Studies on the ecology and disease transmission of the psyllid vectors of the citrus greening disease, with special reference to the South African vector, Trioza erytreae [Del Guercio] [Homoptera : Psyllidae]

Abstract

A comparatively new virus-like disease called greening has become a very serious threat in many citrus producing countries of the world. The disease is transmitted by two insect vectors belonging to the Psyllidae (Homoptera) namely, Trioza erytreae (Del Geurcio), the African vector, and Diaphorina citri Kuw., the Oriental vector. Field studies were made on the ecology, biology and control of T. erytreae in the northern Transvaal and in Swaziland during 1965 - 1970. This vector is extremely fecund but has weak powers of dispersal. The main ecological factors found to regulate populations of this insect were the flushing rhythm and flush quality of citrus, extremes of weather, and natural enemies. Other factors involved included interspecific competition with citrus aphids, and in some seasons intraspecific competition for breeding sites. Fundamentally flushing rhythm determines the potential population density of the vector, while the occurrence and sequence of lethal weather extremes mainly regulate the population size during citrus growth periods. A weather index was found to explain the pest status of the vector in southern Africa and partly accounts for previous outbreaks of vector and disease. A population model is given which explains the seasonal abundance of the insect. Studies were also made on the general biology including sex ratio, egg laying, mating behaviour, and the influence of temperature on the duration of the immature stages. The greening disease represents a severe limiting factor to citrus production in the high lying regions of the eastern Transvaal and Swaziland. The hotter low veld areas are virtually free of disease symptoms and vector populations though present are usually low. Transmission studies showed that the T. erytreae is the principal vector, several other psyllid species feeding on citrus were not found to be transmitters. It appears that a fairly small proportion of field adults are infective and that there is a seasonal fluctuation in transmission efficiency. Single males and females were able to transmit the disease. Unsuccessful attempts were made to screen adults for infectivity using a chromatographic method, and to isolate the causal organism from excised salivary glands. The control of T. erytreae depends an the use of insecticides. Experiments to select a suitable material and to determine it’s correct time of application are described. A spray programme for the control of citrus psylla has been used with apparent success by farmers in the Malkerns district of Swaziland. Comparative field observations and surveys for both Asian greening disease and the Oriental vector are described, and the world distribution of greening and of the two psyllid vectors are given.