A Preliminary Study on the Regularity of Outbreak of Thrips in Asparagus-bean Interplanting Fields

Yanpo CAO Xiaolei CAO Yu TIAN Yingtao SUN Huiling YAN Yajing ZHU Huimin GAO

Abstract [Objectives] This study was conducted to clarify the composition and occurrence regularity of thrips in asparagus-soybean intercropping fields.

[Methods] The species and occurrence dynamics of thrips in asparagus-bean intercropping fields in Luquan District of Shijiazhuang were investigated.

[Results] There were 7 species of thrips in this area， namely， tobacco thrips [Thrips alliorum （Priesner）]， western flower thrips [Frankliniella occidentalis （Pergande）]， Scolothrips takahashii Priesner， Megalurothrips usitatus （Bagrall）， Sussericothrips melilotus Han and Aeolothrips fasciatus. As the dominant species， tobacco thrips had two peak outbreaks in late June and late September， and maintained a high quantity level for a long time. Other thrips had fewer population and different occurrence time， but were greatly affected by temperature， humidity and host crop species.

[Conclusions] This study provides a theoretical basis for the forecast and comprehensive control of thrips in North China.

Key words Thrips; Tobacco thrips; Dynamic

Thrips belong to Thysanoptera of Thripidae. At present， there are more than 7 700 thrips species described in the world. The investigation and research of thrips in China started late， and only more than 570 species were recorded[1-2]. In recent years， thrips have gradually increased from secondary pests to major pests that harm many economic crops. Thrips have been reported to cause serious damage in North China. The affected crops include vegetables， flowers， and food crops.

Thrips have the characteristics of small individuals， easy to hide， fast reproduction， and easy to produce drug resistance， so it is difficult to prevent and control them. Thrips rasping-sucking mouthparts often file the plant epidermal tissue to suck the juice， causing the crop to wilt and the seeds to shrivel. And they have a certain impact on crop quality， such as tobacco thrips， western flower thrips[3]. Thrips secretions can cause disease. Some thrips species can form galls， reducing the ornamental value of garden plants， such as Dendrothrips ornatus. The most important thing is that some thrips can transmit viruses. For instance， tobacco thrips can spread tomato spotted wilt virus on tomatoes， tobacco and potatoes[4]. Many scholars at home and abroad have studied the population dynamics of thrips on different crops， but there are few studies on the composition of thrips species and the effects of spatial and temporal dynamics of thrips under intercropping between different crops. In this study， the population dynamics of thrips in asparagus-soybean intercropping fields in Hebei area was investigated， and the composition and dominant species of thrips were clarified， so as to provide a theoretical basis for the forecast and comprehensive control of thrips in this area.

Materials and Methods

Investigation time， site and environment

The test field was set at the Dahe Experimental Station of Hebei Academy of Agriculture and Forestry Sciences， with an area of 1 050 m2. The host plants were interplanted asparagus and soybean. Asparagus was planted in May 2013. Soybean was planted between the asparagus rows on June 10， 2018， and harvested in late October. No pesticides were sprayed during the test.

Investigation methods

Malaise traps were set in the middle of the test field （containing ethanol bottles， for temporarily storage of the collected insect samples）. From May 15 to November 30， 2018， insects were collected from the ethanol bottles about every 15 d， and the types and numbers of thrips adults collected in the bottles were identified under microscopes.

Specimen identification

The identification mainly referred to classification data such as Hans Chinese Economic Entomology and MOUND[6].

Data analysis

The test data was analyzed using Excel 2003 and SPSS 17.0 software.

Results and Analysis

Thrips species and dominant species

A total of 746 thrips were collected in the area， which were identified as belonging to 7 species， 7 genera and 2 families. There were 5 species of harmful thrips， including tobacco thrips （Thrips alliorum （Priesner））， D. ornatus， western flower thrips （Frankliniella occidentalis （Pergande））， Megalurothrips usitatus and Sussericothrips melilotus Han. There were two kinds of thrips that can be used as natural enemies， Scolothrips takahashii Priesner and Aeolothrips fasciatus.

Among them， the number of tobacco thrips was the highest， 671， accounting for 90% of the total， indicating that tobacco thrips was the dominant species in the asparagus and soybean intercropping fields in Shijiazhuang， followed by thrips 5.36%， S. melilotus 5.09%， A. fasciatus 2.81%， western flower thrips 1.07%， D. ornatus and S. takahashii 0.2% （Table 1）.

Thrips species dynamics

Through investigation， as shown in Fig. 1， it was found that with the changes of seasons and crop growth stages， the species of thrips also changed greatly. Only tobacco thrips occurred in May. In June， there were 266 tobacco thrips， 2 S. melilotus， 11 A. fasciatus and 1 S. takahashii. In July， 11 tobacco thrips， 4 A. fasciatus and 8 western flower thrips were collected. In August， there were 73 tobacco thrips， 4 A. fasciatus， 9 S. melilotus， 1 M. usitatus and 1 S. takahashii. In September， 169 tobacco thrips， 2 A. fasciatus， 21 S. melilotus and 3 M. usitatus were collected. No adult thrips were found after October.

Thrips population dynamics

Among the 746 thrips collected in this study， the number was the highest in mid-to-late June and mid-to-late September， respectively， 171 and 134. The number was less from July to mid August， and the number gradually decreased to 0 in mid-to-late October （Fig. 2）.

Different species of thrips had different occurrence patterns in different seasons （Fig. 3）. Tobacco thrips had two peaks since the start of the survey on May 15. The first peak was 159， collected on June 29， followed by a decrease to 0 on July 16 and then a fluctuated increase， and 123 were collected on September 30， which was the second peak. Only 8 western flower thrips were collected on July 16. For S. melilotus， 2 were collected on June 29， 6 on August 14， 3 on August 28 and 27 from September 14 to October 15. M. usitatus was collected with a number of 1 on August 28and 3 on September 14， respectively. Only 2 D. ornatus were collected during the entire survey period. As to S. takahashii， one was collected on June 29 and August 28， respectively. Two A. fasciatus were collected on June 15， 9 on June 29， 4 on July 31， and 8 since then to September 14.

Discussion

There are many factors that affect the mutual adaptation between thrips and their hosts. Plant types， growth stages， tenderness， and secondary biomass are all important factors for thrips adaptability. There are many kinds of harmful thrips in the asparagus-soybean intercropping fields in Shijiazhuang area， among which tobacco thrips is the dominant species， and the number of other thrips is smaller.

According to the life habits analysis of tobacco thrips， from May 15 to June 29， the temperature gradually increased from about 20 ℃ and the humidity gradually decreased to about 25%， which is conducive to the reproduction of tobacco thrips. In addition， at the end of May and the beginning of June， which was the harvest period of tender asparagus stems， and the young and juicy tender stems were suitable for stabbing and sucking by tobacco thrips， which led to the continuous increase of the number. In July， at the end of asparagus harvesting， the degree of fibrosis of the aboveground mother stems became higher， and the temperature gradually increased， which was not conducive to thrips reproduction， and the density of the insects gradually decreased. From August 15 to September 30， the temperature gradually dropped， the asparagus grew into the second harvest period， having a lot of tender stems， and the soybean entered the flowering and podding period， so that the tobacco thrips obtained a better breeding environment and food source and thus showed the second peak.

According to the living habits of the D. ornatus[7]， a small amount of cloves were planted at the edge of the test field at 500 m. The temperature and humidity at that time were suitable for the propagation of D. ornatus. It is speculated that the D. ornatus were transferred from other fields. Western flower thrips is a flower-perching insect， and it has a positive tendency to a large number of plants with floral fragrance and mainly harms the flowers and tender leaves of legume vegetables[8]. In this study， we did not find a large number of western flower thrips in the asparagus flowering period and soybean flowering period， indicating that western flower thrips had no obvious tendency towards asparagus and soybean flower organs. The optimal growth temperature for S. melilotus is 25 ℃， and the optimum humidity is 50%. It mainly parasitizes on sweet clover， alfalfa， and weeds. In this study， 38 S. melilotus were discovered， which might be caused by alfalfa weeds growing in the test field. A. fasciatus mainly damage cotton and wheat. In this study， only 21 A. fasciatus were found， indicating that asparagus and soybeans were not the most suitable hosts for A. fasciatus， and the A. fasciatus might be transferred from cotton about 1 kg away from the test field.

In addition， two S. takahashii were found in this study. It is a kind of predatory thrips， mainly preying on small insects such as spider mites， and most of them appear on fruit trees and crops with many red spiders. The discovery of S. takahashii provides a basis for the diversity and ecological adaptation of natural enemies in the area.

Yanpo CAO et al. A Preliminary Study on the Regularity of Outbreak of Thrips in Asparagus-bean Interplanting Fields

Conclusions

This study analyzed the species and dominant species， population dynamics， and quantity dynamics of thrips in the asparagus-soybean intercropping fields of Dahe Experimental Station of Hebei Academy of Agricultural and Forestry Sciences. It was found that there were fewer thrips species under this planting pattern， among which there were more harmful thrips and less natural enemies; and tobacco thrips accounted for about 90%， which was the dominant population， and the peak occurred in a bimodal trend. The species and quantity of thrips were greatly affected by factors such as temperature， humidity， and parasitic crops.

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