Wandering Time of the Crawlers of California Red Scale, Aonidiella Aurantii (Mask.) (Homoptera: Diaspididae), on Citrus.

Abstract

The wandering time of the crawlers of California red scale was studied in the laboratory using leaf discs, and by a sampling experiment in the field on lemon trees. A comparison of wandering times on fourteen varieties and species of Citrus showed that crawlers wandered longer on leaves than on fruit of the same species. Wandering times on different Citrus species ranged from 115.4 min on navel orange to 158.2 min on citrange. Wandering time was shown to be affected by the colour of the leaf, with the shortest time on dark green leaf discs and the longest on yellow ones. Crawlers at high densities wandered longer than those at low densities, and there was a linear relationship between density of active crawlers and wandering time. Presence of established scales on the leaf discs increased the wandering time. These features probably increased the movement of crawlers to uninfested parts of individual trees. An experiment combining both temperature and light showed that light had no significant effect on wandering time over the range 100-500 lm/ft2; but increasing temperature, over the range 15-30C, decreased wandering time, in a linear relationship. Additional tests suggested that this relationship did not hold at either 10 or 40C. Wandering time was not affected by wind velocity over the range 0-4 m/sec; it was shorter on dusty than on clean leaves, and did not differ significantly between males and females. Estimates of wandering time in the field were in the range 174-206 min with a mean of 186 min. By comparison, the range in the laboratory experiments was 98-243 min. The distribution of times of emergence and settling ensures that most wandering occurs in the late morning when wind velocities are most favourable for dispersal.