Published online by Cambridge University Press: 05 August 2010
Introduction. The effects of imidacloprid and fenobucarb insecticides on thedynamics of the psyllid Diaphorina citri and on the incidence ofCandidatus Liberibacter asiaticus (Ca. L. a.), theputative causal agent of Huanglongbing disease (HLB), were studied in a field experiment.Materials and methods. The experimental design consisted ofthree independent 0.5-ha Citrus orchards planted with disease-freeHLB-susceptible orange trees, located in a Citrus producing areaseriously affected by HLB. Imidacloprid was applied monthly to the trunks in one orchardat a rate of 0.15 g a.i.·tree–1; fenobucarb was sprayed fortnightly in a secondorchard at a rate of 250 g a.i.·ha–1. The 3rd orchard was managed as a controlwithout insecticide applications. The total number of adult D. citrispecimens and the percentages of trees harbouring psyllid eggs and 5th instar nymphs weremonitored at fortnightly intervals in each orchard. Ca. L. a. incidencewas assessed in each orchard by PCR at 5 months, 12 months and 24 months after planting.Results. Compared with the control, both the fenobucarb and imidaclopridtreatments reduced adult psyllid populations by over 90% and reduced the frequency oftrees harbouring eggs and 5th instar nymphs. Only imidacloprid treatments totallyprevented development of a new generation of adults from eggs. Two years after planting,the prevalence of Ca. L. a. was 0.939, 0.745 and 0.239 in the control andin the orchards treated with fenobucarb and imidacloprid, respectively. Discussionand conclusion. The results indicated that, although both the insecticides usedeffectively reduced D. citri populations by killing adults and nymphs andby affecting or preventing psyllid reproduction in orchards, neither of the twoinsecticide treatments totally prevented transmission of Ca. L.asiaticus. However, due to its long-lasting effect and systemic activity, the imidaclopridtreatment provided the best protection against infections, and delayed and slowed down thespread of the pathogen. Furthermore, it reduced the number of pesticide applicationsneeded and left the way open for biological integrated pest management programmes.