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Diamondback moth resistance to insecticides in Guangdong Province

Xia Feng, Huan-yu Chen, Li-hua Lű

Plant Protection Research Institute, Guangdong Academy of Agricultural Sciences, Guangzhou 510640, China
Corresponding author: lhlu@gdppri.com

Abstract

The diamondback moth (DBM), Plutella xylostella (L.) (Lepidoptera: Yponomeutidae) is one of the most serious and important pests of crucifers in many parts of China, particularly in South China. DBM has developed resistance to all of chemical insecticides commonly used in vegetable growing regions of Guangdong Province. The development of resistance to avermectin and a microbial insecticide, Bacillus thuringiensis Berliner has been observed in the laboratory and field populations of DBM for at least eight years since 1992. The field study indicated that DBM resistance of field populations in both Shenzhen and Guangzhou regions to abamectin, Bacillus thuringiensis and chlorfluazuron increased greatly in the past decade and that the tolerance of the Shenzhen DBM population was higher for all three agents compared with the Guangzhou DBM population. However, there has been no significant change in resistance to dichlorvos, methomyl, fenvalerate and cartap in DBM since 1992. Laboratory studies indicated that susceptible DBM strain (SS) to abamectin had gradually developed resistance when DBM was treated regularly with sub lethal dosage of the preparation. After 20 generations, the induced resistance level was as high as 11.55-fold compared with susceptible. This case is in accordance with the situation in which field population gave rise to high level of resistance (2-, 12-, and 20-fold, in 1992, 1994 and 1996) to abamectin preparation. There was no cross-resistance between abamectin and Bt (Bacillus thuringiensis), dichlorvos, fenvalerate or cartap, but some cross-resistance was found between abamectin and chlorfluazuron. Synergists like SV1 and TPP enhanced the toxicity of abamectin to DBM field strains. Therefore, based on both field and laboratory studies, insecticide rotation has been recommended to vegetable growers and at present, the problem of DBM resistance has been alleviated. Devastation by DBM has been controlled in South China after an insecticide resistance management program was conducted to guide insecticidal application.

Keywords

abamectin, Bacillus thuringiensis

Introduction

The diamondback moth (DBM), Plutella xylostella (L.) (Lepidoptera: Yponomeutidae), is a cosmopolitan pest of cruciferous crops and an important pest of cruciferous vegetables in the Pearl Delta of Guangdong Province in South China. This pest has developed the most severe resistance to insecticides all over the world. It is reported that DBM had resistance to DDT (Ankersmit 1953) and then developed high tolerance to a range of pesticides in the Philippines, Japan, Malaysia, the United States and China. Insecticide resistance in DBM has held the attention of researchers worldwide and three international workshops related to DBM resistance and management have been held in several regions since 1986.

In the turning point of the 1980s and 1990s, DBM in vegetable farms in Pearl Delta, producing for Hong Kong, had built up resistance to almost all chemical insecticides and vegetable production was reduced greatly. In 1989, chlorfluazuron, one of the insect growth regulators, was registered by a Japanese company and used widely in Guangdong Province. However, after six months, a field population of DBM developed high resistance to this new insecticide. Soon afterwards, the Plant Protection Research Institute and Yangzhou Biological experimentation factory collaborated to produce a preparation of Bacillus thuringiensis and chemicals which was used widely in the vegetable production region. Until now, Bt products are still the primary insecticides used in the early stages of cruciferous crops. Avermectins, a novel class of macrocyclic lactones that have demonstrated nematicidal, acaricidal and insecticidal activity, were introduced and applied extensively in the Pearl Delta and DBM damage to crucifers was efficiently controlled.

Based on previous work concerning DBM resistance to biological formulations (Feng et al. 1996), the aims of this experiment were to monitor resistance variation of avermectin and other chemicals, to select for avermectin resistance in DBM and to study the cross resistance spectrum.

Materials and methods

Test insects

Three strains of DBM were studied: a laboratory, insecticide-susceptible strain from the insect toxicity laboratory, South China Agricultural University, maintained in culture in the pesticide laboratory, PPRI, GAAS; a field strain (resistant DBM I) collected in a suburb of Guangzhou city, Guangdong Province, China and subsequently maintained in the pesticide laboratory; a field strain (resistant DBM II) collected in Shenzhen where most Hong Kong Vegetable Farms are located. Frequency of pesticidal application is higher in the area around Shenzhen compared with that around Guangzhou.

The strains were cultured and tested on flowering Chinese cabbage at 24±2°C and 60±5% RH under natural light conditions at the Plant Protection Research Institute, GAAS. Larval instars of DBM were identified by the width of the head capsule.

Chemical insecticides

The following insecticides were used in this study: abamectin (concentrated fermentation liquid), efficient ingredient (B1a); Dynamec® 1.8% abamectin emulsified concentration (EC), DiPel® (Bacillus thuringiensis var. kurstaki, 16,000 IU/mg Wettable powder (WP) (Abbott), dichlorvos 80% EC, fenvalerate 20% EC, cartap 98% WP, dimehypo 18% liquid formulation (L), isoprocarb 20% EC and methomyl 24% L.

Testing technique

Bioassays of bio-preparations of abamectin and Bt products were conducted by dipping flowering Chinese cabbage leaves into an aqueous solution of chemicals for 2-3 min. Five concentrations of each preparation with two replications were tested. After being air-dried, the leaves were put into Petri dishes (diameter 9 cm) and 15 III instar DBM larvae were released per dish. The larvae were cultured in a chamber controlled at 25±2ºC. Mortality of DBM larvae was recorded two days after treatment commenced.

Analysis

Concentration-mortality response data were subjected to probit analysis (Finney 1971) where applicable. All concentration-mortality response data were corrected for control mortality (Abbot 1925) unless indicated otherwise. Data analysis was conducted using DPS (Data processing system) (Tang & Feng 1997). Resistance ratios (RR) were calculated by dividing the LC50 of each field population by the LC50 of the SS population. Synergistic ratios (SR) were calculated by dividing the LC50 of one insecticide with synergist by the LC50 of the insecticide alone.

Results and discussion

Monitoring insecticidal resistance in field populations

As a great deal of avermectin insecticides had been applied in successive years in the Shenzhen vegetable region from 1992 to 1999, DBM resistance to both concentrated preparations by the Chinese pesticide factory and Dynamec® were bioassayed to trace the resistance tendency of field DBM populations. The resistance level of DBM was evaluated according to resistance ratio (Table 1). The bioassay indicated that in 1992, when avermectin began to be used, the value of the resistance ratio was 2 (sensitive level). After one year of numerous and successive use of avermectin, the resistance ratio increased to 5-10 (low level). In 1994, DBM populations developed resistance as high as 12.00 and 14.52 (medium level) separately to the concentrated preparation of avermectin and to Dynamec®. Since then and up to 1999, DBM resistance remained at a medium level. During this period, the value of the resistance ratio reached as high as 22.75 or 25.19. There were slight differences in resistance ratio or lethal dosage between the avermectin concentrated preparation and Dynamec®, although their tendency is the same in general. The variation may result from purity of products with the concentrated preparation containing more active ingredients and Dynamec® containing 80% B1a and 20% B1b.

Table 1. Diamondback moth (DBM) resistance to a concentrated preparation of abamectin and Dynamec® from 1992 to 1999

Year

Population source

LC50 (mg/L)

Resistance ratio

   

abamectin

Dynamec®

abamectin

Dynamec®

1992

Susceptible strain

0.04

0.21

   

1992

Field

0.08

0.50

2.00

2.38

1993

Field

0.30

1.73

7.50

8.24

1994

Field

0.48

3.05

12.00

14.52

1995

Field

0.64

3.57

16.00

17.00

1996

Field

0.81

5.17

20.25

24.62

1997

Field

0.91

5.29

22.75

25.19

1998

Field

0.85

5.10

21.25

24.29

1999

Field

0.82

5.05

20.50

24.05

Evolution of resistance to several insecticides

DBM populations in vegetable farms of both Shenzhen and Guangzhou rapidly developed resistance to the microbial insecticides Dynamec® and DiPel® after numerous and frequent applications. In 1992, resistance ratios of DBM to Dynamec® at both places were 2.18 and 2.38 respectively (Table 2). In 1999, ratios had increased to 9.82 and 24.05. Products of Bacillus thuringiensis were registered and applied earlier than avermectin and so high Bt resistance occurred in DBM populations. In 1992, resistance ratio of DBM to DiPel® at both Shenzhen and Guangzhou were 17.97 and 5.55 and, in 1999, had increased to 44.82 and 12.23. There was little variation in resistance to dichlorvos, fenvalerate, cartap and methomyl throughout the monitoring period. DBM resistance to fenvalerate decreased slightly because of less use in the field. DBM resistance to insect growth regulators (IGRs) increased.

Table 2. Bioassay results of resistance to insecticides in DBM field populations I - Guangzhou and II - Shenzhen, China

Treatment

LC50 of susceptible strain

Resistant DBM I

Resistant DBM II

 

(mg/L)

1992

1999

1992

1999

abamectin concentrated

0.04

1.89

14.75

2.00

20.50

Dynamec®

0.21

2.18

9.82

2.38

24.05

DiPel®

30.33

5.55

12.23

17.97

44.82

dichlorvos

68.39

13.78

13.39

14.95

16.04

methomyl

151.29

7.62

6.57

11.03

10.01

fenvalerate

68.22

43.73

38.69

78.98

61.83

chlorfluazuron

2.74

24.67

39.19

28.20

45.36

cartap

140.09

3.65

3.22

4.47

5.20

Cross resistance

In the experiment to study cross resistance, an abamectin resistant strain was selected from a Bt resistant strain through continuous treatment with a sub-lethal dose of abamectin. The Bt resistant strain had been raised in the laboratory for more than 100 generations. Although its resistance level to Bt products was as high as about 60.82-fold, for abamectin resistance it was only less than 3x the susceptible level (Table 3). When the Bt resistant strain was selected with abamectin instead of Bt, the DBM strain at generation F5 developed abamectin resistance as same as F0 and Bt resistance was 58.02-fold. At generation F10 after selection, abamectin resistance increased to 7.55-fold and Bt resistance decreased to 49.32. At generation F20, abamectin resistance increased to 11.5-fold and Bt resistance decreased to 31.34-fold. It was concluded that there was no cross resistance between abamectin and Bt because Bt resistance declined with enhanced abamectin resistance when high pressure of abamectin was put on the DBM strain.

Table 3. Resistance selection of DBM to abamectin in a Bt resistant strain

Population source

Generation

LC50 (mg/L)

Resistance ratio

   

abamectin

Bt

abamectin

Bt

Susceptible strain

 

0.04

30.29

   

Bt resistant strain

F0

0.09

1842.20

2.29

60.82

Bt resistant strain

F5

0.12

1757.40

3.11

58.02

Bt resistant strain

F10

0.29

1493.90

7.55

49.32

Bt resistant strain

F15

0.34

1137.69

8.95

37.56

Bt resistant strain

F20

0.44

949.36

11.55

31.34

The abamectin resistant strain was used to test for cross resistance between abamectin and several chemical insecticides. There was no cross-resistance between abamectin and dichlorvos, fenvalerate or cartap, but some cross-resistance of DBM populations was found between abamectin and chlorfluazuron (Table 4).

Table 4. Resistance to commonly used insecticides in an abamectin-induced resistant DBM strain

Treatment

LC50 (mg/L)

Resistance ratio

 

Susceptible strain

Resistant strain

 

chlorfluazuron

2.74

55.56

20.28

abamectin

0.04

0.44

11.55

dichlorvos

68.39

253.32

3.70

methomyl

151.29

344.83

2.28

fenvalerate

68.22

127.63

1.87

Synergistic effect

Some of the synergists tested enhanced the toxicity of abamectin towards DBM (Table 5). Among them, SV1 (O, O-diethyl O-phenyl phosphorothioate) was highest with an 8.44-fold increase. TPP caused a 3.70-fold increase in toxicity. S2 and POB (piperonyl butoxide), one of the main synergists for fenvalerate, had little effect on increasing toxicity of abamectin. SV1 is an inhibitor of microsomal oxidases and carboxylesterase in insects. Synergism with SV1 and TPP indicates that these enzymes play a major role in the mechanism of avermectin resistance. These results are similar to other reports (Li et al. 1998, Abro et al. 1988).

Table 5. Effects of synergists on abamectin toxicity to DBM

Treatment

LC50 (mg/L) at 48 h

Synergist ratio

abamectin (ab)

0.73

 

ab + SV1

0.09

8.44

ab +TPP

0.20

3.70

ab + S2

0.22

3.30

ab + PB

0.38

1.92

Discussion

There are many vegetable farms at Shenzhen, Dongguang, Huizhou, Bolo and Zengcheng in Guangdong Province, which are run by investors from Hong Kong. The farms cover a large area of cultivated land on which cruciferous vegetables are grown continuously throughout the year. Pest management personnel have limited plant protection knowledge, insecticides are applied with knapsack equipment and pest control is not effective. In recent years, the products of abamectin were produced and applied more frequently than before and pest managers used them aimlessly without reasonable mixture and rotation. These conditions of use have resulted in high selection pressure of abamectin on DBM field populations and consequently, in the main region of vegetable production, resistance to abamectin in DBM has increased rapidly in the past ten years.

In the Shenzhen region, DBM populations developed resistance to abamectin as much as 10-fold in 1999 compared with resistance levels in 1992. The same situation happened with Bt products. DBM resistance to commonly used chemicals remained stable perhaps because they were used rarely due to low efficacy. In experiments on the abamectin resistance mechanism and selection, SV1 and TPP inhibited activities of carboxylesterase. DBM could develop resistance to abamectin under selection pressure of regular abamectin usage, but its rate of development of resistance was less than that of Bt. An abamectin-induced resistant strain had obvious cross resistance with chlorfluazuron, some with dichlorvos, but none with methomyl and fenvalerate.

References

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Ankersmit GW. 1953. DDT-resistance in Plutella maculipennis (Curtis) (Lep.) in Java. Bulletin of Entomological Research 44, 421-425.

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Li TW, Gao XW, Zheng BZ , Zhu SX & Si SY. 1998. A study on carboxyl esterase activity in insecticide resistant and susceptible populations of diamondback moth, Plutella xylostella, from different regions. Acta Entomologica Sinica 41 (supplement), 26-33.

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