INFLUENCE OF THE MEASUREMENT UNIT OF POPULATIONS IN DENSITY-DEPENDENTFECUNDITY STUDIES OF THE PREDATORY MITE AMBLYSEIUS ABERRANS (OUD) (ACARINA, PHYTOSEIIDAE)

A population of the predatory mite Amblyseius aberras (Oud.) (Acarina, Phytoseiidae) was reared on Vitis virzifera var. Perera plants in the absence of prey. During summer 1993 samples of leaves were collected on 29 different dates. The number of phytoseiids per leaf was positive ly correlated with the leaf area. The regression coefficient between t he variables is significantly different from zero. Nevertheless, when population density is indicated as mites/leaf, the error is too small to affect analysis of data. During the season the growth rate of popul ations fluctuated between -0.15 and +0.57 mites/mite x day. The ovipos itional rate assumed values between 0.005 and 0.17 eggs/female x day. The unit of measurement employed to represent the population density a ppeared to be relevant in some biological studies. The increase in the number of adults per cm(2) of leaf surface is correlated with a decre ase in the ratios juvenile/adults and eggs/adults suggesting the exist ence of a self-regulating mechanism of fecundity or survival in A. abe rrans.