Bioefficacy of Newer Insecticides against Major Sucking Insect Pests of Cowpea [Vigna unguiculata (Linn.) Walp.]

Cowpea [Vigna unguiculata (Linn.) Walp.] is one of the important legume crops grown in Rajasthan that belongs to family Leguminosae. It is used as a green legume, vegetable and fodder as well as green manure crop. Its seeds contain 23.4 per cent protein, 1.8 per cent fat, 60.3 per cent carbohydrate and also rich source of lysine and tryptophan (Singh, 1983). Sardhana and Verma (1986) reported 21 insect pests of different groups damaging the crop from germination to maturity. The important insect species infesting cowpea are aphid, Aphis craccivora Koch; jassid, Empoasca fabae (Harris); thrips, Megaleurothrips distalis Karny; army worm, Mythimna separata (Walker); semilooper, Thysanoplusia orichalcea (Fab.); Leafminer, Phytomyza horticola Meigen and pod borer, Helicoverpa armigera (Hubner) resulting in heavy yield losses (Prasad et al., 1983 and Satpathy et al., 2009). It is a common observation that the sucking insect pests population is brought down by the application of non-systemic insecticides due to high initial kill of sucking insect pests, but some time the surviving individuals soon build up their population because of high rate of multiplication and absence of natural enemies. In the recent years, these pests created a serious threat to agriculture industry due to development of resistance towards regularly used insecticides. In this view there is scope of utilizing the newer pesticide molecules, which are required in small quantity to control the pests and are comparatively environmentally safe and economically effective for control of sucking insect pests in cowpea ecosystem.

The experiment was laid out in a simple randomized block design (RBD) with nine treatments including the untreated control (Table 1), each treatment replicated thrice. Each plot was measured 3.0 m × 2.5 m each at the Agronomy farm of S.K.N College of Agriculture, Jobner during kharif 2018. The seeds of cowpea variety, RC-19 (recommended for this region) were sownon 1^st July, 2018 at row to row and plant to plant distance of 30 cm and 10 cm, respectively. The experiment was conducted as per recommended agronomics practices and fertilized with 20 kg N ha^-1 as a starter dose and 40 kg P₂O₅ ha^-1 in the soil before sowing.
 

 
Treatments and their application
  V= Volume of the insecticide.
C = Concentration required.
A = Amount of spray solution needed.
% a.i. = Percentage of active ingredient of the insecticide.
       
The observations on population of aphid, Aphis craccivora Koch, leafhopper, Empoasca fabae (Harris) and whitefly, Bemisia tabaci (Genn) were recorded early in the morning from (10 cm terminal shoot) for aphid and from Three leaves, viz., one each from top, middle and lower canopy of the plant for leafhopper and whitefly one day before and 1, 3, 7 and 15 days after application of insecticides in both the sprays.The crop was harvested when pods attained full maturity. The harvested plants were kept separately and sun dried. The dried plants were threshed manually, seed were cleaned and weighed.  Seed yield per plot was converted into quintal per hectare.
       
The population data thus recorded were converted to per cent reduction in population using the method utilized by Henderson and Tilton (1955) as under: Where,
Ta = Number of pest after treatment in treated plot.
Tb = Number of pest before treatment in treated plot.
Ca = Number of pest in untreated check after treatment.
Cb = Number of pest in untreated check before treatment.
       
The data were than statistically analyzed. The analysis was carried out by transforming the percentage reduction data into angular transformation values (Gomez and Gomez, 1976). The data of seed yield obtained as a result of application of different treatments were also subjected to analysis of variance.
       
The avoidable loss and increase in seed yield over control was calculated for each treatment by the following formula. The economics of treatments were worked out by computing the cost of insecticides as well as their cost of application. The gross income was worked out by multiplying the yield with the whole sale rate of cowpea seed prevailing at the time of threshing.

The bioefficacy of different treatments were evaluated on the basis of per cent reduction of aphid, leaf hoppers and whitefly population and effect on grain yield, and economics of insecticidal treatments. The differences in population of aphid, leaf hoppers and whitefly recorded before spraying were found to be non-significant among different treatments which indicated that the infestation of aphid, leaf hoppers and whitefly were in homogenous condition. The insecticides viz., imidacloprid 17.8 SL @ 0.005%, thiamethoxam 25 WG @ 0.005%, malathion 50 EC @ 0.05%, chlorantraniliprole 18.5 SC @ 0.005%, emamectin benzoate 5 SG @ 0.002%, acetamipird 20 SP @ 0.004%, azadirachtin 0.15 EC @ 1.5 ml/l and Dimethoate 30 EC @ 0.03% against aphids, leafhopper and whitefly, were evaluated. All of the treatments were applied twice. First spray was given when appearance of the pests and second spray after 3 weeks of the first spray. The data on the mean reduction in the population of pest species were calculated on 1^st, 3^rd, 7^th and 15^th day after each spray.
 
Bioefficacy of newer insecticides on population of aphid, leafhopper and whitefly
 
Among the different insecticides tested (Table 2) maximum reduction (77.03%) was recorded in the treatment of imidacloprid 17.8 SL which was at par with thiamethoxam 25 WG that gave 74.88 per cent reduction followed by acetamiprid 20 SP which recorded 71.94 per cent reduction at one day after spraying.  The treatment of dimethoate 30 EC gave 67.42 per cent reduction followed by chlorantraniliprole 18.5 SC (66.00% reduction) and both were at par with each other. Emamectin benzoate 5 SG (57.87%) and malathion 50 EC gave 55.78 per cent reduction have no significant difference. The minimum reduction of 49.82 per cent was recorded in plots treated with azadirachtin 0.15 EC inferior to all the other insecticidal treatments. Similar trend of mean per cent reduction in aphid population was observed on 3, 7 and 15 days of insecticidal spray. The present findings were in full agreement with Choudhary et al., (2017) who reported that the imidacloprid (0.005%), thiamethoxam (0.005%) and dimethoate (0.03%) were found effective against aphid whereas, the azadirachtin (0.002%) and malathion (0.05%) were found least effective against aphid. In addition, Anandmurthy et al., (2017) reported that application of dinotefuran (0.006%), acetamiprid (0.004%) and dimethoate (0.03%) proved effective in recording minimum aphid population. Likewise, Swarnalata et al., (2015) observed that the efficacy of imidacloprid (0.005%) was found most effective (0.19 aphid index/plant) in cowpea which support the present findings. Jangu (2005) also reported that azadirachtin 5 ml/l was found least effective in reducing the aphid population on cowpea.
 

 
Data presented in (Table 3) indicated that all the treatments were significantly superior in reducing the population of leafhopper in the field. The maximum reduction (76.84%) was recorded in the treatment of imidacloprid 17.8 SL which was at par with thiamethoxam 25 WG that gave 73.61 per cent reduction followed by acetamiprid 20 SP (70.52%) at one day after spraying. The treatment of dimethoate 30 EC gave 65.99 per cent reduction followed by chlorantraniliprole 18.5 SC (65.69% reduction) and both were at par with each other. Emamectin benzoate 5 SG (57.71% reduction) and malathion 50 EC gave 55.19 per cent reduction have no significant difference. Azadirachtin 0.15 EC was recorded as least effective (72.13%) and was at par with malathion 50 EC (73.40%). Similar trend of mean per cent reduction in leafhopper population was observed on 3, 7 and 15 days of insecticidal spray. Similar results have been reported by Hithesh et al., (2022) who found that thiomethoxam+lambda-cyhalothrin (0.007%), imidacloprid (0.006%) and triazophos (0.1%)  proved to be most effective in controlling jassid, whitefly and thrips population on green gram. Khade et al., (2014) also revealed that the imidacloprid 17.8 SL (0.005%) proved superior on the mean per cent reduction of jassids, thrips and aphids population in cowpea crop. Likewise, Choudhari et al., (2015) observed that imidaclorpid (0.015%), acetamiprid (0.025%) and clothianidin (0.025%) found effective against leafhopper, aphid, whitefly, and thrips on Indian bean. Yadav et al., (2015) observed that dimethoate (0.03%), imidacloprid (0.005%) and thiamethoxam (0.025%) proved most effective against sucking insect pests, viz., leaf hopper, E. motti; whitefly, B. tabaci and aphid, A. craccivora of cluster bean which support the present findings.


 
The data on per cent reduction in whitefly population (Table 4) revealed that maximum reduction (73.98%) was recorded in the treatment of imidacloprid 17.8 SL which was at par with thiamethoxam 25 WG that gave 71.21 per cent reduction followed by acetamiprid 20 SP (67.78 %) at one day after spraying. The treatment of dimethoate 30 EC gave 62.66 per cent reduction followed by chlorantraniliprole 18.5 SC (61.41% reduction) and both were at par with each other. Emamectin benzoate 5 SG (54.05% reduction) and malathion 50 EC gave 51.72 per cent reduction have no significant difference. The minimum reduction of 45.23 per cent was recorded in plots treated with azadirachtin 0.15 EC inferior to all the other insecticidal treatments. Similar trend of mean per cent reduction in whitefly population was observed on 3, 7 and 15 days of insecticidal spray. The findings are in line with Jakhar et al., (2022) who found that imidacloprid (0.00 5%) resulted in highest reduction in whitefly, jassid and aphid population followed by dimethoate (0.03%) on indian bean. In addition, Singh et al., (2010) also reported that dimethoate 30 EC (0.03%) most effective followed by imidacloprid 17.8 SL (0.005%) and thiamethoxam 25 WG (0.025%) against whitefly, jassid and thrips on moth bean. Dhamaniya et al., (2005) reported the azadirachtin 5 ml/ l was found least effective for the control of whitefly, jassid and thrips.
 

 
Effect of newer insecticides on seed yield of cowpea
 
The data presented on the (Table 5) reveal that all the plots treated with insecticides gave significantly higher seed yield over control (10.05 q ha^-1). The maximum seed yield of 19.88 q ha^-1 was obtained in the plots treated with imidacloprid 0.005 per cent followed by thiamathoxam 0.005 per cent (18.92 q ha^-1). The seed yield obtained in the treatment of acetamiprid 0.004 per cent was (15.88 q ha^-1). The seed yield (14.68 q ha^-1) obtained in the treatment of dimethoate 0.03 per cent with treatment of chlorantraniliprole 0.005 per cent (13.95 q ha^-1), followed by the treatment emamectin benzoate 0.002 with seed yield of (13.56 q ha^-1). The minimum seed yield of (11.58 q ha^-1) was obtained in the plots treated with azadirachtin 1.5 ml/ l followed by the treatment malathion 0.05 per cent (11.75 q ha^-1). Choudhary et al., (2017) who reported highest grain yield of 20.38 q ha^-1 was recorded in the plots treated with imidacloprid, followed by thiamethoxam (19.32 q ha^-1). The minimum grain yield of 11.98 q ha^-1 was obtained in the plots treated with azadirachtin (0.002 per cent) followed by the treatment malathion 0.05 per cent (12.02 q ha^-1).
 

 
Economics of insecticidal treatments
 
The maximum profit was recorded in imidacloprid 0.005 per cent which gave a benefit : cost ratio of 18.48:1 (Table 6). It was followed by thiamethoxam 0.005 per cent and acetamiprid 0.004 per cent, which resulted in a benefit: cost ratio of 17.30:1 and 9.73:1, respectively. dimethote 0.03 per cent, emamectin benzoate 0.002 per cent, azadirachtin 1.5 ml/l and malathion 0.05 per cent were resulted in 7.42:1, 2.63:1, 1.97:1 and 1.76:1 benefit : Cost ratio, respectively. The lowest benefit: cost ratio of 1.62:1 was recorded from plots treated with chlorantraniliprole 0.005 per cent. Choudhary et al., (2017) reported the highest B: C ratio (19.01:1) was recorded in the treatment of thiamethoxam followed by imidacloprid 0.005 per cent and dimethoate 0.03 per cent, which resulted in a benefit: cost ratio of 16.52 : 1 and 8.74: 1, respectively whereas, lowest benefit : cost ratio of 1.21: 1 was recorded from plots treated with chlorantraniliprole 0.005 per cent support the present results.
 

The overall order of effectiveness of insecticides against aphid, leafhoppers and whitefly were found to be imidacloprid 17.8 SL> thiamethoxam 25 WG> acetamiprid 20 SP> dimethoate 30 EC> chlorantraniliprole 18.5 SC> emamectin benzoate 5 SG> malathion 50 EC> azadirachtin 0.15 EC. The maximum benefit cost ratio was obtained from imidacloprid 17.8 SL treated plots followed by thiamethoxam 25 WG and acetamiprid 20 SP. The newer groups of insecticides are very effective at lower doses and have low risk to non-target organisms and environment, and their adaptability in application methods. So there is scope of utilizing the newer pesticide molecules, for control of sucking insect pests in cowpea ecosystem.

The authors are thankful to the Dean, S.K.N. College of Agriculture, Jobner for providing necessary facilities to accomplish this study.

None.