Spatiotemporal Variation In Sex Ratio Of Megalurothrips Usitatus(Thysanoptera:Thripidae),and Impacts Of Temperature And Insecticides On Its Sex Allocation

Bean flower thrips,Megalurothrips usitatus（Bagnall）（Thysanoptera:Thripidae）,is a major insect pest of Vigna unguiculata in Hainan.The management of the bean flower thrips has relied primarily on the use of insecticides,which indiscriminate use leads to increasingly serious problems covering insecticide resistance,pesticide residue,and environmental contamination.A non-chemical control method for the bean flower thrips is necessary for the safe and sustainable management of cowpea.Thrips population dynamics is very sensitive to variation in sex ratio.Manipulating sex ratio should be an alternative way to control thrips.Like many phytophagous thrips species,the bean flower thrips is capable of both bisexual and arrhenotokous reproduction.So far,however,there is little information about variation and adjustment in sex ratio of the bean flower thrips.In this study,the temporal and spatial patterns of the natural population of the bean flower thrips on cowpea were observed.The effects of temperature and sublethal doses of six insecticides on the sex ratio of the bean flower thrips were evaluated.The main results were as follows:1.Temporal and spatial variation in population sex ratio of the bean flower thrips in cowpea fieldsTemporal and spatial variations in number and sex ratio of adult M.usitatus on cowpea（var.Chunfeng-changjiangdou）in two planting seasons were investigated by three ways,such as using blue sticky cards,sampling parts of cowpea plants,and collecting larvae dropping to the ground for pupation.The adult number increased as cowpea plants grew.Most of adults occurred in the flowers.The blue sticky cards at 64 and 124 cm above ground level trapped more adults than those at other heights.The population sex ratio of the bean flower thrips changed with the growth of cowpea,depending the investigating ways.2.Effects of temperature and duration when the bean flower thrips were paired on the females and their sex allocationEach pairs of newly emerged adults of the bean flower thrips was reared with young cowpea pods at the temperatures of 17,26,and 33℃,respectively.After the period of from24 to 72 h,the females were transferred,and reared singly at 26℃.The reproduction of each female and the development of their offspring were observed daily.The sex ratio of adult offspring produced by the females being paired at 26℃was female-biased,while the females both being paired at 17 and 33℃produced more sons,which indicated that lower or higher temperature during mating decreased the availability or vigor of sperms the females received and stored in their spermatheca.The higher temperature during mating also decreased the fecundity of the treated females,resulting in the reduced number of their adult offspring.The duration for adults being paired at the same temperature did not affect the fecundity of the females and their sex allocation.3.Effects of temperature on the mated females of the bean flower thrips and their sex allocationUnder laboratory conditions of 60%±5%R.H.,a photoperiod of 14L:10D,and different temperatures of 17,20,23,26,30 and 33℃,the females were singly reared with young cowpea pod.The food was replaced every day.The old pods were reared at 26℃until adult offspring emerged.The survival time of the treated females decreased as temperature increased.The females at 23 and 26℃produced adult offspring of199.40-210.35 per female,which was greatest among different treatments.The mated females reared at lower temperature of 17℃produced much more female offspring,whereas the adult sex ratio of offspring by the females at higher temperature of 33℃became male-biased.4.Sublethal effect of insecticides on reproduction and sex allocation of the bean flower thripsUnder laboratory conditions of（26±1）℃,60%±5%R.H.and a photoperiod of 14L∶10D,with young leaves of cowpea as food of the bean flower thrips,the lethal and sublethal concentrations of cartap hydrochloride,spinetoram,avermectin,cyantraniliprole,amitraz,and indoxacarb for the 1-d-old mated adult females were determined using a leaf-disc dipping bioassay method.After 48 h exposure to LC₂₀concentration of each insecticide,the surviving mated or unmated adult females were provided with non-treated leaf discs.The number of eggs and subsequent adult offspring were counted daily until the parental females died.The females treated by LC₂₀concentration of spinetoram and avermectin produced a few adult offspring.Exposure to LC₂₀concentration of cartap hydrochloride,cyantraniliprole,amitraz,and indoxacarb significantly decreased survival time of the parental females,and reduced to some extent the number of adult offspring produced by them,but did not significantly change their sex allocation.In a field test of insecticides,spraying at LC₂₀concentration of cyantraniliprole and indoxacarb did not affect the variation in population sex ratio of the bean flower thrips,but increase of the adults was inhibited by cyantraniliprole treatment.In conclusion,the population sex ratio of the bean flower thrips changed as cowpea plants grew.Temperature was one of major factors affecting sex ratio of the bean flower thrips.The lower temperature experienced by the females during and after mating resulted in male-biased and highly female-biased sex ratio of their offspring,respectively,while the females reared at higher temperature either during mating or after mating produced more male offspring.Cartap hydrochloride,cyantraniliprole,amitraz,and indoxacarb did not exhibit sublethal effect on sex allocation of the bean flower thrips.