Behavioral Responses and Contact Toxicity of Australian Tea Tree Oil and Its Major Constituents Against the Asian Citrus Psyllid, Diaphorina citri Kuwayama

The Asian citrus psyllid Diaphorina citri Kuwayama (Hemiptera: Liviidae) is the vector of the devastating citrus disease Huanglongbing, posing a significant threat to the global citrus industry and necessitating environmentally sound management strategies. This study aimed to evaluate Australian tea tree oil (TTO) and its primary constituents as potential botanical insecticides. Gas chromatography-mass spectrometry (GC-MS) was performed to analyze the chemical profile of commercial TTO, and behavioral effects on D. citri adults were assessed using a Y-tube olfactometer. Direct spray bioassays were conducted to determine contact toxicity. A total of 12 compounds were identified, with TTO being a Terpinen-4-ol chemotype, dominated by Terpinen-4-ol (40.62%), γ-Terpinene (21.46%), and α-Terpinene (10.45%). TTO demonstrated potent, concentration-dependent repellency, achieving 100% repellency at 10 g/L. In contrast, Terpinen-4-ol alone was attractive to psyllids at low concentrations, suggesting synergistic or masking effects within the complex oil blend. TTO and its major constituents also exhibited significant dose- and time-dependent contact toxicity. Although the 72 h LC50 of TTO (19.18 g/L) indicates lower potency compared to conventional insecticides (0.59–1.23 g/L), its combined repellent and toxic properties make it a promising candidate for integrated pest management (IPM) programs aimed at controlling D. citri and mitigating insecticide resistance.