Introduction. The effects of imidacloprid and fenobucarb insecticides on the
dynamics of the psyllid Diaphorina citri and on the incidence of
Candidatus Liberibacter asiaticus (Ca. L. a.), the
putative causal agent of Huanglongbing disease (HLB), were studied in a field experiment.
Materials and methods. The experimental design consisted of
three independent 0.5-ha Citrus orchards planted with disease-free
HLB-susceptible orange trees, located in a Citrus producing area
seriously affected by HLB. Imidacloprid was applied monthly to the trunks in one orchard
at a rate of 0.15 g a.i.·tree–1; fenobucarb was sprayed fortnightly in a second
orchard at a rate of 250 g a.i.·ha–1. The 3rd orchard was managed as a control
without insecticide applications. The total number of adult D. citri
specimens and the percentages of trees harbouring psyllid eggs and 5th instar nymphs were
monitored at fortnightly intervals in each orchard. Ca. L. a. incidence
was 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 imidacloprid
treatments reduced adult psyllid populations by over 90% and reduced the frequency of
trees harbouring eggs and 5th instar nymphs. Only imidacloprid treatments totally
prevented 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 and
in the orchards treated with fenobucarb and imidacloprid, respectively. Discussion
and conclusion. The results indicated that, although both the insecticides used
effectively reduced D. citri populations by killing adults and nymphs and
by affecting or preventing psyllid reproduction in orchards, neither of the two
insecticide treatments totally prevented transmission of Ca. L.
asiaticus. However, due to its long-lasting effect and systemic activity, the imidacloprid
treatment provided the best protection against infections, and delayed and slowed down the
spread of the pathogen. Furthermore, it reduced the number of pesticide applications
needed and left the way open for biological integrated pest management programmes.