Susceptible baseline date and establishment of diagnostic doses of insecticides for detecting resistance in Apolygus lucorum Meyer-Dür

Abstract:【Objective】The mired bug, Apolygus lucorum Meyer-Dür, has gradually become an impor- tant fruit tree pest in Yellow River region of China in recently year. For apple tree, both nymphs and adults suck plant juices through their needle-like mouthparts, and their feeding can induce the stunting of plant young tissue and the abscission of flower buds even young fruits, finally leading to serious yield and quality losses. Currently, management of A. lucorum relies exclusively on chemical insecti- cides, including pyrethroids, organophosphates, and neonicotinoids. Although some of these insecti- cides have been used for a long time and insect susceptibilities are declining in cotton filed, they are still effective in the field for the control of A. lucorum. However, continues and dominant use of chemi- cal sprays will facilitated pesticide resistance in this pest. Therefore, it is urgent and essential to esti- mate insecticide resistance in the mired bug in orchards for proper choice of insecticides. Developing tools for insecticide resistance detection and monitoring is a key component of resistance management. To make clear the sensitivity difference of A. lucorum to frequently-used pesticides in orchard and estab- lish the susceptible baseline date and the diagnostic doses of insecticides for detecting resistance, a se- ries of toxicity of nine insecticides to the susceptible strain of A. lucorum were conducted in this study. 【Methods】The susceptibilities of third instar nymph of A. lucorum laboratory strain were tested by the glass-vial method bioassay. Glass-vial bioassays were conducted in 5-mL glass vials with an internal surface area of 8 cm2. The stock solutions (2 000 g · L- 1) of all insecticides tested were prepared in ace- tone. Then, 5 concentrations of the working solutions were prepared by diluting the stock solution in 0.01% Triton X-100 in acetone. 50 mL working solution was added to the vials that were then manually rolled horizontally until the acetone had completely evaporated. Control vials were treated with acetone and 0.01% Triton X-100. Insecticide was applied to the glass-vials’inner surface assuming that bugs would be exposed to insecticide through direct contact with the treated glass surface. 10 third instar nymphs were transferred onto each vial for each replicate. There were three replicates for each concen- tration of insecticide. Each treatment concentration was replicated three times for a total 90 insects per concentration. Then the glass-vials were plugged with a breathable cap and held vertically in the incuba- tor (light at 25 °C). Mortality was evaluated after treatment 3 h. Replicates with control mortality >10% were excluded from the analysis. Insects were considered to be dead if they were unable to walk or if they could not move when prodded. The LC50 and LC99 values were calculated by Probit analyses using PoloPlus software. The LC50 value was used as the susceptible baseline date. The 2-fold LC99 value was used as the diagnostic dose date.【Results】The mortality dates were revealed via glass-vial method bio- assay are showed in Table 1. The toxicity data of nine insecticides toxicity to A. lucorum from high to low in turn is: Bifenthrin (median lethal concentration, LC50=0.79 mg·L-1), Chlorpyrifos (LC50=3.96 mg·L-1), Lambda- cyhalothrin (LC50=6.25 mg · L- 1), Thiamethoxam (LC50=10.47 mg · L- 1), Fenpropathrin (LC50= 10.91 mg·L-1), Malathion (LC50=29.42 mg·L-1), Acetamiprid (LC50=36.26 mg·L-1), Imidacloprid (LC50= 78.29 mg · L-1) and Sulfoxaflor (LC50=160.89 mg · L-1). Sulfoxaflor showed the lowest toxicity to A. luco- rum while the highest toxicity to A. lucorum is bifenthrin, with a toxicity ratio 204.15. This result indi- cates that bifenthrin could be an effective alternative insecticide for the management of A. lucorum. The susceptible baselines of the eight insecticides to A. lucorum were established based on the LC50 values. The susceptible baseline data of A. lucorum to eight pesticides with the glass-vial bioassay was listed as follow: bifenthrin (4.93 ng·cm-2), chlorpyrifos (24.75 ng·cm-2), lambda-cyhalothrin (39.06 ng·cm-2), thi- amethoxam (65.44 ng · cm- 2), fenpropathrin (68.19 ng · cm- 2), malathion (183.88 ng · cm- 2), acetamiprid (226.63 ng · cm-2) and imidacloprid (489.31 ng · cm-2). The diagnostic doses of the eight insecticides to A. lucorum were established based on the 2-fold LC99 value. The corresponding diagnostic doses of the eight pesticides with the glass-vial bioassay were listed as follows: bifenthrin (19.95 ng·cm-2), chlorpyri- fos (153.91 ng·cm-2), lambda-cyhalothrin (558.95 ng·cm-2), fenpropathrin (588.53 ng·cm-2), malathion (731.76 ng·cm-2), thiamethoxam (1 162.18 ng·cm-2), acetamiprid (2 827.65 ng·cm-2) and imidacloprid (9 167.81 ng·cm-2).【Conclusion】The availability of diagnostic methods that are both accurate and prac- tical is crucial in the development of pesticide resistance monitoring and management programs. The diagnostic doses of nine pesticides established in this study for A. lucorum can be used as the reference for monitoring insecticide resistance.