Hatching from cysts and egg sacs of Heterodera cruciferae and effects of temperature on hatching and development on oilseed rape

Abstract

SUMMARYJuveniles hatched readily from field cysts and very readily from eggs in egg sacs of Heterodera cruciferae, when exposed to oilseed rape root diffusate. They hatched very poorly, however, from white or brown females from which the above egg sacs had been removed. Some hatching occurred at 8 oC but much more occurred at12, 16, 20 and 24 oC, with most at 16 oC.Development of juveniles in roots of oilseed rape occurred throughout the range 8–24 oC, and proceeded faster the higher the temperature. The basal development temperature was taken as 5 oC and the number of day degrees above this temperature required to reach each stage of development was calculated. From invasion of roots to the hatching of F1 juveniles required an average of 680 day degrees, but only 210 day degrees were required for the first appearance of egg sacs on adult females. On this basis, two consecutive generations of H, cruciferae would be possible on autumn‐sown oilseed rape in southern England, but the second would mature fully only after the crop was harvested. In Scotland, two consecutive generations could also occur but the second would be much less mature by harvest: only about 850 day degrees are available compared to almost 1100 in southern England. In practice, however, overlapping generations probably occur due to flushes of hatching of juveniles (i) at sowing, (ii) when soils warm up after winter and (iii) when the first generation completes its development.The proportion of eggs found in egg sacs was never more than 37% and some field cysts contained about 220 eggs; their egg sacs may, therefore, have contained as many as 150 eggs. Any study of population dynamics or damage assessment will require a quantification of the contribution of eggs in egg sacs to population density. Oilseed rape is direct drilled and may, therefore, be more sensitive to a given population density of the nematode than host crops which are transplanted.