Studies On Selection For Tebufenozide Resistance And Resistance Revolution In Spodoptera Exigua (Hübner)

The selection for tebufenozide resistance and resistance revolution in Spodoptera exigua (H(u|¨)bner) were studied in this paper. Main results are summarized as follows:1. After selection for 12 generations by mass and single selection in the laboratory, the selected strain showed 5.24-fold resistance to tebufenozide compared with the susceptible strain. It was beneficial for speeding up resistance selection to alternate a few generations single pair selection in mass selection.2. Tebufenozide had a deep influence on the pupae of S. exigua. The pupation rate of the selected strain was obviously lower than that of the susceptible strain. When the concentration of inoculating larvae with poisonous artificial diet was the same, the pupation rate tended to recover. For example, when the concentrations of F₆ ,F₇ ,F₈ were all o.67 mg/kg, their pupation rates were 3.70% ,9.08% ,11.20% respectively. With the development of the selected generations, the abnormal pupae rate rose slowly at first and the F₇ increased to its maximum—33.81%, the F₈ began to decrease, however, they were all higher than the susceptible strain. The average pupa weight of the selected strain fall, and when the inoculated concentration rose, the falling rate increased greatly. For example, when the inoculated concentration increased from 0.67 mg/kg to 0.80 mg/kg, the female average pupa weight decreased from 89.43 mg to 75.19 mg. And the female pupae rate varied from 46.24% to 59.51%. It indicated that the pressure of choice imposed on the susceptible strain influenced the growth of the selected strain deeply but had little influence on the female and male pupae rate of the susceptible strain.The emergence rates of female and male adults were lower than that in the susceptible strain. The F₅ generation was the lowest, and 79.38%,71.07% respectively lower than the susceptible strain and they came back slowly again. The abnormal adult rate of the selected strain was higher than that in the susceptible strain, as 1.43～3.17 times and 1.11～3.13 times as that in the susceptible strain. It indicated that tebufenozide had some inhibition on the adult emergence of 5. exigua.The LT₅₀ of female and male adults in the selected strain condensed by 1.59d and 2.17d respectively than that in the susceptible strain .However, the total egg production per female was obviously lower than that in the susceptible strain. The F₁₀ generation was the lowest, just as 0.30 times as that in the susceptible strain. And the egg hatching rate changes little, but still lower than the susceptible strain. F₁₀ and F₁₁ generations had great differences with the susceptible strain, respectively amounting to as 56.03% and 55.53% as the susceptible strain. It indicated that tebufenozide's influence on adults of S. exigua demonstrated not only in terms of the cut of the adult longevity but also in terms of the decrease of the total egg production per female and egg hatching rate.3. Compared with the selected generation, the pupation rate, the abnormal pupae rate, the average pupae weight, the emergence rate, the abnormal adult rate, LT₅₀, the total egg production per female and the egg hatching rate in the offspring of the selected generation came back to different degree. For example, compared with the F₆ generation, the total egg production per female in the F₆₍₃₎ and F₆₍₆₎ generations increased to 180.67 and 229.00 respectively. Compared with the F₇ generation, the pupation rate in F₇₍₁₎, F₇₍₂₎, F₇₍₃₎ generations increased to 61.12%, 58.52% and 65.85% respectively, but still could not come back to the susceptible strain level. It indicated that tebufenozide had strong after effect and persistence, and it could influence at least three generations. It needed several generations if the selected strain wanted to come back to the susceptible strain.4. The tebufenozide selected strain had no significant cross-resistance to methoxyfenozide, chlorfluazuron, chlorfenapyr and chlorpyrifos (1.00, 0.89, 0.82 and 0.94-fold respectively), and low level to methomyl(1.45-fold), betacyfluthrin(1.78-fold) and spinosad(2.52-fold).5. A day old eggs of S. exigua treated with tebufenozide showed that, in the susceptible strain and 9th selected generation, hatching IC₅₀ were 179.38, 160.71 mg/kg respectively and larvae survival IC₅₀ were 18.64, 30.53mg/kg respectively. It indicated that tebufenozide inhibited the egg hatching rate and larvae survival rate of S. exigua in the susceptible strain and in the selected strain. But in the terms of influence on the egg hatch rate, it was higher in the susceptible strain than in the selected strain, while in the terms of the larvae survival rate, it was higher in the selected strain than in the susceptible strain.Newly emerged larvae(1st instar)of S. exigua were also treated with tebufenozide. In the susceptible strain and 8th selected generation, the larvae survival rate and the pupation rate fell as tebufenozide concentration increased and the larva development period lengthened. Compared with 0.08 mg/kg, the larvae survival rates were 0.04 and 0.24 times respectively; the pupation rates were 0.05 and 0.21 times respectively and the larva period lengthened 2.71 and 6.19d respectively when the inoculated concentration was 0.40 mg/kg. When the inoculated concentration was the same, the larvae survival rate and the pupation rate in the selected strain were higher than that in the susceptible strain and 1.54—10.01 and 1.70—8.01 times respectively, While the period of larvae development shortened 0.94～2.59d. It indicated that the influence on the selected strain was weaker than that on the susceptible strain.When the 3rd instar larvae of S. exigua were treated with tebufenozide in the susceptible strain and 11th selected generation, compared with 0.20 mg/kg, the pupation rates were 6.00 and 4.60 times when the inoculated concentration was 0.125mg/kg. It showed that the pupation rate fell with the increase of the concentration. The average pupa weight tended to decrease, and in the same concentration, differences between the two strains were not so great. The female pupae rate varied slightly with increase in concentration, but the female pupae rate in the susceptible strain was below 50% while above 50% in the selected strain. The influence on the emergence rate was weak in other treatment concentration except 2.00 mg/kg.Comparison of the relative fitness of tebufenozide on selected and susceptible strains of S. exigua, indicated that the selected strain possessed development and reproductive disadvantages. The selected strain had a fitness of 0.31 relative to the susceptible strain.6.Biochemical study showed that there were significant differences in the activities of CarE and GST among the selected strain and its treatments, natural and susceptible strains. CarE activities of the selected strain and its treatments and natural strain were 1.20, 1.34, 1.94, 1.80 and 2.67-fold respectively; GST activities were 2.34, 5.12, 5.62, 3.76 and 0.97-fold respectively. The results suggested that the enhanced activities of CarE and GST were an important resistance mechanism in S. exigua to tebufenozide.The Km value of the selected strain was 0.83 times lower than that of in the susceptible strain. The significant differences between the two strains suggested that the selected strain had stronger affinity of CarE to a-NA, which probably resulted in the higher activity of CarE. The Km and Vmax values of the natural strain were 2.33 and 4.97 times respectively higher than that of the susceptible strain, suggesting that the quality and quantity of CarE were different. The esterase isoenzymes polyacrylamide gel electrophoresis (PAGE) also showed the changes of the CarE activities and kinetic constants.7.There were significant differences in AchE activities among the selected strain and its treatments, natural and susceptible strains. The ratios compared with the suscepible strain were 1.09—2.33-fold. The results showed that AchE was sensitive to tebufenozide. Maybe the resistance of S. exigua to tebufenozide had no correlation with AchE.8.Compared with the suscepible strain, the activities of CAT, POD and SOD in the selected strain were 0.94, 0.82 and 3.72-fold respectively; the activities in the natural strain were 2.79, 4.33 and 7.40-fold respectively; the activities in the three treatments were 1.43, 1.52 and 4.75-fold respectively, 1.42, 1.44 and 3.54-fold respectively, 2.19, 3.39 and 6.54-fold respectively. Meanwhile the activities of the three treatments increased firstly, then decreased slightly and increased fast again. The changes of SOD activities were more significant than that of CAT and POD activities. The results suggested that the resistance of S. exigua to tebufenozide may be related to SOD but not closely correlated with CAT and POD.9.Compared with the suscepible strain, the PO and chitinase activities of the selected strain in S. exigua increased 37.64% and 27.37% respectively; and that of the natural strain increased 59.63% and 60.29% respectively. The results showed that the resistance of S. exigua to tebufenozide had correlation with PO and chitinase. Studies on kinetic constants of PO showed that the Km and Vmax values of the selected strain were 2.53 and 2.52 times respectively higher than that of in the susceptible strain.The significant differences suggested that the selected strain had lower affinity and higher quantities of PO.