Abstract: By analysing the susceptibility of different development stages of citrus leafminer,Phyllocnistis citrella,to insecticides and investigations into the correlation of leafminer population structure in field and the shoot length,the optimum spraying time indicated by shoot length was discussed.It found that the early stage of leafminer(including egg,the first and second instar larvae)were relatively susceptible to the five insecticides used,and the third instar larvae was relatively tolerant compared with the early stage.The results of estentses activity bioassay by the improved filter paper dot-spot test showed that the estenaes activity of the individuals of different development stages was negatively correlated with their susceptibility to insecticides.The correlation of the population structure in field and the shoot length of sweet orange(citrus chinensis)illustrated that the insect number in early stage was sound dominant in the population when the shoot length was less than 9.5 cm,whereas the inseet number of other stages was relatively even when the shoot length was more than 9.5 cm.The insect number in early stage reached 600 per hundred shoots when the shoot length was 4.5 and 10.5cm.It was emphasized that chemical control of citrus leafminer on sweet orange should be focused on the time when the shoot length was less than 9.5cm and the optimum spraying time was in 5.0 and 11.0cm of shoot length.