.076%) and acetamiprid (0.03%), were most effective, recording significantly lower thrips population levels. Misra et al.

(2003) studied the effect of certain insecticides like dimethoate (0.2%), quinalphos (0.2%), imidacloprid (0.05%) and methyl parathion (0.2%) in the management of Pseudodendrothrips mori in mulberry and the results revealed that imidacloprid (0.05%) was found to be very effective. Poornima (2015) [3] also reported that acetamiprid 20 SP @ 0.25 g and fipronil 5 SC @ 1 ml/l were effective in controlling the thrips.

Conclusion

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). As an alternative to dichlorvos, fipronil, imidacloprid and dimethoate can be used for managing thrips in mulberry.

References

1. Subramanian S, Muthuswami M, Krishnan R, Thangamalar A, Indumathi P. Bioefficacy of botanicals and insecticides against mulberry thrips, Pseudodentrothrips mori Niwa. KA J Agril Sci. 2010;23(1):47-50.