Settling-site selection and survival of two scale insects, Ceroplastes rubens and C. ceriferus, on citrus trees.
Settling-site selection and the resulting survival of the coccids Ceroplastes rubens and C. ceriferus were studied on satsuma trees in Honshu, central Japan, in 1987-88. C. rubens showed most preference for new (current year) twigs as a settling-site: the density of nymphs settling on new twigs was significantly greater than that of those on ≥1-year-old twigs, and few nymphs settled on ≥3-year-old twigs. The mean survival rates from settling until reproduction in the next year were significantly greater on more preferred twigs than on less preferred ones. In C. ceriferus, nymphs significantly preferred 1- and 2-year-old twigs to new and ≥3-year-old ones, and the mean survival rates on the more preferred 1- and 2-year-old twigs were significantly higher than those on less preferred ≥3-year-old twigs. The survival rate on less preferred new twigs, however, was slightly greater than those on 1- and 2-year-old ones. Thus, in both species, it was the preferred twigs which were more profitable sites for survival after settling, except for less preferred new twigs for C. ceriferus. In both species, most mortality was due to growth cessation, which is believed to be related to the twig quality as a food source. Predators and parasitoids were minor mortality factors. Both species showed constant survival rates until the density of settled nymphs exceeded double the "upper-limit" density, whereupon they decreased drastically. Nymphs of C. rubens settling on twigs of high scale density showed a spacing-out distribution, those of C. ceriferus did not. In C. rubens, an increase in preference for originally less profitable twigs at the later stage of the settling season was observed, but not in C. ceriferus. Accordingly, individuals of C. rubens showed a stronger tendency to avoid conspecifics than did C. ceriferus. Although nymphs clearly preferred more profitable sites, their settling-site selection did not agree with the predictions from the ideal free distribution theory. The discrepancies were frequent settling on less profitable sites at the early stage of the settling season, insufficient utilization of the most profitable twigs, and virtually 100% mortality on overcrowded twigs under conditions where unoccupied twigs still remained. These discrepancies are thought to be due to the limited dispersal time of nymphs.