© 1978 by Institute of Chartered Foresters
Distribution of Gilpinia hercyniae (Hymenoptera Diprionidae) Eggs within Sitka Spruce Trees
Forestry Commission, Forest Research Station Wrecclesham, Farnham, Surrey
The distribution of Gilpinia hercyniae (Hartig) eggs was determined in a total sample of 15,360 needles taken from 16, 16–17 year old Sitka spruce trees at Hafren Forest, Powys, Wales. The percentage of needles occupied by eggs was 1·09 per cent. The data indicated significant differences between egg populations in four crown levels and three foliage age classes (1–3 years), with a preference for eggs to be laid at the top of the crown in one year old needles. There was no significant relationship between any of four aspects and egg population. Calculation of the total egg population based on the number of needles in each age and crown level class (mean of 10-tree sample), and assigning probabilities that needles in a given class would be occupied, disclosed that 76 per cent of the population would be in one-year-old needles and 83 per cent would be in the two middle crown levels. Oviposition preference by females could be accounted for in part by positive phototaxis, avoidance of the darker basal and central regions of the tree, rejection of significantly smaller needles in the basal crown level, and by fairly immediate acceptance of peripheral foliage by an alighting female. It is concluded that an effective egg survey need only sample one-year-old needles in the top crown level.