Examination of stability of resistance and cross-resistance patterns to acylurea insect growth regulators in field populations of the diamondback moth,Plutella xylostella,from Malaysia

In teflubenzuron and teflubenzuron/chlorfluazuron laboratory-selected sub-populations of a Cameron Highlands (1988) strain of Plutella xylostella L., resistance to teflubenzuron reached much greater levels than chlorfluazuron but declined at a correspondingly greater rate to give similar resistance levels after 10–13 generations. There was little evidence of cross-resistance to flufenoxuron and none to hexaflumuron in these populations. Reselection of a Cameron Highlands population with teflubenzuron after 22 generations resulted in a significant (P < 0.05) increase in resistance after three generations. In two more recently collected strains from lowland Malaysia, Sawi (1991) and Serdang (1992) high levels of resistance to teflubenzuron were found, but only the former showed any apparent resistance to chlorfluazuron. There was also evidence of resistance to diflubenzuron in the Sawi strain (Serdang not examined). Resistance to teflubenzuron appeared to be unstable in unselected sub-populations of both lowland strains. Selection of the Sawi strain with teflubenzuron did not effect cross-resistance to chlorfluazuron, while attempted selection with chlorfluazuron had no effect on resistance to either compound. Selection of the Sawi strain with diflubenzuron increased resistance to teflubenzuron but had no effect on resistance to chlorfluazuron. In contrast to the Sawi strain, selection of the Serdang strain with teflubenzuron resulted in cross-resistance to chlorfluazuron. Synergist experiments with piperonyl butoxide (PB) and S,S,S-tributyl phosphorotrithioate (DEF) suggested that resistance to teflubenzuron in the lowland strains may involve both microsomal monooxygenases and esterases. The ovicidal activity of teflubenzuron, chlorfluazuron, hexaflumuron and flufenoxuron was consistently less than their activity against second-instar larvae, hexaflumuron, followed by teflubenzuron, showing the greatest relative ovicidal activity. Hexaflumuron was markedly more active than teflubenzuron against eggs from acylurea-selected Cameron Highlands populations and there was some evidence that resistance to teflubenzuron may be expressed at a lower level in eggs than in larvae in one such strain.     

Genetic basis of resistance and studies on cross-resistance in a population of diamondback moth, Plutella xylostella (Lepidoptera: Plutellidae)

The genetic basis of abamectin resistance was studied in a strain of the diamondback moth, Plutella xylostella (L), following laboratory selection of a field population collected at Xuanhua, Hebei Province, China. Data from the testing of F1 progeny from reciprocal crosses between abamectin-resistant and abamectin-susceptible strains indicated that resistance might be autosomal and incompletely recessive with a degree of dominance of -0.13. Chi-squared analyses from the response of a backcross of crossed F1 progeny and the resistant strain and F2 progeny were highly significant, suggesting that the resistance was probably controlled by more than one gene. The results of cross-resistance studies showed that there was little cross-resistance between abamectin and four pyrethroid insecticides (deltamethrin, beta-cypermethrin, fenvalerate and bifenthrin) and no cross-resistance between abamectin and the acylureas chlorfluazuron or flufenoxuron.     

Persistence of insect growth regulators against the rice weevil,sitophilus oryzae,in grain commodities

Ten insect growth regulators (IGRs): four juvenile hormone analogs (JHAs) — fenoxycarb, MV-678, R-20458 and dofenapyn, and six molt inhibitors (MIs) — triflumuron, chlorfluazuron, hexaflumuron, teflubenzuron, Dowco 439 and flufenoxuron; and the organophosphate malathion, were evaluated for residual activity in stored grain for 2 years against the rice weevil,Sitophilus oryzae (L.). Triflumuron, flufenoxuron, teflubenzuron, chlorfluazuron, hexaflumuron and fenoxycarb, each at a concentration as low as 1 ppm a.i., were superior to 10 ppm malathion throughout the experiment. The six MIs, each tested at 50 and 100 ppm a.i., gave complete control for 2 years. Moreover, a concentration as low as 1 ppm a.i. of triflumuron, teflubenzuron, flufenoxuron and chlorfluazuron had good residual activity for 1 year post-treatment and provided almost complete grain protection during this period. Among the JHAs, fenoxycarb had the best residual activity. It gave almost complete control (96.6%) ofS. oryzae at 10 ppm for 18 months post-treatment. The other JHAs at the four test concentrations failed to produce this level of control. The viability of the wheat grain was not affected by any of the insecticidal treatments, with 90% germination recorded in treated and control, grains at all intervals tested.     

Synergism of teflubenzuron and chlorfluazuron in an acylurea-resistant field strain of Plutella xylostella L. (lepidoptera: Yponomeutidae)

Topical application of the synergists piperonyl butoxide (PB) and S,S,S-tributyl phosphorotrithioate (DEF) to second-instar larvae of a standard laboratory strain (FS) and an unselected Malaysian field strain (CH) of the diamondback moth Plutella xylostella had no significant effect on the toxicity of the acylurea insecticides, chlorfluazuron and teflubenzuron, in a subsequent leafdip bioassay. In contrast, pre-treatment with PB or DEF in acylurea-selected subpopulations of the CH strain with varying levels of cross-resistance to chlorfluazuron and teflubenzuron significantly increased (up to 34-fold and 28-fold, respectively) the toxicity of both compounds, suggesting that microsomal monooxygenases and esterases may be involved in resistance. The addition of a mineral oil, ‘Sunspray 6E’, to topically-applied chlorfluazuron consistently reduced its LD₅₀ value, and the effect of the oil appeared to be greatest on the most resistant population of P. xylostella. However, the effects of the oil were not significant (P > 0·05) and further studies are necessary to determine whether a penetration factor is present in the CH strain.     

小菜蛾对阿维菌素的抗性机制及交互抗性研究

用叶片药膜法研究了阿维菌素抗性小菜蛾 Plutella xylostella （L.）品系 对常用药剂的交互抗性谱以及增效醚（PB）和磷酸三苯酯（TPP）的增效作用。小菜蛾对阿 维菌素与高效氯氰菊酯、溴氰菊酯、氰戊菊酯和联苯菊酯等菊酯类药剂间具有比较低的交互 抗性，对后者抗性为3～20倍，对阿维菌素的抗性为575.6倍；对氟虫脲和氟啶脲没有交互抗 性。PB和TPP对阿维菌素分别增效8.2和5.5倍，说明小菜蛾对阿维菌素的抗性可能与多功能 氧化酶（MFO）和羧酸酯酶有关。通过差光谱技术测定了阿维菌素抗性和敏感小菜蛾细胞色 素P450的含量，抗性品系是敏感品系的1.38倍。     

Aqueous solubilities,photolysis rates and partition coefficients of benzoylphenylurea insecticides

The aqueous solubilities and octanol–water partition coefficients (K_OW) of the benzoylphenylurea (BPU) insecticides teflubenzuron, chlorfluazuron, flufenoxuron and hexaflumuron were determined in comparison with the more extensively studied diflubenzuron. Both teflubenzuron and hexaflumuron were substantially less water‐soluble (9.4 (± 0.3) µg litre⁻¹ and 16.2 (± 0.5) µg litre⁻¹ in water, respectively) than the value previously reported for diflubenzuron (89 (± 4) µg litre⁻¹ in water). Log K_OW values for diflubenzuron, teflubenzuron, hexaflumuron, flufenoxuron and chlorfluazuron were 3.8, 5.4, 5.4, 6.2 and 6.6, respectively, as determined using reverse‐phase HPLC. Photodegradation of hexaflumuron, teflubenzuron and diflubenzuron in water indicated hexaflumuron to be the most rapidly degraded of the three compounds at pH 7.0 (t_1/2 = 8.6 (± 0.4) h) and pH 9.0 (t_1/2 = 5 (± 1) h); diflubenzuron was the slowest of the three pesticides to degrade in pH 7.0 (17 (± 4) h) and pH 9.0 (8 (± 2) h) buffered water. In a solar simulator using river water buffered to pH 9.0, teflubenzuron, hexaflumuron and diflubenzuron half‐lives were 20 (± 4), 15 (± 2) and 12 (± 1) h, respectively; dark controls showed no loss of parent BPU over similar time periods. © 2000 Society of Chemical Industry     

Insecticide resistance inFrankliniella occidentalis: Corroboration of laboratory assays with field data and cross-resistance in a cypermethrin-resistant strain

Field efficacy of five insecticides to a greenhouse strain ofFrankliniella occidentalis (Pergande) (Thysanoptera: Thripidae) of known resistance status was studied. In a greenhouse trial, the mortality of aF. occidentalis strain (Uncali) which exhibited resistance only against cypermethrin in laboratory assays, reached 56%, 68%, 82%, 94% and 99%, 24 h after application of abamectin, cypermethrin, endosulfan, malathion and methomyl, respectively. In the laboratory, for larval mortality equal to that achieved in the field, much lower doses — half of the field dose of cypermethrin and endosulfan, and one-third and one-sixth of malathion and methomyl, respectively, were required in topical application. Because of differences between laboratory and field responses ofF. occidentalis to insecticides, a sound resistance evaluation program to determine field efficacy of insecticides is needed. Resistance increased 28.0- and 139.3-fold at LC₅₀ in theF. occidentalis strain subjected to 27 and 44 cycles of cypermethrin selection, respectively. A slight cross-resistance of two- to threefold was found with endosulfan, malathion and methomyl and no cross-resistance was observed with abamectin. http://www.phytoparasitica.org posting August 4, 2008.     

Evidence for Resistance to Bacillus thuringiensis (Bt) subsp. kurstaki HD-1, Bt subsp. aizawai and Abamectin in Field Populations of Plutella xylostella from Malaysia

The efficacy of Bacillus thuringiensis (Bt) subsp. kurstaki HD-1 (‘Dipel’™; Btk; CryIA & CryII) and Bt. subsp. aizawai (‘Florbac’™; Bta; CryIA & CryIC) was assessed against larvae from various field populations of Plutella xylostella (F2 generation) collected in the Cameron Highlands, Malaysia in April 1994 and a lowland population (SERD 2; F10 generation) collected in December 1993. Evidence of resistance to Btk and to a lesser extent Bta is reported in these populations (LC₅₀ Toxicity Ratios [TR]=3–14 and 2–8 respectively), most notably in SERD 2. The first recorded evidence of resistance to abamectin (TR=17–195-fold) in field populations of P. xylostella is also reported. In an unselected sub-population of SERD 2, the TR values for Btk, Bta and abamectin declined 2- to 3-fold (P<0·01) over six generations in the laboratory (F10–F16) while in sub-populations of SERD 2 selected with these products (F11–F15) there was a significant (P<0·01) increase in the TR (15-, 3- and 2·5-fold respectively) when compared with the F10 generation. This suggests the presence of marked resistance to Btk and some resistance to Bta and abamectin. There is also evidence of slight cross-resistance to Btk in the Bta-selected sub-population but no evidence for the reverse selection of resistance or for cross-resistance between Btk and abamectin. Concurrent selection studies (F11–F15) with another sub-population of SERD 2 demonstrated resistance to the acylurea insect growth regulator, teflubenzuron (‘Nomolt’™) (29-fold increase in TR). Based on the selection experiments with SERD 2, estimates of realised heritability (h²) of resistance gave very high values for teflubenzuron and Btk (c.0·7) and moderate values for abamectin and Bta (c.0·3). The results are discussed in relation to integrated pest management (IPM) and insecticide resistance management (IRM) strategies for P. xylostella.     

小菜蛾对阿维菌素的抗性选育及交互抗性研究

用阿维菌素对小菜蛾进行室内抗性汰选,选择压为约杀死种群70％的剂量。连续施药7～8次即表现抗性趋势,汰选至11代,获得抗性指数为80.71的抗性种群(ABM-R)。ABM-R种群对所测试的有机磷杀虫剂乙酰甲胺磷、有机氯类的硫丹、沙蚕毒素类的杀螟丹、氨基甲酸酯类的灭多威、微生物杀虫剂Bt、磺胺脲类衍生物丁醚脲、芳基取代的吡咯杂环化合物虫螨腈和昆虫生长调节剂定虫隆、虫酰肼无交互抗性(R/S为0.73～1.19),而对苯基吡唑类氟虫腈的敏感性却有所上升(R/S为0.22)。增效剂试验显示,PB和唧对ABM-S种群无增效作用,而对ABM-R种群增效作用显著,增效比分别为24.57和13.61,表明多功能氧化酶和羧酸酯酶在抗性机制中可能起重要作用。     

不同药剂对甜菜夜蛾幼虫毒力和田间应用效果研究

利用浸渍法分别测定虫螨腈等不同药剂对甜菜夜蛾3龄及5龄幼虫的毒力，结果表明：不同药剂对甜菜夜蛾幼虫的毒力存在明显差异，一些特异杀虫剂对甜菜夜蛾幼虫的毒力普遍高于试验用常规化学杀虫剂的毒力；同一药剂对不同龄期幼虫毒力的差异程度也不同，如虫螨腈、氟啶脲、氟铃脲、甲胺基阿维菌素、阿维菌素、虫酰肼等药剂对甜菜夜蛾3龄及5龄幼虫毒力的差异程度明显低于顺式氯氰菊酯、丙溴磷、灭多威等化学药剂。同时，作者还对12种药剂分别稀释1000倍对甜菜夜蛾5龄幼虫不同时间的毒力进行了观察，除常规化学农药出现中毒现象较快外，虫螨腈、甲胺基阿维菌素也表现较快中毒症状，田间药效试验进一步验证防治甜菜夜蛾宜选用虫螨腈、氟啶脲、氟铃脲、甲胺基阿维菌素、阿维菌素、虫酰肼等药剂。     

广州、深圳地区小菜蛾对定虫隆、Bt的抗性监测

采用 Potter's 喷雾塔喷雾的方法测定广州和深圳菜区小菜蛾对儿丁质合成抑制剂定虫隆、氟虫脲、伏虫隆、氟铃脲的抗药性。结果表明,深圳菜区小菜蛾对定虫隆产生明显抗性,1990年11月测定抗性倍数为15.0,1991年12月为70.9,对氟虫脲、伏虫隆、氟铃脲具有不同程度交互抗性,抗性倍数分别为26.7,15.5 和5.8。广州菜区小菜蛾对以上药剂均未产生抗性(抗性倍数最高为2.8)。采用浸渍法测定小菜蛾对苏云金杆菌 Bt 抗性的结果表明,深圳菜区小菜蛾对 Bt 的抗性逐年提高,1990年12月测定的抗性倍数为21.1,1991年12月为26.9,1992年3月为35.0,广州菜区小菜蛾具有强的耐药性或低抗药性,抗性倍数8—9。     

The inhibition of chitin synthesis in spodoptera littoralis larvae by flufenoxuron,teflubenzuron and diflubenzuron

The chitin precursor [¹⁴C] N-acetylglucosamine injected into the haemolymph of Spodoptera littoralis (Boisduval) larva was incorporated into the chitin exponentially with time. When caterpillars were injected with precursor at the commencement of feeding on acylurea-treated leaf discs, flufenoxuron, teflubenzuron and diflubenzuron were found to be equally effective inhibitors of chitin synthesis, measured after 21 h. The dose response curves by feeding are not parallel, indicating that the relative potency of the compounds will vary across the dose range. When chitin precursor was injected simultaneously with topically applied diflubenzuron, flufenoxuron or teflubenzuron, all three acylureas were found to be equally effective as inhibitors of chitin synthesis when measured after five hours. The I₅₀values (50% inhibition of chitin synthesis) were not significantly different; average 600 ng, compared with LD₅₀values (50% lethal dose) of ～ 13 ng for flufenoxuron and teflubenzuron but 130 ng for diflubenzuron (topical application). Injection of precursor 24 h after topical application of insecticide gave an I₅₀value which had dropped 670- and 150-fold for flufenoxuron and teflubenzuron respectively but only 20-fold for diflubenzuron. It is postulated that the reason for the low increase in diflubenzuron effectiveness with time was due either to less diflubenzuron than flufenoxuron reaching the site of action, or more probably, a faster rate of metabolism and excretion for diflubenzuron. The lower toxicity of diflubenzuron compared with flufenoxuron and teflubenzuron may not be due to any inherent differences in biochemical effectiveness, but rather to different penetration/metabolism properties.     

Comparative studies on acylurea insect growth regulators and neuroactive insecticides for the control of the armyworm Spodoptera exempta walk

Leaf-dip bioassays with commercial pesticide formulations were used throughout this study. Third-instar larvae of Spodoptera littoralis were bioassayed for comparative purposes. The end-point mortality against the third-instar larvae (L3) of two Spodoptera exempta populations (Bangor and Gent) and a S. littoralis population (NRI) was found to occur at 72 h and 120 h for neuroactive compounds (carbamate, organophosphate and pyrethroid insecticides) and acylureas respectively. Overall, the acylureas were found to be more toxic (up to 240-fold at the LC₅₀ level) than the most active of the neuroactive compounds tested, the pyrethroids. This difference was greatest against S. littoralis, which was markedly less sensitive to the latter compounds. The Gent population of S. exempta was also found to be significantly less sensitive (7-folt at LC₅₀) to the pyrethroid, lambda-cyhalothrin, than the Bangor strain. Two of the most toxic neuroactive and non-neuroactive compounds, the pyrethroid, cypermethrin and the acylurea, teflubenzuron were chosen for further studies with S. exempta (Gent) and S. littoralis. Relatively little difference in the toxicity of teflubenzuron was observed against different larval instars (L2, L3 and L4) of S. exempta when compared with cypermethrin. Choice-chamber-experiments on feeding behaviour showed that when L3 larvae of either species were exposed to both treated and untreated leaves. teflubenzuron had no significant effect on larval distribution, the percentage of each leaf disc consumed or larval weight gain when compared with untreated controls. In contrast, a marked dose-dependent repellent response to cypermethrin-treated leaves was shown by S. littralis after 6 h. together with corresponding reductions in the percentage of the treated leaf consumed and in larval weight gain. Similarly, when L3 S. exempta were exposed only to pesticidetreated leaves, cypermethrin significantly reduced premoult larval weight gain, whereas tefiubenzuron had no premoult effect. However, following exposure of L3 S. exempta and S. littoralis to teflubenzuron for 48 h prior to moulting, L4 which survived the moult showed reduced weight gain compared with controls, the reduction being more pronounced on maize than on artificial diet. This antifeedant effect increased in proportion to the concentration of teflubenzuron originally administered. The effect of exposure time to teflubenzuron was examined by placing L3 S. exempta and S. littoralis on leaves treated with the approximate LC₅₀ or LC₉₅ dose for 6–48 h. The exposure time required to produce appreciable mortality (relative to LC values) within the treated population was found to be 24 h.     

Biochemical mechanisms conferring cross-resistance between tebufenozide and abamectin in Plutella xylostella

Experiments have been carried out to confirm the cross-resistance between abamectin and tebufenozide in Plutella xylostella and demonstrate its mechanism. The results showed that the resistant strain of P. xylostella selected by tebufenozide (RF 99.38) really showed high cross-resistance to abamectin (RF 29.25). When this strain was subjected to resistance decaying treatment, breeding without contacting any insecticides, and abamectin resistance selection for 20 generations, the former resulted in decrease of its resistance to both tebufenozide and abamectin to about one third of the original (RF 35.03 and 11.67, respectively), and the later enhanced its resistance to abamectin dramatically (RF 303.77), but not to tebufenozide(RF 50.04). PBO showed high synergism to abamectin (SR 2.11-12.23), and the synergism ratio positively related to the resistance level among different strains. Enzyme analysis also proved that the activity of cytochrome P450 monooxygenase (MFO) was notable enhanced in the strains resistant to both tebufenozide and abamectin (1.71- to 3.01-fold). Based on discussion, it was concluded that tebufenozide selection could resulted in significant cross-resistance of P. xylostella to abamectin. The major mechanism for the cross-resistance should be the enhancement of MFO activity. For resistance management, tebufenozide and abamectin would not recommend for rotational use.     

五种昆虫生长调节剂对韭菜迟眼蕈蚊的致毒作用

为明确昆虫生长调节剂对韭菜迟眼蕈蚊幼虫的毒力差异、作用特点和田间控制效果,采用药液定量滴加法比较了5种昆虫生长调节剂对其不同龄期幼虫的毒力、LC50对4龄幼虫生长发育和繁殖的影响,以及药剂致毒的温度效应和盆栽防效。结果表明:5种昆虫生长调节剂对韭菜迟眼蕈蚊2龄和4龄幼虫的毒力均以虱螨脲最高,LC50分别为0.173 mg/L和0.295 mg/L,氟铃脲和氟啶脲次之,均高于对照药剂辛硫磷;而吡丙醚和灭蝇胺低于辛硫磷。5种昆虫生长调节剂以LC50处理4龄幼虫后,虱螨脲、氟铃脲和氟啶脲对其存活幼虫后续发育历期、成虫羽化率和繁殖力的影响较大,尤其是单雌产卵量较清水对照分别降低72.6%、57.8%和58.1%;灭蝇胺也可使成虫产卵量降低;吡丙醚可引起卵孵化率下降。该类药剂对其幼虫的毒力基本不受环境温度的影响;虱螨脲、氟铃脲和氟啶脲7.5、15 g a.i./667 m2处理对当代4龄幼虫的防效较低,但对下一代幼虫的防效可达93%以上;虱螨脲、氟铃脲和灭蝇胺与新烟碱类的噻虫胺混用,可显著提高药剂的速效性,且持续控制作用好。     

小菜蛾对唑虫酰胺的抗性监测、抗性生化机制及交互抗性

为了明确小菜蛾Plutella xylostella对唑虫酰胺的抗性特征,采用生物生化方法测定了江西省5个蔬菜产区小菜蛾田间种群对唑虫酰胺的抗性水平,并研究了小菜蛾唑虫酰胺抗性品系对其它药剂的交互抗性和生化抗性机制。结果显示,分宜县和高安市小菜蛾田间种群对唑虫酰胺尚未产生明显抗性,永丰县、德安县和余江县小菜蛾种群对唑虫酰胺产生了低水平的抗性,抗性倍数为5.20~8.20倍;小菜蛾唑虫酰胺抗性品系对阿维菌素、氯虫苯甲酰胺、氟虫双酰胺和茚虫威有中低水平的交互抗性,抗性倍数分别为11.72、3.44、2.77和2.20倍,而对溴虫腈、定虫隆和丁醚脲无交互抗性;增效剂磷酸三苯酯和胡椒基丁醚对小菜蛾唑虫酰胺抗性品系均有显著增效作用,增效倍数分别为3.42倍和2.64倍;唑虫酰胺抗性品系的酯酶和多功能氧化酶活性均显著提高,分别为敏感品系的2.18倍和1.64倍。研究表明,小菜蛾对唑虫酰胺产生抗性可能与酯酶和多功能氧化酶活性的升高有关。     

Speed of action and toxicity of acylurea insect growth regulators against Spodoptera exempta (Walk.) and Spodoptera littoralis (Boisd.) larvae: Effect of inter-moult age

Ultra-low volume (ULV) spray bioassays at droplet densities (20–80 drops cm^?2) corresponding to field deposits were used to determine the efficacy of two acylurea insect growth regulators, teflubenzuron (‘Nomolt®’) and flufenoxuron (‘Cascade®’) against three different age groups of third-instar Spodoptera exempta and Spodoptera littoralis larvae. While no response (mortality and/or abnormal moulting with impaired ability to feed) was observed with one-day-old larvae until 48 h after treatment, two-day-old larvae showed some response (27–93% abnormal moulting and 3–10% mortality when larvae were sprayed; 80–100% abnormal moulting and up to 83% mortality when larvae and plants were sprayed) after 24 h and up to 100% mortality at 120 h. Studies with non-feeding, pre-moult larvae showed that cuticular uptake of acylureas was sufficient to affect an appreciable proportion of larvae after 24 h, with up to 100% mortality at 120 h. More detailed studies with pre-moult S. littoralis larvae showed that, while a few larvae which moulted between 1 and 5 h after treatment showed abnormal moulting to the L4 stage, a much larger proportion of larvae which moulted 6–20 h after treatment were affected. Previous studies with S. exempta have shown that different larval instars show similar levels of sensitivity to acylureas. The present work suggests that, while the age distribution within instars has little influence on the end-point mortality of acylureas, it can profoundly modify the speed of response, the majority of larvae within an instar being effectively controlled at the moult following treatment. The significance of these observations in relation to the crop protection activity of acylureas against field populations of Spodoptera larvae is discussed.     

甜菜夜蛾对虫酰肼的抗药性研究初报

在室内用虫酰肼对甜菜夜蛾幼虫进行抗药性选育,经过12代汰选,甜菜夜蛾对虫酰肼产生了5.47倍的抗性,抗性发展缓慢。该种群对甲氧虫酰肼有一定交互抗性,对有机磷、氨基甲酸酯、拟除虫菊酯类等多种杀虫剂都不存在交互抗性,抗性比值在0.83~1.63倍之间。     

Cross-resistance and biochemical mechanisms of abamectin resistance in the western flower thrips, Frankliniella occidentalis

Abamectin resistance was selected in the western flower thrips [Frankliniella occidentalis (Pergande)] under the laboratory conditions, and cross-resistance patterns and possible resistance mechanisms in the abamectin-resistant strain (ABA-R) were investigated. Compared with the susceptible strain (ABA-S), the ABA-R strain displayed 45.5-fold resistance to abamectin after 15 selection cycles during 18 generations. Rapid reversion of abamectin resistance was observed in the ABA-R strain in the absence of the insecticide selection pressure. Moderate and low levels of cross-resistance to chlorpyrifos (RR 11.4) and lambda-cyhalothrin (3.98) were observed in the ABA-R strain, but no significant cross-resistance was found to spinosad (2.00), acetamiprid (1.47) and chlorfenapyr (0.26). Our studies also showed that the esterase inhibitor S,S,S-tributyl phosphorotrithioate (DEF) and glutathione S-transferase inhibitor diethyl maleate (DEM) were not able to synergize the toxicity of abamectin, whereas the oxidase inhibitor piperonyl butoxide (PBO) conferred a significant synergism on abamectin in the ABA-R strain (SR 3.00). Biochemical analysis showed that cytochrome P450 monooxygenase activity of the ABA-R strain was 6.66-fold higher than that of the ABA-S strain. It appears that enhanced oxidative metabolism mediated by cytochrome P450 monooxygenases was a major mechanism for abamectin resistance in the western flower thrips.     

Differential susceptibility to abamectin and two bioactive avermectin analogs in abamectin-resistant and -susceptible strains of Colorado potato beetle, Leptinotarsa decemlineata (Say) (Coleoptera: Chrysomelidae)

As resistance to currently used insecticides increases in the Colorado potato beetle (CPB), Leptinotarsa decemlineata (Say), abamectin and its 4″-epi and 8,9-oxide analogs may serve as likely replacements if proven effective. We previously selected an abamectin-resistant strain of CPB (AB-F) that is suitable for the determination of cross-resistance to these two bioactive avermectin analogs. Using bioassay and logit analysis, the present work shows that, on average and following normalization by weight, the larval stages of the insecticide-susceptible SS strain are equally sensitive to the toxic action of abamectin and its 4″-epi and 8,9-oxide analogs, indicating that all three compounds retain high bioactivity towards the susceptible larval stages. Abamectin and the two analogs also are similar in toxicity to the larval stages of the AB-F strain. However, the AB-F larval stages are significantly less sensitive on average to these insecticides than the SS larval stages, indicating resistance to abamectin and cross-resistance to both the two analogs tested. Likewise, adults of the AB-F strain are significantly less sensitive to the toxic action of abamectin and the two analogs compared to SS adults, also indicative of resistance and cross-resistance. Abamectin is significantly more toxic, however, to both SS and AB-F adults, respectively, than either of the two analogs. The reduction in sensitivity was particularly evident in SS adults to both the 4″-epi and 8,9-oxide analogs. Additionally, adults of the SS strain are significantly less sensitive to the toxic action of abamectin and the two analogs when compared to SS larval stages. There is no significant differences, however, in the toxicity of these three insecticides, respectively, between larval and adults stages of the AB-F strain. This phenomenon results in lower resistance ratio (RR) values calculated for the two avermectin analogs compared to those calculated for abamectin regardless of the insect stage examined but is particularly evident and significant in the adult stage. This relative decrease in resistance levels is primarily associated with SS adults that are less sensitive to the toxic action of these insecticides. The decrease in abamectin toxicity is apparently due to significantly increased levels of P450 associated with SS adults versus forth instars and to similar levels in adults of the SS and AB-F strains. Because abamectin resistance in CPB is due in large part to enhanced oxidative metabolism of abamectin, it is likely that the SS adults are more tolerant to abamectin due to an enhanced level of oxidative detoxification. Finally, neither abamectin nor the two analogs are structurally protected at the specific molecular locations, C₃″, C₂₄, and C₂₆ carbons, which leads to enhanced oxidative metabolism, resistance and cross-resistance. Structurally-protected avermectin analogs at these vulnerable intramolecular sites are likely to be more effective insecticides in suppressing the development of oxidative detoxification-based resistance to abamectin, as would the use of oxidative synergists.