PHOTOPERIODISM AND SEASONAL ABUNDANCE OF AN APHID, MACROSIPHUM EUPHORBIAE (THOMAS), IN OILSEED FLAX

AbstractThe potato aphid, Macrosiphum euphorbiae (Thomas) (Homoptera: Aphididae), is a host-alternating species and an important pest of Canadian flax. Populations of this aphid are highest in flax when the weather is warm and dry in July. Field-plot studies show that populations of the potato aphid increase rapidly in late July and early August and decline rapidly in mid-August. In some years potato aphids die because of a fungal epizootic or drought-induced senescence of the crop, but usually these factors do not account for the population decline. Field cage studies show that the potato aphid emigrates from the crop in mid-August. The photoperiodic response of the potato aphid and resulting emigration explain the population decline in flax, which occurs in mid-August each year regardless of the growth stage of the crop, population density, or average temperature at the time of the photoperiodic cue. Male and mating female potato aphids, born at the end of August, have time to develop, mate, and lay eggs before temperatures drop below the developmental threshold. The population decline assures that farmers need not sample or control the potato aphid in flax after mid-August.