| In recent years, Hexi area of Gansu Province has become production base of hybrid cornseed because of rapid development of hybrid corn breeding. However, Tetranychus truncatusEhara has become one of important pests in this area due to undertaking contiguouscultivation in large regional scale, seriously affecting yield and quality of hybrid corn seed.Pyridaben was one of mitochondrial electron transport inhibitor (METI) acaricides. Due to itsunique chemical structure, special physiological mechanism and broad-spectrum efficient,pyridaben was widely used in corn field of Hexi area, Gansu Province. However, applyingpyridaben singly for a long time over large areas would cause resistance of T. truncatus.Based on results of resistance selection, the author carried out studies of the biologicalfitness, biochemical mechanisms, and inheritance patterns of the resistance to pyridaben of T.truncatus, to provide theoretical support for application of pyridaben and comprehensivecontrol of T. truncatus. The main results are shown below.1.The15%pyridaben emulsifiable concentrates were used to perform resistanceselection of T. truncatus in the laboratory. After49genneration selection with pyridaben, theLC50of T. truncatus increased from17.08mg/L at F0generation to16253.59mg/L at F49genneration, with resistance ratio being951.6. The resistance development of T. truncatuswere slow at the beginning, faster at earlier stage, stabile at mid-term, and sharply increased atlater period.2.The development and reproduction of the susceptible strain (SS) andpyridaben-resistant strain (Py-R) were observed under16°C,20°C,24°C,28°C,32°C, and36°C, with16h illumination and relative humidity60%±5%. The results indicated thatboth SS and Py-R strains could reach complete generation development within all temperaturerange. The whole developmental duration and average life duration were shortened astemperature rose. Compared with SS strain, the generation time developmental duration, theaverage life span, and reproduction of the Py-R strain shortened under low and suitabletemperature (16°C-28°C). But the development period, life expectancy, and the ovipositionperiod of Py-R strain were longer or greater than SS strain at the high temperature (36°C).The relationship of developmental rate and temperature were fitted withWang-Lan-Ding models. Results showed that the minimum and maximum boundarytemperatures of SS, Py-R strains were10.1°C and39.2°C,13.4°C and41.9°C, respectively. The optimum temperature of SS and Py-R strains were not significantly different. Comparedwith SS strain, the Py-R strain has stronger suitability to extreme high temperature but weakersuitability to lower temperature.3.The life tables of SS and Py-R populations of T. truncatus were made under differenttemperatures. Survival curves showed that survival rate of two populations shortened astemperature rose within range of16-36°C. Reproduction fecundity curves of two populationswere parabolic. Egg production of per female of SS and Py-R population were lower at theearly stage of reproduction, reached to peak later and then decreased over time. Under16°C-32°C, there were difference on survival curves of two populations. The survival rate of Py-Rpopulation fell faster than SS population. Py-R population reached ovipositing peak before SSpopulation. But the survival rate and fecundity of Py-R population was obviously greater thanSS population at36°C. The results indicated that the suitability to high temperature of T.truncatus increased after selected with pyridaben compared with the susceptible strain.The relationship between the population intrinsic rate and the net reproductive rate (R0),the innate capacity of increase (rm), the finite increase rate (λ), population double time (Dt)and temperature was described with parabola equation of SS and Py-R populations, exceptmean generation time (T). The result reflected the general physiological rule, that is, thedevelopmental rate decreased rapidly at high temperature for both SS and Py-R populations.the relative fitness of two populations (SS and Py-R) was determined using netreproductive rate (R0). Results showed that the relative fitness of SS population was less than1under16-28°C, but the fitness of Py-R population was greater than1at32-36°C,indicating that biological fitness of Py-R population was significantly higher than SSpopulation at high temperature, but defected at lower and suitable temperature.The results of the reaction time of T. truncates to disturbance under differenttemperatures showed that the reaction time (t) of SS and Py-R strains was prolonged as themortality rate (M) increaseed. If the mortality rate M was assumued to be caused by usingpesticide, t value represented the time needed for population density of T. truncates to returnto the original level. Assuming the mortality rate (M) was over50%at24°C, the reactiontime (t) of SS and Py-R strains were calculated to be2.9and3.1d, respectively. Assuming themortality rate (M) was over99%at24°C, the time needed for population density of SS andPy-R strains to return to the original level were only19.0and20.3d, respectively. Therefore, frequently use of acaricides in field would result in accelerated resistance of T. truncatus andlose of insecticidal action of acaricides.4. Using synergist bioassay and biochemical assay methods, the synergistic effect ofthree synergists and activity changes of carboxylesterases (CarE), glutathione-s-transferase(GSTs), phosphoesterase, mixed function oxidase (MFO), and acetylcholinesterase (AChE) inboth SS and Py-R strains were investigated. The synergist bioassay showed that the piperonylbutoxide (PBO), triphenyl phosphate (TPP) and diethylmaleate (DEM) enhanced toxicity ofpyridaben to Py-R strains after three synergists were mixed with pyridaben (3:1by weight).PBO and DEM exhibited a higher level of synergism to pyridaben compared with TPP.Synergism ratio of PBO and DEM were25.3and39.0, respectively, with that of TPP beingonly6.85. Therefore it can be concluded preliminarily that the resistance of T. truncates topyridaben was closely related to MFO， CarE， and GSTs, because relative synergisticcoefficient of three synergist were96.0%,85.1%, and97.4%, respectively, all greaterthan0.The activity of CarE, GSTs, and MFO of Py-R strain increased significantly comparedwith the SS strain (P <0.05). However, there was no significant difference (p>0.05) in acidphosphatase (ACP), alkaline phosphatase (ALP) and AChE activities between SS and Py-Rstrains, which was determined by enzyme assay using model substrates.Michaelis constant (Km) values of CarE, GSTs, and MFO of Py-R strain were lower thanSS strain, but the maximal reaction velocity (Vmax) values were higher than SS strain.This result indicated that CarE, GSTs and MFO of Py-R strain had higher affinity withsubstrate, and increase of the metabolic detoxification might be the main resistancemechanism of T. truncates to pyridaben.5. Cross and backcross between resistant (Py-R) and sensitive (SS) strains were used toexplore the inheritance pattern of T.truncates to pyridaben. The estimateddegree of dominance (D) for the F1females from SS♀×R♂crosses and from RR♀×S♂crosses was0.40and0.60, respectively, illustrating an incompletely dominant inheritance ofresistance. The95%confidence limit of this two degree of dominance superposed, showingDSRand DRShad no significant difference. It revealed that the resistance inheritance was notmaternal inheritance. The chi-square (χ2) goodness-of-fit analysis indicated that mortalitiesobserved in F2females (SR♀×S♂and RS♀×R♂) were not significantly different (χ2=9.72, df=9, P>0.05) from expected values for single gene control. The result suggested that theresistance to pyridaben is under monogenic control.6.Using the realized heritability (h2) estimated in quantitative genetics, the realizedheritability of resistance to pyridaben at49generation of T. truncatus was0.11. Assuming theselective pressure was50-90%mortality, it required10-16generation for pyridaben to obtain10-times increase in LC50. Assuming the realized heritability of field was half of that indoor(h2=0.05), it required21-31generation, indicating higher resistance risk for using thepyridaben to control T. truncatus. |
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