2.1. Collection and Preparation of Test Materials: M. piperita was collected from the Rajshahi University campus, identified and kept in the herbarium of the Department of Botany, University of Rajshahi. The plants were chopped into small pieces, dried under shade and powdered using a hand grinder, weighed and placed in separate conical flasks to add Pet. ether, CHCl3 and CH3OH (Merck, Germany) (100 gm × 300 ml × 2 times) for 48 h. Filtration was done by Whatman filter paper (made in USA) at 24h interval in the same flask followed by evaporation until the extract was left. The extracts was then removed to glass vials and preserved in a refrigerator at 40 C with proper labeling.
2.2. Collection and Culture of Test Insect Aphids are highly reproducing insects. At first some mature aphids were collected from affected plants and then released on the new fresh eggplants for further production. They multiply in a good number within a short time. Aphids were collected repeatedly from the culture field with a fine camel hair-brush in a Petri dish and used in the experiments. Mosquito eggs were hatched in stagnant water. They are collected from damp drains with special collecting spoon. Collected mosquito egg-rafts were placed into a new beaker containing pond water and kept it in a dark place inside the lab for hatching. After 24 h, hatched larvae were collected from the hatching tank and used in the experiment.
2.3.1. Dose-mortality Test on Eggplant Aphids For aphids instead of Petri dish fresh eggplant laves were used. Fresh eggplant leaves were collected and placed on a Petri dish. Stalks of each of the leaves were wetted with water soaked cotton to keep them fresh. The leaves were made round (3.6 cm diam.) in shape by a pair of scissors near the stalks. With the help of a CD marker a round circle was drawn. Extract was applied on both the sides of the round shaped leaves with the help of a 1 ml syringe. The concentrations used in this experiments were 0.196, 0.147, 0.098, 0.049, 0.025 mg cm-2 for Pet. ether, CHCl3 and CH3OH extracts, respectively. Three replicates for each of the extracts were maintained. After application of the extractives the leave circles were allowed to dry out as exposed in the air for 30-45 minutes. Then ten insects were released in the middle of each circle. The mortality of the aphids was assessed after 3, 6, 9, 12, 15, 18, 21 and 24h of exposure.
2.3.2. Statistical Analysis: The mortality (%) was corrected using Abbott’s formula (Abbott, 1925).
2.4. Repellent Activity The repellency test was adopted from the method (No. 3) of McDonald et al. (1970) with some modifications. A general concentration for each of the plant extracts was selected as stock dose for repellency and other successive doses were prepared by serial dilution to give 0.393, 0.197, 0.098, 0.049 and 0.025 mg cm-2 concentrations for the extracts. Ten aphids were released in the middle of each of the leaf circle. Insects that settled on each of the nontreated half of the circles were counted after 1 h and then observed repeatedly at hourly intervals for five hours. The orientation was changed in the two remaining replicates to avoid the effects of any external directional stimulus affecting the distribution of the test insects. The average of the counts was converted to percent repellency (PR) using the formula of Talukder and Howse (1993, 1995): PR = (Nc – 5) × 20, where, Nc is the percentage of insects on the untreated half of the circles.
2.5. Larvicidal Activity Test Mosquito rafts (eggs) were collected from the different drains of Rajshahi University then placed into a new beaker containing normal pond water and kept in a dark place inside the laboratory to hatch. After 24 hours, hatched larvae were collected and used in the experiment. The series of doses were 200, 100, 50, 25, 12.5 and 6.25 ppm; 400, 200, 100, 50, 25 and 12.5 ppm; and 800, 400, 200, 100, 50 and 25 ppm for Pet. ether, CHCl3 and CH3OH extracts respectively. Ten freshly hatched larvae were added to each of the test tubes with different concentrations mentioned earlier and mortality was observed after 6, 12, 18, 24 and 30 h of exposure. The data was subjected to Probit analysis.