Studies conducted in the 2000 cropping season at two different localities, Flaeming and Magdeburger Boerde in Germany, have provided new information on cereal aphid (Sitobion avenae (F.), Metopolophium dirhodum (Walker), and Rhopalosiphum padi (L.)) parasitoids in winter wheat; their species composition, relative abundance, hosts, and location effects. The average aphid population density was higher at Magdeburger Boerde and lower at Flaeming. Among the aphid species, Sitobion avenae was more abundant at Flaeming and Metopolophium dirhodum at Magdeburger Boerde. In total, eight species of primary parasitoids were recorded: Aphidius colemani Viereck, Aphidius rhopalosiphi DeStefani Perez, Aphidius uzbekistanicus Luzhetzki, Aphidius ervi Haliday, Aphidius picipes (Nees), Ephedrus plagiator (Nees), Praon gallicum Stary Â, and Praon volucre (Haliday). The predominant parasitoid species were Aphidius colemani and Aphidius rhopalosiphi on Metopolophium dirhodum and Aphidius uzbekistanicus on Sitobion avenae. A low number of hyperparasitoids were also recorded. Aphidius colemani was recorded for the first time in the open winter wheat fields in Germany, although it has been used as a biocontrol agent in glasshouses in many European countries and overseas. An analysis of the aforementioned information shows that Aphidius colemani detected as a parasitoid of cereal aphids in Germany is likely a result of an accidental escape of parasitoids from a glasshouse, as well as their successful overwintering and establishment in the area. This study provides baseline information essential for assessing future changes in aphid parasitoid species guild and dynamics in cereal fields in Germany.U.S.
Investigations were carried out in spring barley, Hordeum vulgare L., fields to determine the abundance of the Russian wheat aphid, Diuraphis noxia (Kurdj.), and associated natural enemies and to evaluate the potential of these natural enemies as biocontrol agents. There was a significant difference in the density of D. noxia among the various sampling dates, the average for the season being 16.7 aphids tiller 1 • Surveys and field countings carried out over several seasons and localities indicated that the pest is highly abundant in the region. Among predators recorded, coccinellids and syrphids were found to be more abundant; however, the coccinellid beetle, Hippodamia variegata Goeze, was the dominant species of all predators. Moreover, syrphid fly larvae were the only predators consistently found feeding on aphids within D. noxia rolled up leaves. The quantitative analyses based on predator units (PU) also showed a highly significant difference among the predator groups, the average for the whole predator community being 17.4 PU m-2 . Among the parasitoids recorded, Aphidius colemani Vier. was the most abundant species; however, their diversity was very low. The rate of parasitism was also generally low, < 13%. In general, the population build up of these natural enemies is slow and maximum population size is reached after the aphid population peaks and the crop has already been damaged.
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