Ineffectiveness of applying additional insecticide sprays at the border of the citrus block for the control of Huanglongbing.

Abstract

Huanglongbing (HLB), mainly associated with the bacterium 'Candidatus Liberibacter asiaticus', is a devastating disease that causes negative impacts on citrus yield and fruit quality. One of the main HLB control measures is based on preventive insecticide sprays, aiming to control its vector, the Asian citrus psyllid (ACP) Diaphorina citri. In blocks on the orchard's periphery, the ACP population is more concentrated in trees closer to the block's edge and gradually decreasing towards its interior. This spread pattern of ACP has motivated growers to perform, empirically based, additional insecticide applications at the border of the block ('AB'), as an additional measure to spraying insecticides in the entire block ('EB'). Hence, this work aimed (i) to evaluate the coverage and efficiency of AB, comparing tree canopy volume, and planting direction; and (ii) to compare ACP population and HLB incidence between blocks treated under EB, and blocks that received 'EB + AB'. Regardless of the operational situation, as the distance increased from the orchard's edge (0m) to the interior, spray coverage exponentially decayed from an average of 21% to less than 1% of coverage, and ACP mortality exponentially decayed from 83% to 7%. In general, the spray coverage and ACP mortality were suitable only for the tree canopy face directly sprayed at the orchard's edge (0m). After 5.5 years, a higher ACP population and number of symptomatic trees were detected in 'EB + AB'. As the 'EB + AB' had no effect in reducing ACP population, or HLB incidence, this additional spraying had no benefit in HLB management. © 2022 Society of Chemical Industry.