Population Dynamics of Sitobion avenae (Hem.: Aphididae) (Fabricius) and its Natural Enemies in a Wheat Fields in Sar-Pole Zahab Regio

Introduction: Aphids are small insects which are famous as pests in agriculture, horticulture and forestry. They cause economic damages directly by feeding plants’ sap and indirectly by transmitting plant viruses. Sitobion avenae (F.) is one the important pests in the United States, Europe and Asia. In Iran, it has been reported on wheat, barley, and the other cereals from the different provinces including Kermanshah. Investigating on the population dynamics of pests is a major factor in evaluation of the pest management. The aim of the present study was to determine the most appropriate life stage of S. avenae for sampling to investigate its population dynamics and identify natural enemies of S. avenae in Sar-Pole Zahab region in Kermanshah province.Materials and Methods: This study was conducted in a wheat field in Sar-Pole Zahab region in Kermanshah province during years 2015 and 2016. None of pesticides were used to control the insect pests during experiments.  Population survey and sampling were began each year in the middle of February and continued weekly until the end of wheat growth season in the middle of June. Sampling pattern was X-shaped and totally 30 samples were collected from the field. The selected plants were separately put in plastic bags and transferred to the laboratory for counting the number of nymphs, winged and wingless adults. To examine probable emergence of parasitoids, the aphids were kept in Petri-dishes in laboratory for two weeks and the results were recorded. A standard insect collecting net was used for sampling of the predators. The collected predators were transferred to the laboratory in glass jars for identification. The aphids and collected natural enemies were sent to Plant Protection Research institution in Tehran for confirmation.Results and Discussion: At the first sampling date in 2015, there were a few adult aphids in the field. However, the population of adults and nymphs were gradually increased and the highest population of aphids was observed between March 12th 2015 and April 30th 2015.The peak density of adult aphids was on April 9th 2015 (9.5 aphids per spike) and for nymphs, it occurred a week earlier, on April 2nd 2015. In the second year, however, the population fluctuation pattern was very different. Although some aphid colonies were seen in the field from 25th of February, 2016, the number of aphids was decreased due to severe cold happened in March, thus no aphids was found in the samples in the next two weeks. The peak population of adult aphids was observed on 14th of April 2016 (36.9 aphids in spike) and then gradually decreased during the next weeks. Coccinella septempunctata L. (Coleoptera: Coccinellidae), Eupeodes corolla (Fabricius) (Diptera: Syrphidae) and Chrysoperla carnea (Stephens) (Neuroptera: Chrysopidae) were reported as aphid’s predators in the field. All predators were present at the field from the first sampling date. In the first year, E. corolla showed two distinct peak in its population during sampling including 19th of March 2015 and 16th of April 2015. In the second year, there were 17 adult flies per net in the first day of sampling, but the number of flies was increased gradually and reached to a maximum of 35.5 flies per net on 14th April 2016. C. septempunctata and C. carnea populations’ dynamics also followed the same patterns and showed two peaks in the first year and one peak in the second year of studies. No parasitoid was emerged from the aphids in Petri dishesConclusion: Due to presence of active predators in the wheat fields of Sar-Pole Zahab region and their high population, using of chemical pesticide should be restricted. Thus, if spray of pesticides is essential, the most appropriate time is when it has the least detrimental effects on natural enemies.