Effects Of Several Pesticides On The Population Growth Of Sesamia Inferens (Walker) And Its Physiological Mechanism

The pink stem borer Sesamia inferens has attained a main pest status in some rice field region of China in recent years. The natural population increase of this insect pest is caused by many factors, among them, the chemical control of S. inferens plays an important role, which mainly involes:1) S. inferens were shared the approximative governance with Chilo suppressalis (Walker) and Cnaphalocrocis medinalis Guenee in production, but the concentration and treatment time of pesticides always do not match with the appropriate control effect of S. inferens, which may cause the increase indirectly of the population of S. inferens.2) Based on the mis-matched chemicals control of S. inferens, the sublethal concentration of some target pesticides will touch S. inferens frequently, which could facilitate the population growth of S. inferens. Therefore, we chose several pesticides, commonly used in paddy, and investigated their effects on the population growth of S. inferens, which including,1) changes of the life table parameters of population growth, and contents of juvenile hormone and molting hormone,2) content changes of the insect-resistance substance of rice after treated with pesticides. The results are as follows:1. Effects of pesticide applications on the population growth of the pink stem borerWith a view to identify whether some pesticide applications, especially at the sublethal concentration, could facilitate the natural population of S. inferens or not, five pesticides, among them three were target pesticides (triazophos, chlorantraniliprole, and chlorpyrifos) at sublethal concentration and the other two were non-target pesticides (pymetrozine and jinggangmycin) at field quantity, were applied at two stages (1st instar or3rd instar larve) of S. inferens respectively, for investigating their effects on the population life history parameters of S. inferens. We found that, at1st instar, with the treatment of triazophos at the sublethal concentration of2mg/L, the pupa weight and the number of egg laid of S. inferens were obviously higher than those on control by26.2%and47%respectively. The application of pymetrozine at100mg/L also enhanced the oviposition significantly. Whereafter, at3rd instar larve, chlorantraniliprole treatment caused an obvious increase of the number of egg laid.2. Effects of pesticides on biochemical substances of riceThe biochemical measurement showed that, contents of flavonoids and phenolic acids in rice plants decreased at different degrees after treated with these pesticide, among which, triazophos reduced the content of flavonoids and phenolic acids by48.5%and22.4%, respectively, relative to control. Moreover, the scanning electron microscopy and energy spectrum analysis indicated that surface silicon contents of rice leaf sheath treated with pesticides varied with different treatments, but most of which didn’t attain a significant difference with control. It is suggested that the applications of some pesticides, e.g. triazophos, at a certain concentration, play an important role to strengthen the population growth of S. inferens in rice field, which is connected with the decreased biochemical resistance of rice once treated with these pesticides.3. Effects of pesticides the hormone levels of the pink stem borerWe selected the pesticides of triazophos, chlorantraniliprole and pymetrozine, applied at three stages (1st instar,3rd instar, and twice treatment at1st and3rd instar, respectively) on S. inferens, and investigated the effects of pesticides on levels of juvenile hormone and molting hormone of S. inferens. Our results showed that, with the treatment at1st instar and3rd instar, the contents of juvenile hormone in4th instar larve of S. inferens was decreased at different degrees. However, twice treatment increased the juvenile hormone level obviously. Triazophos at the sublethal concentration and pymetrozine at100mg/L also increased the content of juvenile hormone of adult to some extent. At3r instar, Chlorantraniliprole at lOmg/L, and Pymetrozine at50mg/L,100mg/L,200mg/L were obviously reduced the content of molting hormone by50.3%and39.4%,51.5%,53.4%, respectively. On the contrary, the application of triazophos increased the molting hormone level. The results show that the sublethal concentration of triazophos could enhance the level of hormone, which is helpful to the population growth of S. inferens.