Original Articles
Resistant levels of Spodoptera exigua to eight various insecticides in Shandong, China
2014 Volume 39 Issue 1 Pages 7-13
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2014 Volume 39 Issue 1 Pages 7-13
The beet armyworm, Spodoptera exigua (Hübner), is a polyphagous insect pest that infests a wide range of crops in tropical and subtropical regions of the world.1) Under favorable conditions, its populations can expand rapidly and move across fields like an army, leading to its name “armyworm”.2) In China, the damage caused by S. exigua has continually increased and S. exigua has attacked numerous economically important crops, causing great losses in agriculture since the 1980s.3) In recent years, mainly due to its development of insecticide resistance and subsequent control failures, its outbreaks have become more common in the province of Shandong, especially in vegetable-growing areas. Although physical control and natural predators may reduce field populations of S. exigua,4) the current level of control is often insufficient to avoid economic damage, especially for high-value crops. This makes insecticide control indispensable for insect pest management (IPM) programs of S. exigua. Due to resistance, many conventional insecticides, such as pyrethroids and organophosphorus insecticides, have failed to provide adequate control in recent years.4–6) Some new insecticides have been rapidly introduced into S. exigua control, which feature low toxicity, high activity, new modes of action, and environmental safety.7,8)
No pesticides can avoid the development of pesticide resistance. For a more effective application of new insecticides, monitoring of resistance in S. exigua to these compounds is crucial. The present study was initiated to determine the resistance of S. exigua to chlorantraniliprole, flubendiamide, emamectin benzoate, chlorfenapyr, spinosyn, indoxacarb, tebufenozide and methoxyfenozid in Shandong, China.
Field populations of S. exigua were collected from five vegetable production bases in Shandong, including Tai’an, Tengzhou, Binzhou, Anqiu, and Zhangqiu (Fig. 1 and Table 1) between July 2011 and October 2012. Third- to fifth-instar larvae were collected by walking through a 3-ha block of a particular crop in a zigzag manner in order to obtain a mixed population. To get sufficient numbers of larvae, in some areas moths were collected using light traps and butterfly nets. The moths were kept in cages with meshed sides to maintain ventilation. The adults were fed on a solution containing sucrose (100 g/L) and a vitamin solution (20 mL/L) in a soaked cotton wool ball. All larvae were fed with a semi-synthetic diet, slightly modified from Mu et al.9) Third-instar larvae of the next generation, which represented the progeny of the field-collected insects, were used for the resistance studies. All instars of the insects were reared in the laboratory at 27±1°C and 50–75% relative humidity (RH) with a 14 hr : 10 hr L : D photoperiod. An insecticide-susceptible colony as a negative control was obtained from the Wuhan Institute of Vegetable Science, China, where it had been maintained in a mass-rearing environment for 30 years.
| Location | Collection date | No. collected | Sites | Host plants | ||
|---|---|---|---|---|---|---|
| Tai’an | July. 2011 | Aug. 2012 | 600 | 500 | 36.18°N, 117.13°E | Cabbage |
| Zhangqiu | Sep. 2011 | Sep. 2012 | 460 | 700 | 36.72°N, 117.53°E | Scallion |
| Anqiu | Aug. 2011 | Sep. 2012 | 520 | 400 | 35.09°N, 117.17°E | Scallion |
| Tengzhou | Sep. 2011 | Oct. 2012 | 500 | 600 | 34.86°N, 117.57°E | Ginger |
| Binzhou | Oct. 2011 | Oct. 2012 | 600 | 850 | 37.36°N, 118.03°E | Ginger |
The following commercial formulations of the insecticides were used in bioassays: chlorantraniliprole (200 g/L SC [suspending agent]; DuPont Agriculture Co., Shanghai), flubendiamide (20% WDG [water dispersible granule]; Janpen Pesticide Co.), chlorfenapyr (100 g/L, SC, BASF AG, Germany), indoxacarb (200 g/L, SC, DuPont Agriculture Co., Shanghai, China), spinosyn (200 g/L, SC, Dow Agro Sci., USA), emamectin benzoate (10 g/L, EC [emulsifiable concentrate], Lvba Chemistry Co., Shandong, China), tebufenozide (200 g/L, SC, Dow Agro Sci., USA), and methoxyfenozide (200 g/L, SC, Dow Agro Sci., USA).
3. BioassaysBioassays were conducted on newly emerged third-instar larvae of S. exigua from the first filial generation of laboratory cultures using a standard leaf-dip bioassay method.10) Serial dilutions as mg/L of the active ingredient of the test compounds were prepared using tap water. Cabbage leaf discs (1-cm diameter) were cut and dipped into the test solutions for 10 sec with gentle agitation, and then allowed to dry between two pieces of paper towel. At least six concentrations and four replications (20 larvae per replication) were used to estimate each concentration’s mortality line. Controls for each insecticide were treated with water. Before and after treatment, larvae were maintained at a constant temperature of 27±1°C and an RH of 50–75% with a photoperiod of 14 hr : 10 hr L : D. Mortality was assessed after 48 hr for general insecticides and after 72 hr for insect growth regulator (IGR) insecticides. Larvae were considered dead if they were unable to move in a coordinated manner when disturbed with the tip of a pencil.11)
4. Statistical analysisData were corrected for control mortality using Abbott’s formula12) before analysis, and data were analyzed using the SAS/STAT® version 6.12 (SAS Institute Inc., 1997). Statistical differences between LC50 values were determined using the presence or absence of overlap in the 95% confidence limits. Resistance ratios (RRs) were calculated by dividing the LC50 of a field population by that of the susceptible population. The resistance level was considered as: none at RR <2-fold, very low at RR=2–10-fold, low at RR=11–20-fold, moderate at RR=21–50-fold, high at RR=51–100-fold, and very high at RR >100-fold.10)
We tested the toxicity of eight various insecticides to the laboratory strain in 2011 (Table 2). The laboratory colony was susceptible to all eight insecticides and was used for calculating the resistance ratios. Emamectin benzoate had the highest toxicity with an LC50 of 0.03 mg/L. Indoxacarb, spinosyn, tebufenozide, methoxyfenozide, chlorantraniliprole, and flubendiamide had LC50 values between 0.20 and 0.63 mg/L. Spinosyn and tebufenozide showed moderate levels of toxicity (LC50=1.23 mg/L and 1.67 mg/L, respectively). Chlorfenapyr was the least toxic among the tested insecticides (LC50=12.3 mg/L).
| Insecticides | Fit of probit line | LC50 (mg/L) | 95% Fiducial limits (Lower–Upper) | ||
|---|---|---|---|---|---|
| Slop±S.E. | χ2 | df | |||
| Emamectin Benzoate | 1.44±0.20 | 3.82 | 5 | 0.0305 | 0.0300–0.0402 |
| Methoxyfenozide | 1.65±0.34 | 4.33 | 4 | 0.201 | 0.184–0.232 |
| Chlorantraniliprole | 1.83±0.59 | 5.67 | 4 | 0.304 | 0.241–0.396 |
| Indoxacarb | 1.97±0.43 | 5.14 | 4 | 0.591 | 0.283–1.25 |
| Flubendiamide | 2.76±0.07 | 3.44 | 5 | 0.632 | 0.472–0.847 |
| Spinosyn | 1.95±0.26 | 5.32 | 4 | 1.23 | 0.862–1.74 |
| Tebufenozide | 1.96±0.35 | 2.45 | 5 | 1.67 | 1.61–1.74 |
| Chlorfenapyr | 2.55±0.24 | 3.42 | 5 | 12.3 | 7.18–21.2 |
Toxicity results of the various chemistry insecticides against different populations are shown in Table 3. In five regions, all populations showed no resistance to chlorfenapyr (RR <2), suggesting that S. exigua was susceptible to chlorfenapyr. Chlorfenapyr has been used extensively in China, yet there are no reports of resistance against it. There were three regions in Tai’an and Anqiu in which low to moderate resistance to emamectin benzoate was observed, although at very low levels (2≤ RR <10).
| Insecticides | Year | Poppulation | Fit of probit line | LC50 (mg/L) | 95% Fiducial limits | Resistance ratios | Resistance level | ||
|---|---|---|---|---|---|---|---|---|---|
| Slop±S.E. | χ2 | df | |||||||
| Chlorfenapyr | 2011 | Tai’an | 3.18±0.46 | 4.23 | 5 | 12.0 | 10.1–14.5 | 0.976 | None |
| Zhangqiu | 4.04±0.40 | 5.14 | 4 | 11.7 | 10.6–13.0 | 0.951 | None | ||
| Anqiu | 1.67±0.24 | 3.38 | 4 | 15.9 | 11.5–23.6 | 1.29 | None | ||
| Tengzhou | 4.00±0.40 | 4.86 | 5 | 11.9 | 10.8–13.2 | 0.967 | None | ||
| Binzhou | 1.89±0.45 | 6.20 | 4 | 12.6 | 9.48–17.0 | 1.02 | None | ||
| 2012 | Tai’an | 2.50±0.32 | 4.02 | 6 | 18.0 | 15.3–21.7 | 1.46 | None | |
| Zhangqiu | 2.66±0.34 | 1.87 | 5 | 13.0 | 11.3–15.0 | 1.06 | None | ||
| Anqiu | 1.58±0.25 | 3.76 | 5 | 7.90 | 5.54–11.2 | 0.642 | None | ||
| Tengzhou | 3.90±0.39 | 1.20 | 4 | 8.80 | 7.18–10.4 | 0.715 | None | ||
| Binzhou | 2.15±0.47 | 3.45 | 5 | 18.7 | 14.7–27.4 | 1.52 | None | ||
| Emamectin Benzoate | 2011 | Tai’an | 1.83±0.27 | 4.22 | 5 | 0.122 | 0.0814–0.149 | 4.00 | Very low |
| Anqiu | 1.83±0.27 | 3.80 | 4 | 0.220 | 0.162–0.303 | 7.21 | Very low | ||
| Zhangqiu | 1.46±0.18 | 2.78 | 4 | 0.424 | 0.321–0.544 | 13.9 | Low | ||
| Tengzhou | 1.54±0.16 | 5.40 | 4 | 0.683 | 0.502–0.971 | 22.4 | Moderate | ||
| Binzhou | 2.63±0.38 | 2.21 | 6 | 0.902 | 0.724–1.00 | 29.6 | Moderate | ||
| 2012 | Tai’an | 2.57±0.26 | 1.90 | 5 | 0.0905 | 0.0721–0.103 | 3.00 | Very low | |
| Anqiu | 1.48±0.25 | 4.36 | 5 | 0.283 | 0.194–0.402 | 9.28 | Very low | ||
| Zhangqiu | 1.53±0.18 | 6.75 | 4 | 0.331 | 0.251–0.420 | 10.9 | Low | ||
| Tengzhou | 1.11±0.15 | 6.23 | 4 | 0.322 | 0.234–0.422 | 10.6 | Low | ||
| Binzhou | 1.47±0.31 | 5.48 | 5 | 1.14 | 0.810–1.78 | 37.4 | Moderate | ||
Resistance ratio (RRa) estimated as RR=LC50 of field strain /LC50 of the insecticide-susceptible strain.
In most tested populations, no resistance to chlorantraniliprole or flubendiamide was found (Table 4). S. exigua from Binzhou revealed moderate resistance to chlorantraniliprole, with resistance levels ranging from 19.8-fold to 28.4-fold. Compared to chlorantraniliprole, flubendiamide, the second diamide insecticide had lower toxicity levels in most populations. In Anqiu, the population showed no resistance to flubendiamide (RR <2.00-fold). The resistance ratio of flubendiamide changed in the other four regions in 2011–2012, increasing from 5.55-fold to 11.5-fold in Binzhou, from 10.3-fold to 16.9-fold in Zhangqiu, from 8.39-fold to 15.4-fold in Tengzhou, and from 18.4-fold to 21.7-fold in Tai’an.
| Insecticides | Year | Poppulation | Fit of probit line | LC50 (mg/L) | 95% Fiducial limits | Resistance ratios | Resistance level | ||
|---|---|---|---|---|---|---|---|---|---|
| Slop±S.E. | χ2 | df | |||||||
| Chlorantraniliprole | 2011 | Anqiu | 1.97±0.32 | 5.24 | 5 | 0.262 | 0.191–0.323 | 0.862 | None |
| Tai’an | 1.66±0.20 | 5.86 | 5 | 1.67 | 1.30–2.18 | 5.49 | Very low | ||
| Tengzhou | 1.06±0.11 | 5.33 | 5 | 1.80 | 1.30–2.44 | 5.92 | Very low | ||
| Zhangqiu | 1.13±0.11 | 6.15 | 4 | 1.98 | 1.47–2.63 | 6.51 | Very low | ||
| Binzhou | 2.95±0.49 | 2.46 | 4 | 6.03 | 4.99–7.23 | 19.8 | Moderate | ||
| 2012 | Anqiu | 2.35±0.47 | 3.42 | 4 | 0.633 | 0.502–0.801 | 2.08 | Very low | |
| Tai’an | 1.73±0.19 | 5.54 | 4 | 2.12 | 1.68–2.61 | 6.97 | Very low | ||
| Tengzhou | 0.852±0.11 | 5.53 | 5 | 4.42 | 3.06–6.67 | 14.5 | Low | ||
| Zhangqiu | 1.10±0.11 | 4.34 | 4 | 5.02 | 3.72–7.05 | 16.5 | Low | ||
| Binzhou | 2.36±0.48 | 2.75 | 5 | 8.63 | 6.96–11.7 | 28.4 | Moderate | ||
| Flubendiamide | 2011 | Anqiu | 1.22±0.28 | 6.44 | 4 | 0.771 | 0.510–1.11 | 1.22 | None |
| Binzhou | 2.28±0.48 | 3.25 | 4 | 3.51 | 2.51–4.38 | 5.55 | Very low | ||
| Tengzhou | 1.81±0.22 | 5.62 | 6 | 5.30 | 4.34–6.30 | 8.39 | Very low | ||
| Zhangqiu | 2.02±0.23 | 2.67 | 4 | 6.53 | 5.34–7.85 | 10.3 | Low | ||
| Tai’an | 1.71±0.32 | 4.53 | 4 | 11.6 | 8.62–17.0 | 18.4 | Moderate | ||
| 2012 | Anqiu | 1.22±0.28 | 3.44 | 4 | 1.03 | 0.82–1.24 | 1.63 | None | |
| Binzhou | 1.80±0.46 | 2.34 | 4 | 7.26 | 5.51–11.8 | 11.5 | Low | ||
| Tengzhou | 1.79±0.22 | 2.65 | 4 | 9.73 | 8.16–11.9 | 15.4 | Low | ||
| Zhangqiu | 1.87±0.24 | 5.36 | 4 | 10.7 | 8.77–13.2 | 16.9 | Low | ||
| Tai’an | 2.00±0.28 | 4.59 | 5 | 13.7 | 11.1–17.6 | 21.7 | Moderate | ||
Five populations from Shandong had moderate resistance levels, two had high resistance levels, two showed low resistance levels, and two had very low resistance levels (Table 5). No significant changes toward indoxacarb were found in the five regions in 2011–2012. Tests with spinosyn (Table 5) revealed differences in the five regions. S. exigua showed a high resistance to spinosyn in Tai’an and Binzhou, with 86.2-fold and 68.1-fold resistance ratios, respectively, in 2012. Moderate resistance levels were found in Zhangqiu and Anqiu (20≤ RR <50). The populations collected from Tengzhou showed low or very low resistance in 2011–2012.
| Insecticides | Year | Poppulation | Fit of probit line | LC50 (mg/L) | 95% Fiducial limits | Resistance ratios | Resistance level | ||
|---|---|---|---|---|---|---|---|---|---|
| Slop±S.E. | χ2 | df | |||||||
| Indoxacarb | 2011 | Zhangqiu | 1.44±0.16 | 4.45 | 5 | 5.45 | 3.62–8.35 | 9.22 | Very low |
| Tengzhou | 1.36±0.16 | 3,33 | 4 | 7.66 | 5.03–12.5 | 13.0 | Low | ||
| Tai’an | 2.93±0.51 | 1.20 | 4 | 12.1 | 9.98–16.0 | 20.5 | Moderate | ||
| Anqiu | 2.00±0.32 | 5.17 | 5 | 17.6 | 13.2–22.0 | 29.8 | Moderate | ||
| Binzhou | 1.94±0.42 | 3.78 | 5 | 37.3 | 28.6–50.4 | 63.1 | High | ||
| 2012 | Tengzhou | 2.84±0.32 | 4.07 | 5 | 5.10 | 4.38–5.78 | 8.63 | Very low | |
| Zhangqiu | 1.51±0.17 | 1.98 | 4 | 6.20 | 4.20–9.69 | 10.5 | Low | ||
| Tai’an | 3.43±0.42 | 4.20 | 6 | 19.7 | 17.4–22.7 | 33.3 | Moderate | ||
| Anqiu | 1.66±0.30 | 3.67 | 4 | 28.9 | 22.1–38.6 | 48.9 | Moderate | ||
| Binzhou | 2.14±0.43 | 6.45 | 4 | 49.7 | 39.1–70.3 | 84.1 | High | ||
| Spinosyn | 2011 | Tengzhou | 1.77±0.22 | 4.48 | 5 | 11.4 | 8.93–14.0 | 9.27 | Low |
| Zhangqiu | 2.30±0.47 | 1.88 | 4 | 25.4 | 19.2–31.7 | 20.7 | Moderate | ||
| Anqiu | 2.49±0.34 | 5.63 | 4 | 27.0 | 22.9–31.2 | 22.0 | Moderate | ||
| Binzhou | 1.84±0.30 | 4.47 | 5 | 48.0 | 37.5–62.0 | 39.0 | Moderate | ||
| Tai’an | 2.44±0.50 | 3.77 | 6 | 93.0 | 75.1–128 | 75.6 | High | ||
| 2012 | Tengzhou | 1.86±0.24 | 5.12 | 5 | 18.8 | 15.4–23.3 | 15.3 | Low | |
| Anqiu | 3.25± 0.37 | 4.66 | 4 | 25.8 | 22.7–29.1 | 21.0 | Moderate | ||
| Zhangqiu | 3.10±0.50 | 1.10 | 4 | 33.9 | 28.5–40.8 | 27.6 | Moderate | ||
| Binzhou | 2.47±0.48 | 2.70 | 5 | 83.8 | 68.2–111 | 68.1 | High | ||
| Tai’an | 2.53±0.39 | 2.50 | 6 | 106 | 89.1–137 | 86.2 | High | ||
Tebufenozide and methoxyfenozide are insect growth regulators. They have been used widely in Shandong, especially in vegetable-growing areas. Our data (Table 6) revealed high and very high resistance ratios to tebufenozide. S. exigua from all ten populations tested between 2011 and 2012 showed very high resistance to methoxyfenozide, especially in Zhangqiu and Tengzhou (RR=672- and 612-fold in 2011 and 567- and 622-fold in 2012, respectively).
| Insecticides | Year | Poppulation | Fit of probit line | LC50 (mg/L) | 95% Fiducial limits | Resistance ratios | Resistance level | ||
|---|---|---|---|---|---|---|---|---|---|
| Slop±S.E. | χ2 | df | |||||||
| Tebufenozide | 2011 | Anqiu | 1.46±0.30 | 3.78 | 5 | 86.9 | 49.9–124 | 52.0 | High |
| Tai’an | 3.33±0.57 | 4.50 | 4 | 96.3 | 79.4–113 | 57.7 | High | ||
| Zhangqiu | 1.45±0.22 | 5.34 | 4 | 134 | 101–199 | 80.2 | High | ||
| Tengzhou | 1.54±0.23 | 4.12 | 5 | 152 | 115–227 | 91.0 | High | ||
| Binzhou | 1.10±0.30 | 5.32 | 5 | 203 | 128–375 | 122 | Very high | ||
| 2012 | Tengzhou | 1.88±0.20 | 6.44 | 4 | 28.2 | 23.4–33.7 | 16.9 | Low | |
| Zhangqiu | 1.50±0.22 | 5.57 | 4 | 97.5 | 76.2–133 | 58.4 | High | ||
| Tai’an | 4.25±0.51 | 1.22 | 5 | 140 | 127–156 | 83.8 | High | ||
| Anqiu | 1.56±0.31 | 3.50 | 4 | 199 | 143–292 | 119 | Very high | ||
| Binzhou | 1.64±0.32 | 7.45 | 4 | 210 | 154–304 | 126 | Very high | ||
| Methoxy- fenozide | 2011 | Anqiu | 2.08±0.33 | 3.93 | 5 | 21.0 | 16.3–27.8 | 104 | Very high |
| Binzhou | 1.52±0.26 | 4.24 | 4 | 27.4 | 18.5–36.8 | 136 | Very high | ||
| Tai’an | 2.26±0.30 | 2.40 | 4 | 39.8 | 31.9–49.5 | 198 | Very high | ||
| Tengzhou | 2.54±0.35 | 5.31 | 6 | 123 | 107–144 | 612 | Very high | ||
| Zhangqiu | 2.56±0.35 | 4.41 | 5 | 135 | 117–158 | 672 | Very high | ||
| 2012 | Anqiu | 1.52±0.30 | 2.34 | 4 | 26.3 | 18.8–40.5 | 131 | Very high | |
| Tai’an | 4.30±0.45 | 5.57 | 5 | 27.9 | 25.1–31.0 | 139 | Very high | ||
| Binzhou | 1.57±0.24 | 2.27 | 4 | 52.5 | 39.3–71.4 | 261 | Very high | ||
| Zhangqiu | 2.07±0.33 | 1.56 | 5 | 114 | 94.4–136 | 567 | Very high | ||
| Tengzhou | 3.29±0.37 | 3.67 | 5 | 125 | 111–141 | 622 | Very high | ||
The present study was conducted between 2011 and 2012, and demonstrated that the Shandong populations of S. exigua have varying degrees of resistance to eight insecticides. However, no resistance to chlorfenapyr was found in Shandong. In China, S. exigua populations resistant to conventional insecticides (beta-cypermethrin, chlorpyrifos and methomyl),4) emamectin benzoate, indoxacarb, spinosyn,8) tebufenozide, chlorantraniliprole, and flubendiamide have been found.13) This suggests that populations of this species in Shandong have the potential to develop resistance to a wide range of compounds. In recent years, concurrent outbreaks of this pest in Shandong have mostly been associated with the development of resistance to various insecticides. In Shandong, conventional insecticides, such as organophosphorus compounds, pyrethroids, and carbamate insecticides, have been widely used for pest control for more than 20 years. However, due to long-term usage, the control effect on S. exigua in the field has decreased rapidly. Many new pesticides with high toxicity to S. exigua have been favorably received by farmers.
The development rate of resistance by insects relies on the factors of selection pressure by insecticides, initial frequency of resistant genotypes, and the fitness of individual insects. No resistance to chlorfenapyr was found in Shandong populations, perhaps because it is very difficult to develop resistance against this compound. Zhang et al.14) indicated that an S. exigua strain with very low levels of resistance (RR=4.72-fold) to chlorfenapyr was obtained after selection for 12 generations in the laboratory. The spider mite Tetranychus urticae has been shown to reach 10-fold levels of resistance to chlorfenapyr after treatment for 12 generations.15) Therefore, chlorfenapyr should be applied for population management of S. exigua.
Emamectin benzoate belongs to the avermectin group of pesticides, which act as chloride channel activators.16) Although moderate resistance to emamectin benzoate was found in Binzhou, the LC50 value was very low (LC50 <1.20 mg/L). No increase was observed from 2011 to 2012, indicating that emamectin benzoate could also be used to manage S. exigua. Zhou et al.4) found that S. exigua showed low resistance to emamectin benzoate in Shandong between 2008 and 2010. Likewise, in Pakistan, emamectin benzoate is considered an effective insecticide for most populations, which exhibited very low to low levels of resistance in nine populations and medium levels of resistance in three out of 15 populations.17) Emamectin benzoate is thus considered an effective tool for the management of S. exigua.
Indoxacarb acts as a voltage-dependent sodium channel blocker, and it belongs to the group of oxadiazine insecticides.18) Many pests have the potential to develop resistance to indoxacarb. After selection against indoxacarb for six generations, a field population of Plutella xylostella evolved a 2595-fold increased resistance as compared to a susceptible population.19) Musca domestica20) and S. exigua5) can also develop high levels of resistance to indoxacarb in a relatively short period of time. Indoxacarb resistance increased in Shandong, with moderate levels of resistance found in four populations and high resistance levels found in two out of 10 populations tested. Therefore, it should be applied only sparsely.
Spinosad belongs to the spinosyn group and acts on nicotinic acetylcholine and gamma-aminobutyric acid (GABA) receptors.20,21) It has been found to be effective against pests of the orders Lepidoptera, Diptera, Thysanoptera, and some species of Coleoptera and Orthoptera.22,23) In our study, eight out of 10 populations of S. exigua exhibited moderate to high resistance to spinosad, while only two populations showed low resistance levels. Although spinosad has a novel action mechanism and no cross-resistance to many different insecticides, many pest species have still generated different levels of resistance. In Mexico, Osorio et al.22) demonstrated that S. exigua exhibited low to moderate resistance to spinosad, increasing from 16-fold to 37-fold. Similarly, Liriomyza trifolii produced high to very high resistance to spinosad in greenhouse ornamental plants in the United States.24)
Tebufenozide and methoxyfenozide are insect growth regulators, which are highly effective against Lepidopteran pests and have an excellent environmental and mammalian toxicological profile.25) In recent years, tebufenozide and methoxyfenozide have been widely used to control S. exigua in vegetable fields in Shandong. However, as they were overused to control insects, their effects on S. exigua decreased rapidly. Almost all populations in Shandong exhibited high to very high resistance ratios. Jia and Shen indicated that a tebufenozide-resistant strain of S. exigua had a high cross-resistance (RR=77.4-fold) to methoxyfenozide, and it has proved difficult to recover sensitivity to tebufenozide during a short period.7) The pesticide selection pressure and cross-resistance were the main causes of resistance to tebufenozide and methoxyfenozide. Hence, the application of this type of pesticide should be halted and replaced by using pesticides without cross-resistance to avoid or delay the development of pesticide resistance.
Diamide insecticides have emerged as one of the most promising new classes of insecticide chemistry due to their excellent insecticidal efficacy and high margins of mammalian safety. Chlorantraniliprole and flubendiamide, the first two insecticides from this class, demonstrate exceptional activity across a broad range of Lepidopteran pests.26) Chlorantraniliprole seemed to be an effective insecticide for most of the populations because it exhibited low or very low levels of resistance in eight populations and medium levels of resistance in two out of 10 populations tested. Although chlorantraniliprole is still an effective tool for management of S. exigua, the resistance ratios increased from 2011 to 2012. Lai et al.27) showed that the risk of developing resistance to chlorantraniliprole exists in S. exigua after continuous application and discovered that the mechanisms associated with the resistance of S. exigua to chlorantraniliprole were dependent on the activities of mixed-function oxidases and esterases. Xing et al.28) considered cytochrome P450 monooxygenase to play a dominant role in the resistance of P. xylostella to chlorantraniliprole. Similar to chlorantraniliprole, flubendiamide resistance of S. exigua increased in Shandong. This is interesting because flubendiamide is rarely used in Shandong. Flubendiamide resistance in S. exigua might be a typical case of cross-resistance acquired chlorantraniliprole. Flubendiamide has the same target and mechanism as chlorantraniliprole,6) and cross-resistance between chlorantraniliprole and flubendiamide in P. xylostella has been reported previously.29)
Chemical control plays an important role in the control of S. exigua in Shandong. Plant growers are already experiencing failures in controlling S. exigua. Conventional insecticides have been replaced with new and more potent compounds that employ novel modes of action. However, some S. exigua populations have developed resistance to some of the new insecticides on different levels. Using chemicals is insufficient for successful control and pest management; biological and physical control measures should be applied based on the occurrence, regularity, and ecology of S. exigua. Using radar, a simultaneously operated searchlight trap, and a ground light trap at a site in Langfang in 2002, Feng et al.30) observed the migration of S. exigua and indicated that the insect was a high-altitude nocturnal windborne migrant in northern China. Thus, trapping and killing the adults in UV light traps is an effective control measure during the eclosion and migration period. The authors have observed natural predators of S. exigua in the field, such as Harmonia axyridis, Chrysoperla sinica, Snelleniua manila and Telenomus remus Nixon, as they collected larvae and pupae from most of the areas during the study period. Due to their high selectivity, new insecticides, such as chlorantraniliprole, flubendiamide, spinosyn, tebufenozide and methoxyfenozide, should help to protect these natural predators.
This work was supported by a grant from the Special Research Funding for Public Benefit Industries from the Ministry of Agriculture of China (200803007).
