| Summary: | Various biotic and abiotic factors prevented aphids on grain crops from reaching their potential on fields under observation in Manitoba in 1968 and 1969. These included wind, rain, predators, parasites and fungus disease. In some cases wind or rain or a combination of wind and rain with other factors caused population decline before plants ripened. Laboratory studies on the effectiveness of the common lady beetle species found preying on aphids on grain crops in Manitoba revealed that Hippodamia tredecimpunctata tibialis and H. convergens were effective aphid predators. They had a high voracity and fecundity and they developed on all the five main aphid species studied. In field collections, the former made up 52% in 1968 and 62.4% in 1969 of all lady beetle species found, and the latter constituted 43.8% and 14.62%. Surveys of aphid predators and parasites in the field revealed that Coccinellids were the most abundant aphid predaors fo11owed by Syrphids and Chrysopids. The most commonly found primary parasites of aphids on grain crops were Aphelinus mali, Aphidius avenaphis and Lysiphlebus testaceipes. The effectiveness of primary parasites was reduced by the hyperparasites Asaphes fletcheri, Pachyneuron siphonophorae, Charips sp., Lygocerus sp., and Alloxysta sp.
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