3.1 Description of the Field Experiment The experiments were carried out in Boro rice in the Entomology Field Laboratory, Bangladesh Agricultural University. The period of the study was from January to June 2014. The soil of the experimental area was under Brahmaputra alluvial tract with sandy loam texture having good irrigation and drainage facilities. The experimental area was under the subtropical climate which is characterized by high temperature, high humidity and heavy precipitation with occasional gusty winds in kharif season. The experiment was conducted with the rice variety of BINA dhan5. The experiments were laid out in a Randomized Complete Block Design (RCBD) with three replications of each treatment. Each replication represented a block. Each block was divided into 6 unit plots. The total number of treatments was 6. The total number of the unit plots was 18. The unit plot size was 2m × 2.5m (5m2), where plot-plot distances were 0.5m, and block-block distance was 1m. Plots were allocated randomly and they were separated in such a way so that impact of treatments can be quantified. Treatments were as T1= Malathion 57 EC, T2= Chlorpyrifos 20 EC, T3= Imidacloprid 17.8 SL, T4= Volume flexi 300 SC, T5= Azadiractin (Neem oil 2%) and T6= Control.
Seeds of the tested variety were collected from the Agronomy Seed Store and those were dipped in bucket for 24 hours. The seeds were then taken out of water and kept thickly in gunny bags. After 72 hours the seeds were sown. A seedling nursery was prepared puddling the soil. Sprouted seeds were sown in the wet nursery bed. Irrigation and other facilities were given in the seedbed as and when necessary. The land was ploughed with a power tiller and leveled with the ladder. The field layout was done according to the design immediately after land preparation. The phosphate, potassium and gypsum fertilizers were applied in the experimental plots at the rate of 100, 70 and 30 kg/ha, respectively in the form of the Triple superphosphate, Muriate of potash and Sulphate respectively. Urea, as a source of nitrogen, was applied as per experimental specification. The entire amount of TSP, MP and Gypsum was broadcast and incorporated into the soil at final land preparation. Urea at the rate of 135 kg/ha was applied as an experimental specification as top dressing in three installments at 15, 30 and 50 days after transplanting (DAT). Seedlings were transplanted in the experimental plots at the rate of 3-4 seedlings per hill. The plant spacing was followed viz. 25 cm x 15 cm. Except for insecticidal application, all the other agronomic practices were common to the six treatments and followed as per recommended package of practices. Intercultural operations were maintained properly. No chemical pesticides were used allowing the pest to multiply and cause damage in pesticide-free rice fields.
For the present study, five insecticides namely Malathion 57 EC, Voliam flexi 300 SC, Imidachloropid 17.8 SL, Chloropyriphos 20 EC, Azadiractin 2% were bought from locally available pesticide dealer. The treatments comprised of synthetic insecticides were tested at the doses of Malathion 57 EC-450 ml/acre, Chlorpyrifos 20 EC-400 ml/acre, Imidacloprid 17.8 SL-14g/acre, Voliam flexi 300 SC-100ml/acre, Azadiractin-2ml/L respectively and untreated control. Insecticide treatments were started on March 08, 2014 (spray 1) when both nymph and adult suck the milk of juice from the developing grain in the milky stage of the plant. Insecticides were applied with a knapsack hand pressurized sprayer. Spraying was done at 4.30 p.m. to avoid bright sun and drift caused by strong wind.
3.2 Procedure of data collection
3.2.1 Panicle infestation: The number of infested panicles per hill was recorded. Then the number of total panicle (infested and uninfested) was recorded. There were average 22 panicles per hill. Percentage infested panicle was counted from the following formula:
% of infested panicle = [total no. of infested panicle/total no. of panicle (infested+uninfested)] × 100
3.2.2 Hill infestation: Number of infested hill per plot was recorded. Then the number of total hill (infested and uninfested) was recorded. There were 80 hills per plot. Percentage infested hill was counted from following formula:
% of infested hill =[ total no. of infested hill/total no. of hill (infested+uninfested)] × 100
3.2.3 Grain infestation: Number of infested grain per plot was recorded. Then the number of total grain (infested and uninfested) was recorded. There were 220 grains per panicle. Percentage infested grain was counted from following formula:
% of infested grain =[ total no. of infested grain/total no. of grain (infested+uninfested)] × 100
3.2.4 Partial grain infestation: Number of partial infested grain per plot was recorded. Then the number of total grain (infested and uninfested) was recorded. There were 220 grains per panicle. Percentage of partial infested grain was counted from following formula:
% of partial infested grain =[ total no. of partial infested grain/total no. of grain (infested+uninfested)] × 100
3.3 Measurement of the efficacy of five insecticides: The efficacy of five insecticides viz. Malathion 57 EC, Voliam flexi 300 SC, Imidachloropid 17.8 SL, Chloropyriphos 20 EC, Azadiractin 2%, each having a single dose along with control was tested against rice bug, Leptocorisa acuta on standing rice plant at the place of Bangladesh Agricultural University Campus, Mymensingh. The trial was conducted in a Randomized Complete Block Design and was replicated 3 times. Each insecticide was tested at the doses of Malathion 57 EC-450 ml/acre, Chlorpyrifos 20 EC-400 ml/acre, Imidacloprid 17.8 SL-14g/acre, Voliam flexi 300 SC-100ml/acre and Azadiractin-2ml/L respectively and efficacy doses on rice bug was compared. Pretreatment data were recorded one day before the application of insecticides. For recording, the data 10 hills and 25 panicles were randomly selected from the plot for respective insecticide treatment and 10 hills and 25 panicles were observed from each plot. Data of damage symptoms like hill infestation, panicle infestation, grain infestation etc. were recorded after 7, 14, 21 days after spraying of insecticides. The yield of treated plots was recorded and compared for their difference. The recorded data were compiled and tabulated for statistical analysis. Analysis of variance was done with the help of the computer package MSTAT. The mean differences among the treatments were analyzed as per test with Duncan’s Multiple Range Test (DMRT) and Least Significance