Toxicity and Field Efficacy of Six Insecticides on Alfalfa Thrips (Thysanoptera: Thripidae)

Lan LUO Hong JIANG Zhonglin YUAN

Abstract Alfalfa thrips, especially the birds-foot trefoil thrips (Odontothrips loti Haliday), is a major pest in the alfalfa field. Toxicity and field efficacy of 6 insecticides were determined by leaf disk in tube method and foliar spray, respectively. The results showed LC50 values of chlorfenapyr 24% SC, lambda-cyhalothrin 5% EW, sulfoxaflor 22% SC, dinotefuran 20% SG, spinetoram 60 g/L SC and tolfenpyrad 30% SC to Odontothrips loti were 0.11, 0.62, 2.92, 4.24, 10.47 and 13.42 mg/L, respectively. The control effects of tolfenpyrad 30% SC, spinetoram 60 g/L SC, sulfoxaflor 22% SC, lambda-cyhalothrin 5% EW and dinotefuran 20% SG against alfalfa thrips were more than 80% after 1 d treatment, showing good readily availability. The control of spinetoram 60 g/L SC and dinotefuran 20% SG was 74.63% and 75.65% after 7 d treatment, showing a long persistence effect respectively. Therefore, spinetoram 60 g/L SC and dinotefuran 20% SG can be used as the first option to control alfalfa thrips.

Key words Alfalfa thrips; Insecticides; Toxicity; Field efficacy

Received: November 23, 2020  Accepted: January 2, 2021

Supported by Shandong Provincial Modern Agricultural Industry Technology System Forage Innovation Team (SDAIT-23-01).

Lan LUO (1965-), female, P. R. China, professor, devoted to agrochemical teaching and research.

*Corresponding author. E-mail: zhongliny@163.com.

Thrips are the important pests on alfalfa field, mainly include of Odontothrips loti Haliday, Frankliniella intonsa (Trybom) and Thrips tabaci Lindeman, etc.［1］. O. loti is the dominant population on northwestern and northern China (O. loti about over 90%)［2］, accounting for 90%-100%［1-3］. Especially, from 2001 to 2018, the cultivated area of alfalfa in China increased from 2.85 million to 4.377 million hm2［4］ since the China proposed the "Development Action of Revitalizing Dairy Industry and Alfalfa". In worldwide, O. loti thrips is mainly distributed in the United States, Russia, Lithuania, Siberian, Switzerland, France, Estonia, Ukraine, Georgia, Yugoslavia, Denmark, Hungary, Austria, Sweden, Romania, Germany, Poland, Netherlands, Italy, Czech republic, Finland, Slovakia, Britain, Japan, Mongolia, and Northwest and Northern China［5］. The known hosts are Medicago sativa, Melilotus officinalis and Trifolium sp.［6］.

The damage begin from the turning green of alfalfa, and it mainly impinges on the leaves, buds and flowers. After the young leaves are damaged, they appear spots, curl and even die, which seriously threaten the yield and quality of alfalfa. Due to its smaller, multiple generations of occurrence and serious harm, chemical control is still the main control method for thrips in current, including botanical and microbial insecticides［7-8］. However, long-term and frequent using of insecticides is likely to lead resistance［9］. In order to delay insecticides resistance and ensure the yield and quality of alfalfa, it is very important to screen the high efficiency, low toxicity and low residue agents suitable for alfalfa production. The use of insecticides on alfalfa has not been reported in China. In this study, 6 insecticides with different mechanisms of action were selected, and toxicity of the target insects was determined by using O. loti as the target insect. Field efficacy test was carried out on mixed populations to study the effect of different agents on alfalfa thrips. This study provides a scientific basis for effective prevention and control of alfalfa thrips.

Materials and Methods

The tested insects

During the experiment, the thrips of O. loti were collected from the alfalfa field of Jiaozhou Experimental Base of Qingdao Agricultural University, and healthy adults were selected for indoor toxicity determination.

Reagents

Chlorfenapyr 24% SC ［Basf (China) Co., Ltd.］; lambda-cyhalothrin 5% ES (Hainan Zhengye Zhongnong Hi-tech Co. Ltd.); sulfoxaflor 22% SC (Dow AgroSciences LLC, USA); dinotefuran 20% SP (Mitsui AGRO chemicals Co., Ltd.); spinetoram 60 g/L SC (AgroSciences LLC, USA); tolfenpyrad 30% SC (Jiangsu Aijin Agrochemical Co., Ltd.).

Test methods

Leaf disk bioassays to estimate LC50 values

The toxicity was determined with leaf disks by the tube method［10］. On the basis of the pre-test, the tested insecticides were diluted into 5 series of concentrations and filled into 2 ml centrifuge tubes, stand still for 4 hours and pour out the liquids, let it dry for later use, respectively. Fresh disk leaves of alfalfa were dipped in each concentration of insecticide for 10 s, then put on filter paper for drying, the dried disk leaves were clipped into the centrifuge tubes, one piece per tube, respectively. Each concentration was repeated for 3 times, and the treatment of water-immersed leaves was taken as a blank control. The adults of O. loti were sucked into the centrifuge tube, and each tube had 20 repetitions.

200 mesh gauze was used to cover the 2 ml centrifuge tubes, which were put into a climate chamber with the indoor temperature (25±1) ℃, relative humidity (75%±5%), and photoperiod (L/D) for 16/8 h. After 24 h, the number of deaths of O. loti was checked when lightly touching individual bodies with the tip of a brush, those who were immobile were regarded as dead.

Field efficacy test methods

Field trials of alfalfa thrips were conducted in accordance with national standards (NT/T 1464.6-2007)［11］. In this experiment, a total of 6 treatments were set up, including chlorfenapyr 24% SC 90 g a.i./hm2, tolfenpyrad 30% SC 102 g a.i./hm2, spinetoram 6% SC 10 g a.i./hm2, sulfoxaflor 22% SC 75 g a.i./hm2, lambda-cyhalothrin 5% EW 20 g a.i./hm2 and dinotefuran 20% SG 30 g a.i./hm2, with water as control. Each part covered an area of 36 m2, with 4 replications. All plots were in random arrangement, and 2 m protection lines were set between the plots. The electric sprayer was used to spray evenly on all parts of alfalfa. The investigation method of alfalfa thrips was carried out by referring literature ［8］. This experiment was conducted for 5 times,  including the base of insect population before treatment,  1, 3, 5 and 7 d after treatment, respectively. The number of surviving insects was investigated, and the control effect was calculated according to the control effects［11］.

Data processing

Excel was used to calculate the mortality and correct the mortality rate. The concentration to logarithm and inhibition to probility rate was transformed to obtain the toxicity regression equation, and the LC50 and 95% confidence limit of the lethal concentration were calculated［12］. DPS v6.55 software was used for data analysis, and new complex range test was used for significance test.

Results and Analysis

Leaf disk bioassays to estimate LC50 values

All of the 6 insecticides had good affect to O. loti, and the activity was positively correlated with the mass concentration (Table 1). The LC50 of chlorfenapyr 24% SC, lambda-cyhalothrin 5% EW, sulfoxaflor 22% SC, dinotefuran 20% SG, spinetoram 60 g/L SC and tolfenpyrad 30% SC were 0.11, 0.62, 2.92, 4.24, 10.47 and 13.42 mg/L, respectively. The inhibition rate of tolfenpyrad 24% SC and lambda-cyhalothrin 5% EW were better to O. loti.

Control effect of insecticides on alfalfa thrips

All the 6 insecticides had certain control effects on the mixed populations of alfalfa thrips (Table 2). After treatment on 1 d, the control effects of tolfenpyrad 30% SC, spinetoram 60 g/L SC, sulfoxaflor 22% SC, lambda-cyhalothrin 5% EW, and dinotefuran 20% SG were over 80% at all above selected concentrations, and tolfenpyrad 30% SC with other 4 kinds of insecticides had significant effect, while sulfoxaflor 22% SC was lowest in control effect. Three days after treatment, the control effect on alfalfa thrips was above 80% except sulfoxaflor 22% SC (of which the control effect was 72.04%), and sulfoxaflor 22% SC was significantly different from other five insecticides. After 5 d, the control effect of spinetoram 60 g/L SC and dinotefuran 20% SG were 79.29% and 80.28% respectively, with no significant difference and they are the highest, the control effect of chlorfenapyr 24% SC, tolfenpyrad 30% SC and lambda-cyhalothrin 5% EW were all more than 70%, and there had no significant differenct among the 3 insecticides, yet sulfoxaflor 22% SC showed a control effect less than 60% and had significant differences from other 5 insecticides. After 7 d treatment, the control effects of spinetoram 60 g/L SC and dinotefuran 20% SG were about 75%, with no significant differences; the control effects of chlorfenapyr 24% SC, tolfenpyrad 30% SC and lambda-cyhalothrin 5% EW were over 60%, with no significant differences, but the control effect of sulfoxaflor 22% SC was less than 40%, with significant differences from other insecticides. Therefore, it can be concluded that 60 g/L spinetoram SC and 20% dinotefuran SG had significant control effects on thrips.

Conclusions and Discussion

Thrips are important piercing-sucking mouthparts pests on alfalfa, which can harm the whole growth period of alfalfa. In addition to strengthening the role of agricultural and biological control in the whole control system, it is particularly important to select insecticides with high efficiency, low toxicity and low residue and apply pesticide with different action mechanisms alternately to delay the emergence of insect resistance. There have been reports on the study of O. loti at home and abroad. In foreign country, VSrteiu et al.［5］ used 6 insecticides in field experiments, and finally obtained the best effect with 10% Beta cypermethrin. In China, Hou et al.［10］ determined the toxicity of 5 pesticides on O. loti and the results showed that abamectin had a good control effect on it and the LC50 was 0.97 g/ml. Zhang et al.［13］ showed that the application of 4.5% Beta cypermethrin and 15% abamectin + chlorpyrifos could effectively control alfalfa thrips, with its quick effect and long duration of efficacy. The results obtained by Zhao et al.［14］ showed that azadirachtin and matrine, the plant derived pesticides, had a lower control effect on thrips than the Beta cypermethrin, and the control effect was similar to that of acetamiprid. In order to evaluate the control effect and safety, Luo et al.［7,15］ used field spray method and gas chromatography to determine the control effects of 6 insecticides on alfalfa thrips, as well as the residue decomposition dynamics and final residues in alfalfa in spring. The results showed that the half-lives of imidacloprid, cypermethrin, chlorpyrifos, acetamiprid and pymetrozine in alfalfa were 4.78, 3.69, 2.48, 5.36 and 2.25 d, respectively. The maximum residue limits (MRL) of imidacloprid, chlorpyrifos, cyprothrin, acetamiprid and pymetrozine in alfalfa are 0.5, 1.0, 0.5, 0.4, 0.1 and 0.1 mg/kg respectively, and the safety intervals are 7, 14, 7, 7 and 7 d, respectively, according to the pesticide residue limits requirements of China and the Codex Alimentarius Commission (CAC). Cui［16］ applied 3 pesticides to control thrips in alfalfa fields, and the results showed that imidacloprid 70% WG and acetamiprid 20% WP had significant control effects on alfalfa thrips. Tao et al.［17］ selected 4 insecticides for control thrips, and Beta cypermethrin had the best control effect, long duration of efficacy and low cost.

In this paper, the laboratory toxicity of 6 insecticides on O. loti and the field experiment of mixed populations of alfalfa thirps were determined, which provided the basis for scientific use of alfalfa thirps.

Among the 6 insecticides tested, spinetoram 60 g/L SC and dinotefuran 20% SG were the best, followed by tolfenpyrad 30% SC, lambda-cyhalothrin 5% EW and chlorfenapyr 24% SC, and sulfoxaflor 22% SC was the lowest. Spinetoram 60 g/L SC had a better effect and widerspread use of prevention and treatment on thrips［18］, dinotefuran was the third generation of new neonicotinoid insecticides, the main effect on the acetylcholine receptor of insects［19］. Chlorfenapyr as a new type of pyrrole compound, acting on the insect cells mitochondria, and with systemic insecticide［20］. Lambda-cyhalothrin belongs to pyrethroid insecticides, with high efficiency, broad spectrum, fast-acting insecticide［21］. Tolfenpyrad belongs to a new heterocyclic insecticide with unique way of action［22］. All of these pesticides are poor persistence, can be chosen according to in alfalfa insect pest control. No harm to alfalfa was found when the 6 insecticides were used on alfalfa at test concentrations.

Acknowlegements

We thank LYU Zhaozhi from Qingdao Agricultural University on early versions of this manuscript. We also thank our laboratory colleagues for help and valuable critical opinions.

Lan LUO et al. Toxicity and Field Efficacy of Six Insecticides on Alfalfa Thrips (Thysanoptera: Thripidae)

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