It mainly uses the Potter spray tower structure, and the air flow generated by the external air compressor passes through the nozzle, forming a pressure difference to attract the Chinese medicine liquid into the nozzle to atomize, and the atomized droplets are small and evenly distributed on the surface of the test insect body.Because the Potter spray tower has the advantage of small error in the test results (South China Agricultural University, 1983), many studies use the Potter spray method to conduct contact killing tests of pesticides on insects, including two-spotted spider mites (James and Price, 2002) and ladybugslarvae (Michaud, 2001), parasitic wasp (Brunner, et al, 2001), etc.However, the determination of the spray amount of Potter spray tower has always been based on experience and lacks theoretical basis.This will inevitably affect the exchange of technology promotion and research results.This article attempts to use Diamond Moth as the test insect and 2.5% deltamethrin emulsion as the test agent to conduct virulence bioassays, explore the impact of spray amount on insect virility determination, and find the best spray amount.

Test materials and methods
2.1 Test equipment Potter spray tower: as shown in Figure 1, produced by Burkard Manufacturing Co., Ltd., UK.Among them, the cylinder body is 68.5 cm long, the upper diameter of the cylinder is 15.5 cm, and the lower diameter is 11.5 cm.The maximum bearing pressure is 206.8kPa*, and the maximum capacity of the liquid liquid tube is 7mL.
2.2 Test agent 2.5% deltamethrin emulsion (provided by the Third Pesticide Factory of Wujin City, Jiangsu Province).
2.3 Test insects Diamond moth (Plutella xylostella L.), indoor feeding at the Biotechnology Center of the Pesticide Testing Institute of the Ministry of Agriculture, sensitive population, feeding temperature (26±1℃), light conditions L:D=14:10.During the experiment, 2nd instar larvae of the same size were selected.
2.4 Test method 2.5% deltamethrin emulsion was prepared into a liquid with active ingredients of 3.125, 6.25, 12.5, 25, 50 and 100 mg/L, respectively, with clean water as the control.The prepared medicinal solution was measured and sprayed with 0.5, 1, 2, 3, 4 and 5 mL per spray.The spray pressure is 68.9kPa*.After 24 hours, the number of dead insects was investigated, the mortality rate of insects was calculated, and LC50 was calculated using POLO software.

discuss
As an accurate spray device, Potter spray towers have a wide range of applications, including insecticides (Brunner, et al., 2001), fungicides (Michaud, 2001), and surfactants (Goodwin and McBrydie, 2000).However, the selection of spray volume often depends on experience or traditional laboratory practices, and lacks proof of data.From previous papers, spray volumes range from 1 mL to 5 mL.Among them, Michaud (2001) used the Potter spray tower to treat pesticide selectivity for parasitic natural enemies in a study on the effects of eight fungicides on two types of ladybug natural enemies.The spray volume of Potter tower was 2mL/time; Brunner et al. (2001) used 4mL/time Potter tower spray volume to treat parasitic natural enemies of Hymenoptera to observe their contact antivirus properties.Zhang Zepu (1993) recommends using a spray volume of 5mL.As we have demonstrated in the article, different spray amounts from 0.5 to 5 mL lead to obvious differences in the measured LC50 values, which will inevitably affect technical promotion and academic exchange.Potter spraying method, except for the droplets falling on the cylinder wall, only 15% to 20% of the dose usually settles on the surface of the Petri dish (Potter, 1952).In this experiment, when the spray amount was 0.5mL, the amount of medicine used to settle into the Petri dish and dip on the surface of the insect was too small, making it difficult to show a good contact killing effect; when the spray amount reached or exceeded 4mL, too muchThe spray amount accumulates the mist droplets into a stream, and the test insects immerse it, affecting the measurement results.The results of the experiment prove that the LC50 value in the spray range of 1~3mL is relatively stable, and the LC50 value changes in the stable trend.To sum up the above results, when spraying diamondback moth larvae, we recommend using a spray volume of 1~3mL.