The Asian citrus psyllid Diaphorina citri Kuwayama (Hemiptera: Liviidae) is the most widespread and devastating pest species in citrus orchards and is the natural vector of the phloem-limited bacterium that causes Huanglongbing (HLB) disease. Thus, reducing the population of D. citri is an important means to prevent the spread of HLB disease. Due to the long-term use of chemical control, biological control has become the most promising strategy. In this study, a novel highly pathogenic fungal strain was isolated from naturally infected cadavers of adult D. citri. The species was identified as Aspergillus fijiensis using morphological identification and phylogenetic analysis and assigned the strain name GDIZM-1. Tests to detect aflatoxin B₁ demonstrated that A. fijiensis GDIZM-1 is a non-aflatoxin B₁ producer. The pathogenicity of the strain against D. citri was determined under laboratory and greenhouse conditions. The results of the laboratory study indicated that nymphs from the 1st to 5th instar and adults of D. citri were infected by A. fijiensis GDIZM-1. The mortality of nymphs and adults of D. citri caused by infection with A. fijiensis increased with the concentration of the conidial suspension and exposure time, and the median lethal concentration (LC₅₀) and median lethal time (LT₅₀) values gradually decreased. The mortality of D. citri for all instars was higher than 70%, with high pathogenicity at the 7th day post treatment with 1 × 10⁸ conidia/mL. The results of the greenhouse pathogenicity tests showed that the survival of D. citri adults was 3.33% on the 14th day post-treatment with 1 × 10⁸ conidia/mL, which was significantly lower than that after treatment with the Metarhizium anisopliae GDIZMMa-3 strain and sterile water. The results of the present study revealed that the isolate of A. fijiensis GDIZM-1 was effective against D. citri and it provides a basis for the development of a new microbial pesticide against D. citri after validation of these results in the field.

Overusing chemicals to manage Diaphorina citri has created insecticide resistance and negative impacts on the natural ecosystem. This has prompted the need to develop new methods of control. In the present study, we evaluated the toxicity of plant spray oil, Metarhizium anisopliae, and their combined formulations against D. citri adults through laboratory bioassays. We tested varying concentrations and application doses of the individual agents and their mixtures to determine synergistic effects. The optimal mixing ratio of M. anisopliae and plant spray oil for enhanced pest control efficacy was also identified. The results showed that the control effect of plant spray oil and M. anisopliae on D. citri became more pronounced as the concentration increased. When the two were mixed in different proportions, the synergistic effect of plant spray oil was the strongest when the mixing ratio was 5:5. Field trials demonstrated that a combined treatment of plant spray oil emulsion (9.10 g/L) and M. anisopliae (1 × 10⁸ spores/mL) achieved over 80% relative control efficacy against adult D. citri by the 6th day post-treatment. Its efficacy further increased to 96.28% by the 8th day, demonstrating its potential to replace chemical control methods. This study provides a practical example for exploring biopesticides and leveraging the synergistic effects of biogenic pesticides in preventing and controlling pests.