Submitted to: International Organization of Citrus Virologists Proceedings
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/11/2001
Publication Date: 2/2/2002
Citation: Gottwald, T., Abreu-Rodriguez, E., Yokomi, R., Stansly, P., Riley, T. 2002. Effect of Chemical of Aphid Vectors Control and Cross-Protection on the Increase and Spread of CTV. Proceedings of the 15th Conference of the International Organization of Citrus Virologists, p.117-130. Interpretive Summary: Citrus Tristeza Virus (CTV) is a serious disease of citrus with over 1.5 million acres of citrus in the United States and several million worldwide. Two control methods for CTV were examined in this series of experiments. First, the effect of chemical insecticides on CTV increase and spread was examined to determine if chemical control had an appreciable effect on the virus epidemic. Only a slight effect with one chemical was seen and the reduction was determined to not be economically feasible relative to the amount of control achieved. Second we examined the use a immunization, or cross-control, to limit infection. In this case trees were immunized with a mild strain of CTV to inhibit infection by a severe form of the virus. Very little effect was seen and thus this method was not feasible with the existing strains of the virus that existed in Puerto Rico.
Technical Abstract: The recent spread of the aphid vector Toxoptera citricida throughout Florida and the Caribbean has raised interest in the possibilities of reducing rate of spread of severe citrus tristeza virus (CTV) through insecticidal control and cross protection. In order to evaluate these practices, six, 210-tree plots were established at the University of Puerto Rico farm at Isabella, to study the effect of various chemical control strategies on the spread of Citrus tristeza virus (CTV) where the brown citrus aphid, Toxoptera citricida, and other aphid vectors are present. Plots were established with healthy nursery trees, with a line-source of CTV-infected trees at one end of each plot to serve as inoculum sources. Tree status was determined prior to planting and assayed annually throughout the experiment by DAS-I ELISA. Planting materials were obtained locally and CTV-infected trees were determined to be of two isolate serotypes (positive or negative for antibody MCA13. Plots were untreated or treated with either Acephate or Imidacloprid. Aphid populations were visually counted on each tree and yellow and green water traps were used to estimate flying aphid populations. CTV increase (the number of trees infected through time) and spread (the relative distances between infected trees and their spatial pattern) and aphid population dynamics were monitored for 5 years from 1994 to 1999. Results indicated that insecticidal control of aphid populations with either acephate or imidacloprid had little effort on CTV increase and spread. In addition, imidacloprid appeared to cause a secondary outbreak of citrus red mite, Panonychus citri (McGregor) and may also have exacerbated damage from the root weevil Diaprepes abbreviatus L. Acephate appeared to greatly reduce Diaprepes damage to roots and foliage, and thus promoted more vigorous trees that flushed more often. In addition, high winds associated with tropical storms occurred and plots next to a windbreak were also more vigorous. Faster-growing trees were more attractive to aphid vectors and resulted in a more rapid CTV increase in acephate-treated plots. Toxoptera citricida, Aphis gossypii, and other vector species were noted either in colonies on the trees or in water traps. Sufficient migratory aphids apparently visited each plot, and transmitted CTV regardless of chemical treatment. Although aphid colonization was suppressed to some extent by chemical treatments, feeding activity by migratory aphids even on chemically treated trees occurred prior to death, and seemed to be sufficient for CTV acquisition and transmission. Thus, in these experiments, there was little benefit of vector population suppression via chemical control on CTV increase.