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Settling‐site selection and survival of two scale insects, Ceroplastes rubens and C. ceriferus , on citrus trees
Author(s) -
Itioka Takao,
Inoue Tamiji
Publication year - 1991
Publication title -
population ecology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.819
H-Index - 59
eISSN - 1438-390X
pISSN - 1438-3896
DOI - 10.1007/bf02514575
Subject(s) - biology , nymph , settling , botany , horticulture , predation , zoology , ecology , environmental engineering , engineering
Summary We studied settling‐site selection and the resulting survival of two sessile scale insects, Ceroplastes rubens and C. ceriferus , in the citrus tree, Citrus unshiu , in central Japan. C. rubens preferred 0‐year‐old twigs most as a settling‐site; the density of nymphs settling on 0‐year‐old twigs was significantly higher than 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 higher on more preferred twigs than on less preferred ones. In C. ceriferus , nymphs significantly preferred 1‐ and 2‐year‐old twigs to 0‐ 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. However, the survival rate on less preferred 0‐year‐old twigs was slightly higher than those on 1‐ and 2‐year‐old ones. Thus, in both species of scale, it was the preferred twigs which were more profitable sites for survival after settling, except for less preferred 0‐year‐old twigs for C. ceriferus . In both scale 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 of the two scales clearly preferred more profitable sites, their settling‐site selection did not agree with the predictions from the ideal free distribution theory (Fretwell and Lucas, 1970). The discrepancies were (1) frequent settling on less profitable sites at the early stage of the settling season, (2) insufficient utilization of the most profitable twigs, and (3) virtually 100% mortality on overcrowded twigs under conditions where unoccupied profitable twigs still remained. These discrepancies are thought due to the limited dispersal time of nymphs.