Survey of Cabbage aphid " Brevicoryne brassicae"(L.) population flactuation and its dominant natural enemies.

Canola is one of the most important oilseeds in Iran. Its seeds have more than 40% oil. Figures show that 90% of oil consumed in our country is imported. Therefore, to achieve self sufficiency in oil production, cultivation of this crop has been promoted as a pivotal strategy. As a result, the expansion of canola cultivation in Iran has caused the resurgence of some pests. Among all, the cabbage aphid (Brevicoryne brassicae) and the mustard aphid (Lipaphis erysimi) are considered damageful. The application of chemical pesticides against the pests, besides being less effective, has caused environmental contaminations, residues in the crop, resistance occurrence in the pests, and mortalities on the natural enemies. Therefore, a study was conducted to collect and evaluate the role of natural enemies of the aphids in canola fields. The survey was conducted in five main cultivation suck as Tehran, Khuzestan, Golestan, west Azerbaijan and Sistan provinces during 2002-2004. The obtained results showed that cabbage aphid (B. brassicae) was dominant species in most areas; however, mustard aphid (L. erysimi) was dominant species only in Khuzestan province. In Tehran, Golestan and west Azerbaijan, peak of aphid population was observed between flowering and capsule forming stages. It means, the population of aphids increased from late in April to late in May which coincided with the canola's flowering stage and started reducing early in June onwards which coincided with the plant capsule forming stage. During the observation in Tehran, it appeared that there were several important natural enemies attacking aphids among which there were coccinellids such as Coccinella septempuncutata and Hippodamia vareigata with 15 and 18 % population frequency and syphids such as Episyrphus balteatus, Eupoedes correllae with 13 and 8 % population frequency and the parasitoid wasp Diaeretiella rapae with 37 % population frequency in 2004. Morover, Chrysoperla carnea, Aphidoletes aphidimyza and Leucopis sp. were collected as active natural enemies. In Khuzestan, mustard aphid was observed on canola mid in Feb and its population reached at maximum early in March. Finally, it reduced with the tempareture increase late in March. The most important natural enemies on the aphids were predators like Coccinella septempuncutata and Eupodes coralle), parasitoids, Diaeretiella rapae, and pathogens (Entomophthoraceae). There were six species of predators and a parasitoid active on cabbage aphid in Golastan. E. balteatus and D. rapae were dominant natural enemies in this province. Plus natural enemies, temperature conditions were also effective on controlling the canola aphids. In Azerbaijan, two coccinellids and six syrphid species were collected as natural enemies of canola aphids in the province. Among all, the D. rapae, C. septempunctata and E. balteatus were important natural enemies with 74, 98 and 28.4% population frequencies, respectively. However, there were 2 hyperpasitoid species collected from mummified aphids, too. The survey in Sistan showed that the cabbage aphid was the dominat species. The pest appeared in mid march and its population built up to early in April. The resutts revealed that there were 3 coccinellids, 3 syrphids, 2 chrysopids and a parasditoid, Ephedrus persicae, attacking the canola aphid colonies. C. septempunctata, E. balteatus were dominant species with 83.4 and 68.3% population frequencies, respectively.