Screening of Effective Insecticides Against Diaghania Pyloalis in Mulberry Fields

: In recent years, with the rapid development of the sericulture industry, the pests of mulberry trees are also increasing. Diaghania pyloalis Walker is one of the main pests of mulberry trees, has strong fecundity and very large food intake, especially in Sichuan and Chongqing areas. In order to find out the insecticides which can effectively control D. pyloalis , the screening test of high - efficiency control efficacy was carried out. In the summer of 2020, the 3rd instar larvae of D. pyloalis were selected as the test objects, and the indoor bioassay of 10 insecticides on D. pyloalis was carried out, and the results of the indoor bioassay were tested by field experiments in the autumn. The results show that 2000 times liquid of Dursban, Spinetoram, Chlorantraniliprole, and 1000 times liquid of Dichlorvos and Phoxim had the best control effect on D. pyloalis . Secondly, 2000 times liquid of Imidacloprid and Chlorfenapyr had the better control effect on D. pyloalis . Finally 2000 times liquid of Pymetrozine and Cyromazine had poor control effect, and 1000 times liquid of Azadirachtin had the worst control effect on D. pyloalis . To control D. pyloalis , 2000 dilution of Dursban and Spinetoram, and 1000 times liquid of Dichlorvos can be used in sericultural production. When using Chlorantraniliprole to control D. pyloalis , the leaves of mulberry should be picked with long interval to avoid affecting the quality of cocoons, and it is not recommended to use Cyromazine, Azadirachtin to control D. pyloalis .


INTRODUCTION
With the continuous development of silkworm industry, the planting area of mulberry trees also increases. According to the statistics of Chongqing Silk Net, the mulberry field area of Chongqing ranked fifth in China in 2019, Mulberry trees provide abundant food sources for insect pests such as D. Pyloalis, with the damage of pests, the quality and yield of mulberry leaves are greatly reduced, and the silkworm industry is also affected. (Cao, 2020) D. Pyloalis, is commonly known as mulberry caterpillar, oil worm, leaf roller, etc., belongs to Pyralidae in Lepidoptera, and is one of the main pests of mulberry trees. In addition to larvae eating mulberry leaves, Microsporidia infected by D. Pyloalis will also contaminate mulberry leaves through insect feces, Silkworms will get pebrine through eating mulberry leaves, pebrine influence seriously on the production of silkworm (Zhu, 2016). The mulberry moth has gradually become the primary pest in mulberry fields because of its wide spread, strong fecundity capacity, terrible diffusion ability (Yang, 2019). The pest occurred in sericultural areas except Xinjiang, especially in Jiangsu, Zhejiang, Guangdong, Anhui, Jiangxi, Sichuan, Hunan, Hubei, Guangxi and Chongqing (Zhao, 2013). For example, the damage rate of D. Pyloalis in Yizhou District of Guangxi was as high as 86% in 2019 (Wei, 2020), The study on the occurrence regularity and control method of D. Pyloalis has become an important subject in the mulberry protection field in China (Zhou, 2021). Therefore, this study is carried out screening of high-efficiency and low-toxicity insecticides for controlling D. pyloalis, providing reference for the future control and providing more scientific and reasonable measures for field prevention and control.

Experiment Materials
The experiment was aried out at the Ganning Base of the Chongqing Three Gorges Academy of Agricultural Sciences. The experimental site is 325 m above sea level. Flat terrain, uniform fertility and consistent field management were chosen for experiments. The test soil was sandy loam soil, with thickness equal to or higher than 1.0 m.
The tested mulberry varieties were HuSang and Da Shi. Insecticides were selected for testing listed in table 1.

2.2.1.Laboratory virulence test of Diaghania pyloalis
In the summer of 2020, the larvae of D. Pyloalis were reared with fresh mulberry leaves after hatching and collectively reared before the third instar. The third instar larvae were selected as the experimental objects to conduct the laboratory virulence test of 10 insecticides (see the top 10 insecticides in table 1) against D. Pyloalis. Five gradients were set for each insecticide, and water treatment was the blank control. The ripe mulberry leaves were soaked in the corresponding agent for 1 min, dried naturally, and placed in the crisper box. 30 mulberry moth larvae were fed with mulberry leaves in each treatmentarea, and fed continuously for 7 days. Each treatment was repeated 3 times. The number of poisoning deaths of D. Pyloalis was investigated daily.

2.2.2.Field control experiment of Diaghania pyloalis
In the autumn of 2020, 10 insecticides (see table 1) were selected to conduct field control experiments on D.
Pyloalis, but Bifenthrin and Thiamethoxam were not selected for the experiment because of their high toxicity to silkworm (Ren, 2020). The dilution times of Azadirachtin, Dichlorvos and Phoxim were 1000 times, and the dilution times of other insecticides were 2000 times. Water treatment was blank control, and each treatment was repeated 3 times and randomly arranged. Before the experimental control, 10 mulberry trees were randomly selected in each plot, and the number of larvae on the labeled mulberry branches was investigated. The number of remaining insects was investigated at 1, 2, 5 and 7 days after the treatment. The rate of pest decline in each pesticide treatment area was calculated and the control effect was corrected.

2.2.3.Data analysis
Data were processed and statistically analysed using IBM SPSS 16.0, and the method of analysis of variance for survey data was One-Way ANOVO, denoted as x, n=3.
The following is the calculation formula: Mortality rate (%) = the number of dead insects / total number of insects before application×100; Insects population decline rate (%) = (total number of insects before application -the number of live insects after application) / total number of insects before application×100; Field control effect (%) = (insects decline rate in treatment area -control area insects decline rate) / (100 -insects decline rate in control area) ×100.

Indoor toxicity of different insecticides against Diaghania pyloalis
The virulence test was carried out on D. pyloalis, the results showed that eight insecticides had the best control effect against D. pyloalis (shown in table 2). First of all, Bifenthrin, Thiamethoxam, and Dursban had the best control effect against D. pyloalis, and all the mulberry moth larvaedied after continuous feeding mulberry leaves for 24 hours. There was no significant difference in the control effect of the three treatments at different concentrations, which was significantly higher than that of other treatments. Secondly, Spinetoram, Chlorfenapyr, Imidacloprid, Chlorantraniliprole and Cyromazine had better control effect against D. pyloalis, and there were significant differences in the control effects at different concentrations. All the larvae of Chlorfenapyr and Spinetoram treatments died after continuous feeding mulberry leaves for 48 hours. All the larvae of Imidacloprid treatments at 1000~1500 times liquid died after continuous feeding mulberry leaves for48 hours, and all the larvae of Imidacloprid treatments at 2000~2500 times liquid died after continuous feeding mulberry leaves for 72 hours. All the larvae of Chlorantraniliprole treatments died after continuous feeding mulberry leaves for 72. All the larvae of Cyromazine at 1500~3000 times liquid died after feeding mulberry leaves for 96 hours. Finally, Pymetrozine and Azadirachtin had poor control effect against D. pyloalis, and Azadirachtin had the worst control effect, the larvae survived in the high concentration area which were continuously fed for 7 days.

Field protection effects of different insecticides against Diaghania pyloalis
In the autumn of 2020, Bifenthrin and Thiamethoxam were not selected because of their highly toxic to silkworms, 10 pesticides (shown in   (Chen, 2010), the insecticide residue in mulberry leaves for a long time when Chlorantraniliprole was sprayed on mulberry trees, and mulberry leaves used for rearing silkworms will do great harm to silkworms. Dursban is a commonly used insecticide in mulberry fields for pest control with low insecticide residue (Lan, 2021). The control effects of 2000 times solution of 25%Pymetrozine SA, 80% Cyromazine WDG, and 1000 times solution of 0.3% Azadirachtin Oil were poor, in particular, the control effect of 0.3% Azadirachtin Oil was the worst, because larvae still survived after feeding for 7 days in the high concentration area.

Conclusion
The 3rd instar larvae of D. pyloalis were selected as the experiment objects, and the indoor bioassay of 10 insecticides on D. pyloalis was carried out, and the results of the indoor bioassay were tested by field experiments. The results show that 2000 dilution of Dursban and Spinetoram, and 1000 times liquid of Dichlorvos can be used for D. pyloalis control in sericultural production. When using Chlorantraniliprole to control D. pyloalis, the leaves of mulberry should be picked with long interval to avoid affecting the quality of cocoons, and it is not recommended to use Cyromazine, Azadirachtin to control D. pyloalis. In addition, the integrated control technology such as agricultural control, biological control and physical control can be used in production to against D. pyloalis.

ACKNOWLEDGMENTS
These experiments were supported by technical system of modern high-efficiency agriculture (sericulture) in