Weather-induced mortality of the citrus psylla, trioza erytreae (del guercio) (homoptera: psyllidae), a vector of greening virus, in some citrus producing areas of Southern Africa

Abstract

A saturation deficit index (S.D.I.) is defined for the purpose of assessing the regional weather-induced mortality of T. erytreae in southern Africa. The S.D.I. is based on a regression curve which reliably depicts the combined effects of temperature and humidity on the mortality of eggs and first instar nymphs in the field. Series of S.D.I. values are computed for eight representative weather stations, for periods ranging from three to twelve years. Consideration of the accumulated effect, the duration and also the seasonal distribution of near lethal S.D.I. values, clearly explains the known geographic distribution of T. erytreae. Despite some incompleteness of data, 30-year series of annual accumulated values of the S.D.I. in a defined lethal range are estimated for two areas. These values throw light on past outbreaks of the insect itself and of greening, the virus disease transmitted by T. erytreae.