Spray volumes and frequencies of insecticide applications for suppressing Diaphorina citri populations in orchards

Abstract

The Asian citrus psyllid, Diaphorina citri, is the most important and widespread vector of the Asiatic form of huanglongbing (HLB), the most serious citrus tree disease. In orchards, psyllids are commonly controlled with insecticides. The aims of this study were to assess the residual efficacy and coverage of insecticides applied at different spray volumes, the appropriate spray volumes for different application frequencies, and the cost/benefits. Three field experiments, based on the tree row volume (TRV) concept, were undertaken in Valencia sweet orange [Citrus × aurantium L. synonym of Citrus sinensis (L.) Osbeck] orchards. In the first experiment, we determined the mortality of a set of adult D. citri confined to branches with dry residues of dimethoate sprayed at 25 mL, 40 mL, and 70 mL/m3 of tree canopy. The mortality of adults confined with residues from days 0–7 exceeded 80% in all spray volumes tested. The spray coverage on water-sensitive papers ranged from 32 to 53%. Overall, the 25 mL and 40 mL of spray/m3 showed potential to reduce the water required for spraying and insecticide rates per hectare by up to 64% and management costs by 40%. In the second and third experiments, spray volumes evaluated in the first experiment were applied at three application frequencies at 7- (four sprayings: dimethoate, imidacloprid, bifenthrin, and dimethoate), 14- (two sprayings: dimethoate and imidacloprid), and 28-day (one spraying of dimethoate) intervals. The spray applications at 7-day intervals resulted in high D. citri mortality (≥75%), irrespective of the spray volume. The 40 mL/m3 and 70 mL/m3 spray volumes resulted in high psyllid mortality in 50% of the assessments when applied at 14-day spray intervals, and low psyllid mortality (<40%) in 75% of the assessments at 28-day spray intervals. These results suggested that the 28-day spray program is not suitable for limiting the primary and, quite possibly, the secondary spread of HLB. Overall, the results of the present study could form the basis of decision-making by citrus growers in Brazil, and elsewhere, regarding seasonal spray volumes and frequencies suitable for suppressing D. citri populations.