Effect Of Teflubenzuron Ingestion On Larval Performance And Chitin Content In Leptinotarsa Decemlineata

At present,unreasonable pesticide usage leads to that Leptinotarsa decemlineata(Say)have developed resistance to many pesticides.To achieve consistent,long-term,integrated management(IPM)of the Colorado potato beetle and to ensure the safe and sustainable development of potato and other crops in China,research assessing the potential of novel,IPM-compatible insecticides is essential.Teflubenzuron may be a potential benzoylurea insecticide to control L.decmlineata larvae.It belongs to benzoyl urea insecticide,an efficient chitin synthesis inhibitor.The main role of teflubenzuron is to inhibit chitin biosynthesis.As a result,the benzoyl urea affects larval development,pupation and adult emergence.Teflubenzuron has higher target bio-selectivity.It exhibits little harm to other animals such as humans and livestocks.It causes little environmental pollution.In this study,the effects of teflubenzuron on L.decemlineata larvae were determined.The results are as follows.1.The negative effects of teflubenzuron ingestion on the larvaeThe newly-ecdysed larvae are allowed to feed three days on foliage treated with teflubenzuron at the concentration of 2.5 mg/L for second-instar larvae,and 5.0 mg/L for the fourth larval instars,and an additinal 4 days on normal leaves.Feeding of teflubenzuron inhibited foliage consumption.Whereas the control larvae reach the fourth larval instars five days after the initiation of bioassay,about 50%of the treated survivors remain at the third-instar stage.Approximatly 50%of the treated larvae arrested at the final stage of the ecdysis.A part of larvae(bout 70%)do not shed the old cuticle on the abdomens.The remaining 30%animals fail to cast the old head capsule.These symptoms are followed by death of the larvae 5-6 days after initiation of experiment.For the fourth-instar larvae,the developing duration were lengthened for the larvae having consumed one of the five concentrations of teflubenzuron solutions.All the teflubenzuron-fed fourth-instar larvae successfully buried into soil.Whereas the control larvae were pupated seven days after initiation of bioassay,many treated prepupae failed to ecdyse.They were completely wrapped in larval cuticle.After nine and eleven days after initiation of the bioassay,the beetles were somewhat withered,dried and darkened.Moreover,approximately 10%of beetles had expanded abodomens.2.Stomach toxicities of teflubenzuron to larvaeThe LC50 values for teflubenzuron were were 0.58,0.63,1.06 and 1.89 mg/L respectively for the first-,through fourth-instar larvae.The LC50 values for cyhalothrin,chlorantraniliprole,spinosad,fipronil and abamectin were 0.872,0.305,0.402,0.003 and 0.008 mg/L for the third-instar larvae;they were 0.615,0.873,1.014,0.020 and 0.018 mg/L for the fourth-instar larvae.According to the relative toxicity index(RTI,determined by comparing teflubenzuron LC50 value with the LC50 value of other insecticide),the six insecticides could be categorized into two groups.Teflubenzuron,cyhalothrin,chlorantraniliprole and spinosad belonged to the first category,with RTI less than 10.0.Fipronil and abamectin fell into the second group,with RTI more than 90.0.The results show that Teflubenzuron is a good larvae of stomach poisonous insecticides,and their larvae activity is comparable to that of cyhalothrin,chlorantraniliprole and spinosad.3.Teflubenzuron ingestion by the fourth larval instars affects pupation and adult emergenceFor the fourth-instar larvae fed teflubenzuron solution at the concentrations of 10.00,5.00,2.50,1.25 and 0.63 mg/L,the pupation rates were found to be 0.06,0.12,0.18,0.37 and 0.49 respectively,whereas that in control was 0.98.All the treated larvae had significantly lower pupation rates than control.Similarly,the emergence rates were respectively 0.08,0.14,0.24,0.32 and 0.44 for the larvae fed one of the five concentrations of teflubenzuron solutions.All of them were significantly lower than control.4.Teflubenzuron ingestion reduced chitin contents in the ectodermally-derived tissuesFor the larvae fed teflubenzuron solutions at the concentrations of 10.00,5.00,2.50,1.25 and 0.63 mg/L,the chitin contents in the body carcass(without midgut)and integument specimen were reduced.Our previous results revealed that in the larvae of L.decemlineata uridine diphosphate-N-acetylglucosamine-pyrophosphorylase 1(LdUAPl)and chitin synthase Aa(LdChSAa)were mainly responsible for chitin biosynthesis in ectodermally-derived tissues.The expression levels of LdUAPl and LdChSAa were measured after feeding of teflubenzuron for 12,24 and 36 hours.For LdUAPI,the transcript levels at the three time points were significantly downregulated.In contrast,the transcription of LdChSAa was only inhibited after feeding teflubenzuron for 36 hours.5.Teflubenzuron ingestion had little effect on chitin content in the midgut peritrophic matrixOur previous results demonstrated that LdUAP2 and LdChSB were involved in chitin biosynthesis in the midgut peritrophic matrix(PM).In this study,we tested their expression levels after teflubenzuron ingestion.Feeding of teflubenzuron had little effect on the chitin content in the midgut PM,compared with that in control.Consistent with the chitin content,teflubenzuron ingestion did not affect the transcript levels of LdUAP2 and LdChSB,except the mRNA level of LdChSB after feeding teflubenzuron for 36 hours.