The Toxic Effects And Mechanism Of Insecticides To The Eggs Of 3 Scarab Species (Coleoptera: Scarabaeidae)

Anomala corpulenta Motschulsky,Holotrichia oblita Faldermann and Holotrichia parallela Motschulsky are dominant species of domestic scarabs,both adult and larva significantly affect quantity and yield..To date,most studies focused on the control of larva and adult.Although take managements at egg stage could reduce or even avoid loss,rarely studies focused on it.So the aim of this paper is to make sure the possibility of control scarab eggs and find the best insecticides.To determine the toxicity value of different insecticides on the eggs of three Scarab species,A.corpulenta,H.oblita and H.parallela were choosed as the material.In addition,we detected the toxicity difference of insecticides against the three scarab eggs,and distinguished the symptoms and stage when the eggs died after insecticides treatment.As well,the CarE and GSTs activities of three scarab eggs affected by insecticides were assayed.Furthermore,the morphological characteristics and ultra microstructure of eggs were analyzed comparatively.Ultimately,the lasting ovicidal effect and integrated control effect of the typically insecticides against A.corpulenta eggs were evaluated.The results followed:1.By observing the morphology and ultra microstructure,we found that the 3 scarabs eggs were all white,prolate spheroid,but difference of size is significant.The order of egg size as follows A.corpulenta egg.<H.oblita<H.parallela.By Scanning electron microscopy?SEM?,we found that the eggshell of the 3 scarab eggs.The A.corpulenta eggshell is glabrous and has no secretions,but the two Melolonthidae eggshell is rough and with sticky secretions.We also found that the chorion micripsculpture was also distinct between the 3 species eggs.It was Knob-like protuberances in A.corpulenta egg and was reticulate in Melolonthine eggs after removing secretions with ultrasonic cleaning instrument.The micripsculpture of 2Melolonthine eggs is higher than A.corpulenta.In addition,the micripsculpture of H.oblita is hexagon and evenly distributed.But it in H.parallela egg is messy and no any shape.2.By observing the embryonic development in 25?,the result showed that the egg period is distinctive,A.corpulenta,H.oblita,H parallela,which is 9.95d,12.85d and 10.10d respectively.Embryonic development can be divided into five stages:blastoderm-germband stage,appendage-generating stage,blastokinesis stage,dorsal enclosure stage and mature stage.The shortest and longest stages are blastokinesis stage and appendage-generating stage in A.corpulenta egg,1.05d and 2.95d respectively.The blastokinesis stage and dorsal enclosure stage are shorter than other satges in H.oblita.and all 5 stages of H.parallela have no significant difference,all in 1.80-2.10d.In embryo development process,the long diamete,short diameter and weight of egg are increase continuously with the egg absorbing water.The peak period of long diamete in 3 scarab eggs is dorsal enclosure stage,appendage-generating stage and blastokinesis stage,respectively.The peak period of short diameter is appendage-generating stage,blastokinesis stage and appendage-generating stage.And the peak period of weight are all appendage-generating stage.3.Toxicity of some insecticides to the egg of white grub showed that tebufenozide had the highest toxicity with a LC₅₀ of 8.620 mg/kg.Based on the LC₅₀ values,the order of toxicity levels of the seven insecticides tested against the egg of A.corpulenta was as follows:tebufenozide>clothianidin>imidacloprid>thiamethoxam>phoxim?the control insecticide?>Chlorantraniliprole>fipronil.And the order of toxicity levels of insecticides to the egg at different stages was as follows:1-old-egg>5-old-egg>9-old-egg.4.The sensitivity of three scarab eggs to different insecticides showed that A.corpulenta egg is more sensitive than 2 melolonthidae eggs.And the virulence ratio of new neonicotinoid insecticide thiamethoxazole to 3 species eggs is 1:5.89:7.26.In addition,The order of virulence difference is that thiamethoxazole>tebufenozide>phoxim.5.The death symptoms of egg which is trated with insecticide showed that the stage where insecticide play a role of 3 eggs is different.After exposed to thiamethoxazole,the embryonic development of A.corpulenta stopped in blastokinesis stage,and that of H.oblita,H.parallela stopped indorsal enclosure stage;After exposed to clothianidin,the embryonic development of A.corpulenta,H.oblita and H.parallela stopped in indorsal enclosure stage,mature stage and mature stage,respectively.After exposed to phoxim,the embryonic development of 3 species eggs all stopped in mature stage.In addition,The stages where thiamethoxazole and clothianidin played a role were early than that of phoxim.6.The results of enzyme activity affected by clothianidin,phoxim on 3 scarabs eggs showed that clothianidin had significant effects on CarE and GSTs,whitch indicated that these two metabolic enzymes play an important role in the process when eggs resist to clothianidin.And,the effect of clothianidin to two enzymes in A.corpulenta is obviously higher than in two Melolonthine eggs.After 5d exposure,the inhibition rate of CarE was63.42%in A.corpulenta and it was less than 30%in two Melolonthine eggs.In addition,GSTs activity increased by 53.46?A.corpulenta egg?,15.48?H.oblita egg?and 17.00%?H.parallela egg?.The effect of phoxim on two metabolic of 3 species eggs enzymes was not significant.Under the stress of pharmacy,the activity of the two detoxification enzymes in the three eggs showed the same trend with the sensitivity of the eggs to insecticide.7.The lasting combined effects showed that,3 insecticides have a good comprehensive control effect on A.corpulenta egg,but there is a difference in persistence.At 10d treated,the comprehensive control effect of 3 insecticides eached above 90%;But 10d treated,the comprehensive control of clothianidin is the best.The control of clothianidin reached 95.88%at a dose of 33.33 mg a.i./kg after treated 30d.And at the same dose,the tebufenozide controlled egg and larvae reached 75.00%at 20d after treaded.However,the control of phoxim is lowest.So,the clothianidin is worth applying to the peanut field at the egg stage of A.corpulenta.It has a good comprehensive effect that 3 insecticides treated eggs in short term,but the effect of thiamethoxam is more stable and longer.So,the clothianidin is worth applying to control the scarabs.