Effect Of Insecticide And Heat Stress On Two Species Of Trichogramma

As important egg parasitoids of many species of lepidopterous pests,Trichogramma are successfully used in inundative and inoculative biological control program around the world.However,in the agro-ecological system,the pest control efficacy of this parasitoid is likely to be influenced by insecticide application and heat.In this thesis,effect of insecticide and heat on the behavior and quality in Trichogramma chilonis Ishiii（Hymenoptera:Trichogrammatidae）and Trichogrammatoidea bactrae Nagaraja（Hymenoptera:Trichogrammatidae）were evaluated in the laboratory through video tracking and naked-eye observation.Both T.chilonis and T.bactrae are natural enemies of diamondback moth,Plutella xylostella（L.）（Lepidoptera:Plutellidae）.The insecticides are beta-cypermethrin and spinosad,both of them are commonly used insecticides to control many vegetable and crop pests including diamondback moth in China.The results will make some realistic recommendations for field applications of these two parasitoids.In order to evaluate the effect of insecticides on the behavior of T.chilonis,parasitoids were exposed to LC₁ and LC₂₀ of tested insecticide for 24 h prior to experiment.For assessing the effect of heat on the development and reproduction of parasitoid T.bactrae,heat stress temperature（33,36,and 39°C）,duration of exposure（2,4,6,and 8 h）,and developmental stage during exposure（embryo-first instar larvae,second instar larvae,prepupae,and pupae）were used as three factors.The main results are as follows:1 Mate choice and host discrimination behavior of T.chilonisMate recognition capacity of male T.chilonis was realized by perceiving the sex pheromone of females.When offered two females of different species,male T.chilonis could distinguish the conspecific female from T.bactrae,a species that has overlapping hosts with T.chilonis.When placed with two conspecific females of different mating status,male T.chilonis preferred mating with the virgin female to the mated female.Trichogramma chilonis females could distinguish unparasitized host eggs from parasitized ones（parasitized by conspecific females or heterospecific females）.When exposed to unparasitized and parasitized host eggs,T.chilonis females preferred to stay on and lay eggs in the unparasitized host eggs.When T.chilonis females were only provided with parasitized host eggs（parasitized by T.chilonis and T.bactrae females）,conspecific superparasitism occurred more often than heterospecific superparasitism.Furthermore,during host egg discrimination period,significantly more antennal rejections were employed by T.chilonis females than ovipositor rejections.2 Effect of insecticides on sex pheromone communication and mating behavior in T.chilonisThe mating behavior of T.chilonis was classified into three phases:mate detection,courtship,and copulation.Based on median lethal concentration(LC₅₀)values,T.chilonis adullts were more susceptible to spinosad than beta-cypermethrin,with LC₅₀ values of1.0030 mg L^-1for males exposed to spinosad,0.4863 mg L^-1for females exposed to spinosad,2.8572 mg L^-1for males exposed to beta-cypermethrin,and 2.2887 mg L^-1for females exposed to beta-cypermethrin.The lower lethal concentration（LC₁）values of tested insecticides were:0.0649 mg L^-1for males exposed to spinosad,0.0318 mg L^-1for females exposed to spinosad,0.0779 mg L^-1for males exposed to beta-cypermethrin,and0.4138 mg L^-1for females exposed to beta-cypermethrin.And the lethal concentration(LC₂₀)values of tested insecticides were:0.3726 mg L^-1for males exposed to spinosad,0.1813 mg L^-1for females exposed to spinosad,0.7761 mg L^-1for males exposed to beta-cypermethrin,and 1.2327 mg L^-1for females exposed to beta-cypermethrin.Insecticide exposure at LC₁ and LC₂₀ had no negative effect on the survival and fecundity of T.chilonis,but it could cause an alteration in the sex pheromone communication and mating behavior of surviving T.chilonis.When only males were exposed to insecticide,beta-cypermethrin LC₂₀ exposure induced significant decrease in sex pheromone perception by males,and significant increase in males’courtship and copulation frequency;beta-cypermethrin LC₁ exposure caused significant decrease in males’courtship and copulation frequency.Males exposed to spinosad LC₂₀ exhibited significantly slower initiated courtship and significantly shorter total copulation duration,and the males exposed to spinosad LC₁ exhibited significantly increased sex pheromone perception,compared with control.When only females were exposed to insecticide,no significant difference was observed in their sex pheromone emission or mating behavior between control females and females exposed to insecticide.When both males and females were exposed to insecticide,significantly more times of courtship and copulation were conducted by treated pairs that survived beta-cypermethrin LC₂₀ or spinosad LC₂₀ than control pairs.Insecticide exposure had no significant effect on T.chilonis males’fertilization capacity(sex ratio of F₁ offspring).3 Effect of insecticide on male specific sex pheromone discrimination and mate choice in T.chilonisRegardless of insecticide exposure condition,T.chilonis males could discriminate the sex pheromone emitted by their conspecific virgin females accurately.Beta-cypermethrin LC₂₀ exposure induced decreases in male conspecific sex pheromone discrimination and mating rate in T.chilonis;and these decreases were not due to the lower locomotor activity of the surviving T.chilonis males.Spinosad LC₂₀ exposure caused a significant decrease in male locomotor activity of T.chilonis,but didn’t affect male specific sex pheromone discrimination（conspecific sex pheromone discrimination or virgin sex pheromone discrimination）or mating rate.However,there was no significant difference in specific sex pheromone discrimination,mate choice,and locomotor activity between control males and males exposed to the LC₁ of insecticide（beta-cypermethrin or spinosad）.4 Effect of insecticide on oviposition and host discrimination behavior in T.chilonisDuring oviposition period,when the number of host eggs was limited（only one host egg）,T.chilonis females exposed to beta-cypermethrin LC₂₀（62.1%）exhibited significantly lower oviposition rate than control（85.7%）.Beta-cypermethrin LC₁ or LC₂₀ exposure caused significantly higher feeding and re-drilling rate,and significantly longer post-oviposition duration of surviving T.chilonis females,compare with control females.Spinosad LC₂₀ treatment not only decreased the wasp oviposition rate,but also significantly extended the oviposition duration.When the number of host eggs was six,similarly T.chilonis females exposed to spinosad LC₂₀ exhibited a significantly lower oviposition rate（79.2%）than control ones（100%）,there was no significant difference in the oviposition rate between control females and females exposed to beta-cypermethrin LC₁（100%）,beta-cypermethrin LC₂₀（96.4%）,or spinosad LC₁（92.3%）.Trichogramma chilonis females’parasitic ability was not significantly affected by insecticide treatment.During the 15-min observation period,the number of host eggs parasitized by females exposed to aceton（control）,beta-cypermethrin LC₁,beta-cypermethrin LC₂₀,spinosad LC₁,and spinosad LC₂₀ were 3.8±0.2,4.0±0.3,3.5±0.3,4.0±1.4,and 3.7±0.4,respectively,and their successful parasitism rate were 93.8±3.8,93.9±3.7,96.8±2.0,94.0±2.7,and90.3±3.2%,respectively.During oviposition period,less self-superparasitism was conducted by both control and insecticide-treated T.chilonis females.In the host discrimination experiment,females exposed to LC₂₀ of both tested insecticides（beta-cypermethrin and spinosad）spent significantly more time on extra-patch area.Both control females and females exposed to beta-cypermethrin LC₁ or LC₂₀,or spinosad LC₁spent significantly more time on the unparasitized eggs and parasitized more unparasitized host eggs,except the females that survived spinosad LC₂₀.5 Effect of heat stress on the quality of T.bactraeThe emergence rate of T.bactrae adults decreased with the increase in exposure time of heat stress.When exposed to 39°C for 8h during pupal stage,only 19.90%adults emerged from host eggs,and more than 14%were deformed（wings were folded or incomplete）.Parasitoid females exposed to 39°C for 8h as prepupae only lived for 1.45days and parasitized about 23.5 host eggs.In addition,the heat stress treatments also influence surviving female’s ability of daughter production,when exposed to 39°C for 8h during embryo-first instar larval stage,the sex ratio of their F₁ offspring（percentage of daughter offspring）was only 28.20%.Moreover,life-table parameters of T.bactrae were also influenced by exposure to heat stress temperatures during each preimaginal developmental stage.Based on the results,we suggest that even the LC₁ and LC₂₀ of tested insecticides（beta-cypermethrin and spinosad）could affect the sex pheromone communication and mating behavior in T.chilonis.In addition,LC₁ and LC₂₀ of tested insecticides had negative effects on the oviposition and host discrimination behavior in T.chilonis.Furthermore,beta-cypermethrin LC₂₀ exposure was harmful to male-specific sex pheromone discrimination and mating rate in T.chilonis.For T.bactrae,even the short-term high temperature（especially at 39°C）could also negatively affect the quality and pest control efficacy of this parasitoid.Thus,in the integrated pest management program,the use of beta-cypermethrin and spinosad should be carefully evaluated;and T.bactrae may be more effectively controlling lepidopterous pests during cooler weather conditions.