Effects Of Five Insecticides On The Population Of The Small Brown Planthopper Laodelphax Striatellus(Fallén)

The five insecticides which were used frequently in the paddy were used in the experiments.Five insecticides were fipronil chlorpyrifos triazophos imidacloprid and cyhalothrin.The effects of five insecticides on the sensivity variation and productivity of the small brown planthopper(SBPH), Laodelphax striatellus(Fallén) were researched.We also studied the SBPH population dynamic influenced by five insecticides for middle and long period.The main results were showed as follows.1 We mensurated the five insecticides'susceptibilities of different instar nymphs of the SBPH in the laboratory.The results revealed that the susceptibilities decreased from the lower instar nymphs to the bigger ones.There was no difference among 1～3 instar nymphs,and the latter's mutation was significiant.For example,the LC50 values of fipronil and chlorpyrifos to the adult SBPH were as 17.25 and 21.50 times as to the 1st instar nymphs.The toxicity of fipronil and chlorpyrifos to SBPH was higher than others,and high-point for prevention and cure was during 1～3 instar nymphs'period.The order of five insecticides'toxic activity to SBPH from big to small was fipronil> chlorpyrifos>cyhalothrin>imidacloprid>triazophos.2 Laboratory deterimenation of the toxic activity of five insecticides to Araneus ventricosus was done by the slip-slide method.The results showed that the vertebrate selectivity ratios(VSR) were less than 1.00. That is to say, the five insecticides'toxicity to the spiders was higher than to SBPH. The order of five insecticides'toxic activity to spider from big to small was fipronil> chlorpyrifos>cyhalothrin imidacloprid triazophos.3 The five insecticides'toxic activity to the 3rd instar nymphs was determined by rice-stem-dipping method.Both the 3rd instar nymphs and adults of SBPH were treated with the doseages.The doseages which were used in the experiment were LC50 LC75 and LC90 of the 3rd instar nymphs.The surivivals were reared on the rice seeding which was treated with the same doses above.And then divided into single pair.We could see that cyhalothrin and triazophos were able to prolong the span of the female adults and increased the number of eggs. So the total number of instar nymphs'hatching was more than the control and treated by cyhalothrin was more than the control significiantly. The effect on the span by chlorpyrifos was not clear, but it also could make the total number of instar nymphs'hatching greater.Fipronil and imidacloprid could cut down the the span of the female adult so that the total number of instar nymphs'hatching was less than the control.4 The sublethal doses, LC10 and LC20 of the 3rd instar nymphs and adults, were got from the experiment. We treated the 3rd instar nymphs and adults with the doses above. The surivivals were reared on the rice seeding and divided into single pair which the male adults were macropterous and the female were brachypterous. It could be seen that both the span and total number of eggs or egg-masses had no difference between the 3rd instar nymphs and adults which were treated by five insecticides. The total numbers of eggs treated by fipronil and imidacloprid were less than the control. That is to say,the resurgence ratios were less than 1.00.5 From the experiment of the SBPH population dynamic influenced by five insecticides for middle and long period, we could see that the prevention and cure effect of fipronil(39.9mL/667m2,0.15mg/L)and chlorpyrifos(59.9mL/667m2,0.96mg/L) was better than others at the beginning. The quantity of SBPH population which was treated by fipronil was less than the control all the time. But the quantity of SBPH population which was treated by chlorpyrifos mounted up and was more than the control form the experiment of concrete pool and potter plant. The quantity's trend of SBPH population which was treated by cyhalothrin (99.8mL/667m2,12.95mg/L) and triazophos (98.6mL/667m2,29.67mg/L)was the same as the one treated by chlorpyrifos. The quantity of SBPH population which was treated by imidacloprid (53.2mL/667m2,34.96mg/L) was less than the control in some sort. There were two main reasons for the SBPH's resurgence by using the pesticides , chlorpyrifos cyhalothrin and triazophos. The first one was these three insecticides could stimulate the productivity of both instar nymphs and adults of SBPH. The second one was the toxicity of these three insecticides to the nature enemy, Araneus ventricosus, was more than the toxicity to SBPH out and away.