| Summary: | <i>Candidatus</i> Liberibacter asiaticus (<i>C</i>Las) is a phloem-limited bacterium that is associated with the Huanglongbing (HLB) disease of citrus and transmitted by the psyllid, <i>Diaphorina citri</i>. There are no curative methods to control HLB and the prevention of new infections is essential for HLB management. Therefore, the objective of our study was to determine the effects of systemic insecticides, such as the neonicotinoids imidacloprid, thiamethoxam, and a mixture of thiamethoxam and chlorantraniliprole (diamide) on the probing behavior of <i>C</i>Las-infected <i>D. citri</i> and their effect on <i>C</i>Las transmission. The electrical penetration graph (EPG-DC) technique was used to monitor the stylet penetration activities of <i>C</i>Las-infected <i>D. citri</i> on sweet orange [<i>Citrus sinensis</i> (L.) Osbeck] ‘Valencia’ treated with systemic insecticides. Systemic insecticides disrupted the probing behavior of <i>C</i>Las-infected <i>D. citri</i>, in a way that affected <i>C</i>Las transmission efficiency, particularly by negatively affecting the stylet activities related to the phloem phase. All insecticides reduced (by 57–73%) the proportion of psyllids that exhibited sustainable phloem ingestion (waveform E2 > 10 min), with significant differences observed on plants treated with thiamethoxam and thiamethoxam + chlorantraniliprole. The transmission rate of <i>C</i>Las with high inoculum pressure (five <i>C</i>Las-infected <i>D. citri</i> per plant and a seven-day inoculation access period) to untreated control plants was 93%. In contrast, <i>C</i>Las transmission was reduced to 38.8% when test plants were protected by systemic insecticides. Our results indicated that all insecticides tested presented a potential to reduce <i>C</i>Las inoculation by an average of 59%; therefore, these insecticides can be used to reduce the spread of HLB.
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