Studying, the Insecticidal Effects of Melia azedarach and Citrus limonum Extracts on Two Aphid Species

Wheat (Triticum aestivum L.) is the most principal plant food for 35 percent of the world's population, and canola (Brassica napus L.) is one of the most important brassicaceous crops that play a major role in the development of edible oil. The greenbug, Schizaphis graminum (Rondani) and cabbage aphid, Brevicoryne brassicae (L.) are the main pests of wheat and canola, respectively, which can considerably limit profitable production of these crops either through direct feeding or via transmission of plant pathogenic viruses. Although chemical control is the most effective and easiest way to control aphids, but this method causes problems such as pesticide residues in food and environment, and development of resistance to insecticides. The utilization of plant extracts is an environmentally safe method that can be used in control of these aphids. Among these, the products of the Melia seed (Melia azedarach Linnaeus) and lemon peel (Citrus limonum Risso) can be noted. Negative associations between phenolic compounds present in plant species and aphid’s invasion have been recorded for some aphid species. In this study, our goal was to determine the amount of phenol in plant extracts of Melia seed and lemon peel and evaluate the toxicity of these compounds on the wheat aphid and cabbage aphid in various doses after different time periods.
This research was conducted in a growth chamber (temperature 25 ± 1˚C, 65± 5% RH and a photoperiod of 16L: 8D). S. graminum and B. brassicae were bred on wheat (Pishtaz cultivar) and canola (Hyola401 cultivar), respectively. The extraction of Melia seed and lemon peel was carried out and then contact toxicity bioassay was done to evaluate the insecticidal effects of these extracts on nymphs of wheat and cabbage aphids using a completely randomized design. The leaves of wheat and canola plants, impregnated with three different concentrations of each extract (10, 50 and 80 g/ml) and dried. Thereafter the leaves were individually placed on agar gel 7.0% in Petri-dishes with a diameter of 6 cm. Then same age nymphs of wheat and cabbage aphids were separately released in Petri-dishes (20 aphids in each Petri-dish), and the number of dead nymphs was recorded after 12, 24 and 36 hours. This experiment was performed in three replicates for each treatment. Furthermore, the amount of phenolic compounds in two extract samples (Melia seed and lemon peel) was determined by Folin ciocalteu method, and the absorbance was measured at 725 nm using a spectrophotometer. The lethal effects of two tested extracts for each of aphid species (in each tested concentration and time period) were analyzed using T-test in SPSS v22 software. Variables of the mortality percent in three different concentrations and amount of total phenolic were evaluated using the one-way analysis of variance in SPSS. The comparison of differences between treatments means was done using Tukey’s test (P