Efficacy of Trunk Injected Imidacloprid and Oxytetracycline in Managing Huanglongbing and Asian Citrus Psyllid in Infected Sweet Orange (Citrus sinensis) Trees

Abstract

Traditional foliar spray and soil drench applications of crop protection compounds have been ineffective at managing huanglongbing (HLB) in citrus. Trunk injection is a technique that delivers crop protection compounds directly into the tree vasculature, which optimizes compound availability while minimizing drift, runoff, and damage to non-target organisms. Five-year-old HLB-affected ‘Valencia’ sweet orange (Citrus sinensis) trees were injected with the insecticide imidacloprid or the antibacterial oxytetracycline in October 2020 and April 2021. Trees were monitored for tree health, psyllid colonization, bacterial titers, fruit quality, fruit drop, and yield for two production seasons. Imidacloprid injection caused 63% mortality of psyllid adults within one week and reduced progeny survival by 80%, though the efficacy waned within two months. Injection with oxytetracycline significantly improved tree health, reduced bacterial titers, and reduced preharvest fruit drop by more than 3-fold with corresponding increases in yield. Residue dynamics varied by injected compound and tissue analyzed. These results suggest trunk injection could be an effective delivery method for existing or novel therapeutics targeting either the insect vector or the pathogen causing HLB.

UV‐LASER adjuvant–surfactant‐facilitated delivery of mobile dsRNA to tomato plant vasculature and evidence of biological activity by gene knockdown in the potato psyllid

“…Foliar sprays are generally preferred for applying agrichemicals such as foliar nutrients and pesticides; however, efficient foliar penetration and entry of these compounds into the plant vasculature by foliar spraying is highly inefficient because the waxy cuticular surface of leaves serves as a natural barrier to the entry of foreign molecules. 13,71,72 Nanoparticles of ∼50 nm 73 as well as bio-recognition motifs of peptides have been shown to facilitate foliar delivery of smallmolecule agrochemical cargo. 74 Alternative approaches explored have been abrasion, high-pressure celite and alumina sprays, and biolistic or gene-gun delivery that requires pressures as great as 8 bar applied at a distance of 4 cm from the plant surface.…”

“…75 Regardless of the approach, delivery of compounds or molecules of interest to the plant vascular system without resulting in wilting, triggering plant defenses, or facilitating entry of pathogens or potentially toxic substances, requires temporary disruption of the cuticular layer of leaves or stems. 76 Diffusion of compounds through the leaf cuticle has been identified as the rate-limiting step in foliar delivery 13,71,72 of dsRNA molecules capable of triggering RNAi in insects. 13,73,74 To overcome this problem, laser-light technology has been shown to aid foliar penetration and phloem delivery of small fluorophores to citrus leaves.…”

“…This approach offers an advantage over invasive trunk injections that can increase the plant's vulnerability to biotic and abiotic stress. 71 Optimal UV-LASER parameters were established experimentally by inflicting minute wounds over an area of 0.6 mm 2 on the midvein of a tomato plant leaflet (Table 1; Set 5). Different FAM-labeled dsRNAs of 100, 300 and 600 bp were delivered to the tomato plant abaxial phloem cells, xylem vessels, and phloem sieve tubes depending on the UV-LASER adjuvant/surfactant regimes.…”

“…This requires the delivery of pesticides to the plant phloem 43,70 where psyllids feed. Foliar sprays are generally preferred for applying agrichemicals such as foliar nutrients and pesticides; however, efficient foliar penetration and entry of these compounds into the plant vasculature by foliar spraying is highly inefficient because the waxy cuticular surface of leaves serves as a natural barrier to the entry of foreign molecules 13,71,72 …”

“…Diffusion of compounds through the leaf cuticle has been identified as the rate‐limiting step in foliar delivery 13,71,72 of dsRNA molecules capable of triggering RNAi in insects 13,73,74 . To overcome this problem, laser‐light technology has been shown to aid foliar penetration and phloem delivery of small fluorophores to citrus leaves 13 .…”