Efficacy of chemical insecticides on percent mortality of mulberry thrips under laboratory condition

Introduction

Mulberry silkworm is monophagous in nature; therefore, mulberry is the only food plant of silkworm, Bombyx mori. Hence quality of mulberry leaf has a predominant influence on the growth and development of silkworm and the quality of cocoons. The production of quality mulberry leaves is based on several factors such as cultivation practices, variety, biotic and abiotic factors (Krishnaswami et al. 1970) [5] . The production of good quality leaves in required quantity is influenced by pests (18%), diseases (24%), weeds (7%) and others (51%) (Govindaiah et al. 2005) [4] . Since mulberry is prone to several insect pest which hampers the production of good quality as well as quantity of mulberry leaves. Mulberry thrips Pseudodendrothrips mori is one of the most serious sucking pests on mulberry during summer, which leads to significant reduction in leaf yield and quality therefore it is necessary to find the best chemical insecticide for its management without affecting leaf quality and quantity, in order to get good rearing performance. Under this investigation, attempts were made to find the best insecticides at certain concentrations to achieve the above-mentioned objectives.

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. . Healthy and succulent fresh leaves were selected and harvested from mulberry plants and were brought to the laboratory. Insecticide solution at different concentrations were prepared and kept ready in glass beakers (5000 ml). Harvested succulent mulberry leaves were selected and with the help of forceps they were dipped into the insecticide solution for 2-3 seconds, after shade drying, the leaves were placed on watersoaked cotton which were kept on a petri plate to maintain the moisture of the leaves. Thirty numbers of second instar nymphs were carefully transferred on to the leaves on each petri-plate with the help of camel hair brush. The petri plates were kept at normal room temperature (27 °C) and RH (56%). Using binocular microscope mortality was recorded after every 24 hours till 5 days after treatment. Paintbrush is used to stimulate individual thrips, recording those that are incapacitated or fail to show any signs of movement as 'dead'. The total number of dead thrips per each petri plate were observed and recorded. Treatment details of the laboratory bioassay study is provided in Table 2. (IRAC 2010) [2] .

Results and Discussion

Fig 1:

Efficacy of insecticides on mortality (%) of thrips under laboratory condition T1 -Acephate 75 SP @ 0.5 g / l T2 -Acephate 75 SP @ 1 g / l T3 -Acephate 75 SP @ 1.5 g / l T4 -Acetamiprid 20 SP @ 0.1 g / l T5 -Acetamiprid 20 SP @ 0.2 g / l T6 -Acetamiprid 20 SP @ 0.3 g / l T7 -Dimethoate 30 EC @ 1 ml / l T8 -Dimethoate 30 EC @ 2 ml / l T9 -Dimethoate 30 EC @ 3 ml / l T10 -Fipronil 5 SC @ 1 ml / l T11 -Fipronil 5 SC @ 2 ml / l T12 -Fipronil 5 SC @ 3 ml / l T13 -Imidacloprid 17.8 SC @ 0.15 ml/l T14 -Imidacloprid 17.8 SC @ 0.3 ml/l T15 -Imidacloprid 17.8 SC @ 0.45 ml/l T16 -Dichlorvos 76 EC @ 1.31 ml/l T17 -Dichlorvos 76 EC @ 2.63 ml/l T18 -Dichlorvos 76 EC @ 3.94 ml/l T19 -Control (untreated)

The efficacy of chemical insecticides at different concentrations against mulberry thrips (Pseudodendrothrips mori) on V-1 mulberry leaves under laboratory condition revealed significant results (Table 3; Fig 1). All the insecticides showed positive result on mortality of mulberry thrips from the first day till fifth day of the treatment. Among the treatments T3 (acephate 1.5 g / l), T5 (Acetamiprid 0.2 g / l), T6 (Acetamiprid 0.3 g / l), T8 (dimethoate 2 ml / l), T9 (dimethoate 3 ml / l), T11 (fipronil 2 ml / l), T12 (Fipronil 3 ml/ l), T8 (Dimethoate 2 ml / l), T9 (dimethoate 3 ml / l), T11 (Fipronil 2 ml / l), T12 (fipronil 3 ml / l), T15 (imidacloprid 0.45 ml / l) and T18 (Dichlorvos 3.94 ml / l) showed 100 per cent mortality. The lowest mortality of mulberry thrips was observed in T1 (Acephate 0.5 g / l) (73.33%) and T13 (imidacloprid 0.15 ml / l) (73.33%). T3 (Acephate 1.5 g / l) showed the highest mortality of 100 per cent at the concentration of 1.5 g / l followed by mortality of 98.89 per cent at 1 g / l concentration. Among the acephate treated leaves least mortality was observed in 0.5 g / l concentration (73.33%). Among acetamiprid treated leaves, T5 (Acetamiprid) @ 0.2 g / l and T6 (acetamiprid) @ 0.3 g / l showed 100 per cent mortality, followed by T4 (Acetamiprid) @ 0.1 g / l. Among dimethoate treated leaves T8 (Dimethoate) @ 2 ml / l and T9 (Dimethoate) @ 3 ml / l showed 100 per cent mortality, followed by T7 (dimethoate) @ 1 ml / l (80%). Among fipronil treated leaves, T11 (Fipronil) @ 2 ml / l and T12 (Fipronil) @ 3 ml / l concentration resulted in 100 per cent mortality, followed by T10 (Fipronil) @ 1 ml / l (84.44%). T14 (Imidacloprid) @ 0.3 ml / l showed 98.89 per cent mortality, followed by T15 (Imidacloprid) 0.45 ml / l (100%). Among the imidacloprid treated leaves least mortality was observed in T13 (Imidacloprid) @ 0.15 ml / l (73.33%). Dichlorvos 76 EC being used as the standard check recorded 100 percent mortality of thrips when treated @ 3.94 ml / l (T18), followed by 2.63 ml / l (T17) (97.78%) and @ 1.31 ml / l (T16) concentration (85.56%). Among the treatments, control recorded least mortality of thrips (10%). Amongst the lowest concentrations of chemical insecticides assessed dichlorvos 76 EC @ 1.31 ml / l recorded highest mortality of thrips (85.56%) followed by fipronil @ 1 ml / l (84.44%), acetamiprid @ 0.1 g / l (82.22%), dimethoate @ 1 ml / l (80%) and the lowest mortality was observed in imidacloprid @ 0.15 ml / l (73.33%) and acephate @ 0.5 g / l (73.33%). When the chemical insecticides were assessed at the recommended dosage (As per label claim) on other crops, acetamiprid @ 0.2 g / l, dimethoate @ 2 ml / l and fipronil @ 2 ml / l showed 100 percent mortality of mulberry thrips. Recommended dosage of acephate 1 g / l and imidacloprid 0.3 ml / l showed 98.89% mortality of mulberry thrips followed by dichlorvos @ 2.63 ml / l (97.78%). Through laboratory bioassay, it was possible to shortlist the insecticides and their concentrations effective against thrips for field evaluation. (Table . 3, Fig. 1). Similarly, Byrne et al. (2007) [3] reported that in bioassay with first and second flush leaves, imidacloprid (273 mg a. i., pot -1 ) was the most effective insecticide, providing at least 70 per cent mortality of thrips for 14 weeks.

Summary and Conclusion

The effect of insecticides at different concentrations against mulberry thrips on V-1 mulberry variety under laboratory condition revealed that all the chemical insecticides recorded 100 per cent mortality when used at above recommended concentrations. However, the chemical insecticides viz., acetamiprid @ 0.2 g / l, dimethoate @ 2 ml / l and fipronil @ 2 ml / l showed 100 percent mortality of mulberry thrips when used at recommended dosage and below recommended dosage. Effective insecticides after evaluating under the lab condition were taken up for thrips management under field condition.

References

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