Effects Of Two Benzoyl Urea Chitin Synthesis Inhibitors On The Development Of Tribolium Castaneum

Tribolium castaneum is a widely distributed storage pest.It damages bread,flour,cereals and dried fruit,but it does not erode completely dry grains.The scent glands of this insect will continuously secrete carbolic acid,contaminate grains and flour,and cause serious food economic losses to most of the countries in the world.Compared with other storage pests,Tribolium castaneum has higher pesticide resistance,and this resistance continues to increase at exponential times.At the same time,due to its short reproductive cycle,low requirement for living environmental conditions,small size,and convenient feeding of groups,it has become a model organism in the laboratory for the toxicity studies of various insecticides.Mixing chemical protectants with grains is one of the main methods for preventing and controlling Tribolium castaneum.However,this control method is seriously harmful to human health and ecological safety.Although those chemicals have high insecticidal effects and are widely used,these protectants still need to be replaced by new insecticides which are non-targeted and environmentally friendly and meet the principles of pest comprehensive management.Using insect growth regulators(IGRs)is a good way to achieve this demand.These compounds have strong effects on storage pests and even strains that are resistant to organic insecticides.At the same time,the toxicity to mammals and other non-target organisms is very low,and they are easy to degrade in the environment.This feature makes it a good alternative to replace conventional pest control measures.Chitin is a natural polysaccharide that is widely distributed in insect cuticle,tracheal wall,and midgut peritrophic membrane.It exists only in fungi,nematodes and insects,but not exists in plants and advanced vertebrates.It means that insecticides targeting chitin have high selectivity to pests.Chitin synthesis inhibitors are IGRs that inhibit the synthesis of chitin in the body,thus affecting the normal physiological and biochemical activities of pests,and killing the pests.Benzoylurea insecticides are a class of efficient chitin synthesis inhibitors.The main role of benzoylurea is to affect larval development,pupation and adult emergence to cause larval death.Teflubenzuron and novaluron are both benzoylurea insecticides with significant inhibitory effect on Coleoptera.In the present paper,we measured the effects of teflubenzuron and novaluron on the development of Tribolium castaneum larvae.1.The negative effects of teflubenzuron ingestion on Tribolium castaneumThe fifth instar larvae were respectively treated with 0.1 mg/L,1 mg/L,1 mg/L,10 mg/L,50 mg/L,and 100 mg/L teflubenzuron solutions,acetone solution was set as control.Within 18 days after treatment,the mortality rate was as high as 98%or more at the concentration of 10 mg/L,almost all the treated larvae failed to pupate.When the concentration of teflubenzuron was less than 1 mg/L,the mortality rate was less than 35%and pupation rate was above 55%.At.the concentration of 1 mg/L,teflubenzuron inhibited pupation and emergence.The initiation of pupation was delayed by 1-2 days compared with the control group.Some larvae died within 6 days after treatment with teflubenzuron.Some remained as larvae that were wrapped in the old epidermis.Some emerged as adults.The wings were abnormally developed;and left un-tanning.It cannot be folded to the mesonotum.And the terminals of adults remained in the pupa state,and were completely or partially wrapped in larval cuticle.2.The negative effects of teflubenzuron on gene expression of enzymes related to chitin biosynthesisUsing qRT-PCR,we found that the expression levels of most of enzyme encoding genes related to chitin synthesis were reduced in the larvae two days after the treatment of teflubenzuron.For example,the transcript levels of TcChS1b and TcChS2 genes were decreased dramatically and hardly to detect in the samples of the larva.Four days after the treatment of teflubenzuron,TcTrel-3,TcTrel-4 and TcTPS increased significantly.It is hypothesized accordingly that the target of the action of teflubenzuron is chitin synthetase.3.The negative effects of novaluron ingestion on Tribolium castaneumAt the concentration higher than 10 mg/L,novaluron caused up to 100%larval mortalities.When the concentration of novaluron was less than 1 mg/L,the mortality rate was less than 45%and pupation rate was above 50%.After analyzing the bioassay data,0.1 mg/L was selected as the appropriate treatment concentration to observe the effects of novaluron on the development and metamorphosis of Tribolium castaneumAt the concentration of 0.1 mg/L,novaluron led to the larval lethality at the fourth day of treatment.The initiation of pupation did not delay.At the higher concentration,novaluron caused the larval death at the second day after treatment.The effects of novaluron on pupation and emergence were similar to those of teflubenzuron.Novaluron inhibited the normal development of wings.And the old cuticle was difficult to ecdyse,the pupation and emergence processes were influenced.4.The negative effects of novaluron on gene expression of enzymes related to chitin biosynthesisTwo days after novaluron treatment,the expression levels of most of enzyme encoding genes related to chitin synthesis was reduced in the treated larvae.TcTrel-2 decreased significantly.We proposed that trehalase is the target of action of novaluron.