Population structure and occurrence of Bactrocera dorsalis in peach orchards

Abstract:【Objective】The oriental fruit fly, Bactrocera dorsalis Hendel is one kind of serious fruit-bor-ing pests, which makes the damaged fruit yellow before ripening, causes the fruit to rot and drop, and seriously affects the yield and quality of fruit. In recent years, it has occurred in the peach orchards and has a tendency to be aggravated, but we still do not know its occurrence trend in peach orchards in vari-ous regions of China. In China, peach is the third largest deciduous fruit tree after apple and pear, and both the cultivation area and yield of peach in China rank the first in the world. The present experiment aimed to clarify the occurrence of B. dorsalis populations in different peach producing areas, and to pro-vide a reference for formulating effective early warning and comprehensive prevention measures. It is of great significance to ensuring the quality-safety production of peach.【Methods】We selected 17 test sites in the main peach producing areas of China, which covered various types of peach production ar-eas, such as Around Bohai sea (Dalian, Liaoning province; Changli, Hebei province; Qingdao, Shan-dong province), Northern area (Beijing; Shijiazhuang, Hebei province; Tai’an, Shandong province), Yel-low River Basin (Zhengzhou, Hennan province; Xi’an, Shanxi province; Lanzhou, Gansu province), Yangtze River Basin (Nanjing, Jiangsu province; Hangzhou, Zhejiang province; Wuhan city, Hubei province), Southwest area (Chengdu, Sichuang province; Kunming, Yunnan province; Guizhou prov-ince) and Southern area (Fuzhou, Fujian province; Guilin, Guangxi province). From 2012 to 2019, the occurrence of B. dorsalis in peach orchards was continuously monitored and investigated by using sex attractant traps (17 test sites were uniformly supplied, and the traps were bought from the market). Five  traps were hung in each peach orchard, and the distance between the traps was at least 20 m. After B.dorsalis was found, we investigated and recorded the number every 5-7 days. Then the samples were brought back to the laboratory for morphological and molecular identification, and the phylogenetic re-lationship analysis was performed on samples from different regions.【Results】Firstly, the samples col-lected from each test site were identified. The results of morphological examination showed that the samples from Beijing and Wuhan were quite different from the type of specimens, but the molecular results showed that the samples were B. dorsalis. Based on the morphological and molecular biological identification results, the B. dorsalis from different producing areas were identified as B. dorsalis com-plex. It was confirmed that the insect sample from Beijing, Nanjing, Wuhan, Shijiazhuang and Chengdu were B. dorsalis, and there was maximum genetic distance between Shijiazhuang population and other four populations; the insect sample from Zhengzhou was a mixed population of B. dorsalis and Bactrocera correcta Bezzi. Secondly, according to the trapping results for 8 consecutive years (2012 to 2019),almost all of the main peach producing areas were harmed by B. dorsalis. Especially in the production areas of the Yangtze River basin such as Nanjing, Hangzhou and Wuhan, the occurrence of B. dorsalis continued to increase since 2017. Southwest production areas such as Chengdu, Kunming and Guizhou,and southern production areas such as Fuzhou and Guilin showed the same trend as or even more seri- ous than the Yangtze River basin.【Conclusion】The morphological changes of B. dorsalis and its relat-ed species were complex, and there were many geographical species. We found B. dorsalis complex in the samples from different regions. In addition, the samples were also mixed with other fruit flies, like B. correcta. Therefore, it was necessary to strengthen the detection of fruit flies in peach orchards to confirm the proportion of B. dorsalis, which was helpful for pest control. Since 2012, the damage of B. dorsalis has been found in northern peach orchards, however, the occurrence period and population number vary from year to year, which mainly harms late-ripening peaches. As we know, many studies have shown that the overwintering northern boundary of B. dorsalis is at (30±2)°N, and areas to the north beyond cannot survive in the winter normally. So where does the initial source of the B. dorsalis that harm northern peach orchards come from? The specific reasons for the serious occurrence of B. dorsalis in peach orchards in Henan and Hebei provinces need to be further studied; in southern peach or-chards, B. dorsalis is a common pest, which is more harmful to mid- and late-ripening peaches. B. dorsalis has gradually become a common pest in peach orchards, especially in recent three years, it has caused serious damage in most of the main peach production areas. With the increasing frequency and complexity of international trade activities, the probability of fruit fly pests spreading is increasing. At the same time, B. dorsalis has been removed from the quarantine objects in the provincial quarantine of fruits and vegetables in China. These changes will increase the threat of fruit flies to ensuring the quali-ty-safety production of fruits and vegetables in northern China.