Preliminary analysis of a field population of citrus red scale, Aonidiella aurantii (Maskell), and the measurement and expression of stage duration and reproduction for life tables

Abstract

AbstractA population of Aonidiella aurantii (Mask.) was studied on 72 Clanor orange trees in an orchard of 1800 15-year-old trees growing at an altitude of 280 m at Tshaneni, Swaziland. This orchard had not been sprayed with toxic chemicals for 18 months before observations started. Trees were banded with ‘Formex’ compound to control ants during the observation period which lasted from February 1972 to June 1975, and during which only nutritional and fungicide sprays were applied. Samples taken at 2–3 week intervals, measured three parameters: live adult female scale intensity on leaves, twigs and fruit; predation and parasitism levels; and adult parasite and predator intensities. A logarithmic transformation and 90% confidence limits showed that short-term fluctuations of adult female scales within each season were real, not due to sampling variation, and represented generations. These generations, more or less discrete in spring, became increasingly overlapped as the season advanced. Suction machine samples showed that the endoparasites Compendia bifasciata How. and Habrolepis rouxi Comp., were closely synchronised with these generations but were not otherwise density dependent. The hyperparasite Marietta javensis (How.) was more closely correlated with C. bifasciata than with H. rouxi. Aphytis spp. and total predator numbers apparently responded in a density dependent manner to seasonal scale increases. Predation predominated on twigs, Aphytis parasitism on leaves and fruit, while endoparasitism passively followed the adult female scale peaks, firstly on leaves and twigs, later in the season on fruit.A method for making life tables is proposed, based upon the expression of stage duration and reproduction in terms of a physiological time-scale. For this purpose, stage duration and reproduction were measured on orange plants at ambient temperature in an insectary over one year. A development threshold of 12°C was established and stage duration expressed in hour-degrees (h°) above 12°C. The greatest factor affecting daily crawler production per female was reproductive age. Accordingly, based upon the per cent contribution of each reproductive age group to the rate of increase, r, only the first 35 days of reproduction were considered, and the mean crawlers/female/day was plotted against over this period. Development and reproduction were fastest on fruit, slowest on leaves, and because the population behaved so differently on leaves, twigs and fruit, these sites should be analysed separately. Migration experiments showed negligible net exchange of crawlers between sites.