Effect of contact insecticides on the transmission of ‘ <i>Candidatus</i> Liberibacter asiaticus’ by <i>Diaphorina citri</i> during sweet orange shoot development

Insecticide applications are commonly recommended for managing <i>Diaphorina citri</i>, the vector of huanglongbing (HLB), but their effectiveness in reducing transmission of '<i>Candidatus</i> Liberibacter asiaticus' (CLas), especially during continuous psyllid influx and shoot growth, remains unclear. This study evaluated the efficacy of foliar application of thiamethoxam and spinetoram in reducing CLas transmission in sweet orange seedlings. Two experiments were conducted up to 13th days after first spray. In experiment #1, CLas-positive psyllids were confined in seedlings every two days. Treatments were: control; S14 - one spray at day 0; and S7 - sprays at days 0 and 7. In experiment #2, psyllids were released for free choice within a screenhouse every two days. Treatments were: control; S14; S7; and S3 - sprays at day 0 and every 3 days. Sprays on day 0 were over unfolded leaves (V2 stage). Psyllid mortality was assessed in experiment #1. Psyllid occupancy (percentage of seedlings with at least one psyllid) and abundance (number of psyllids per seedling) were assessed in experiment #2. After 6 months, seedlings were tested for CLas by qPCR. In experiment #1, cumulative psyllid mortality was higher in sprayed treatments than control for both insecticides, but HLB incidence was similar. In experiment #2, both psyllid occupancy and abundance decreased with shorter spray intervals, with the lowest HLB incidence in S3. These findings suggest that reducing spray intervals can limit CLas transmission during periods of shoot growth and continuous vector pressure. However, CLas transmission was not entirely prevented, highlighting the need for integrated, area-wide management strategies.