Abstract: The damage of citrus leafminer (Phyllocnistis citrella) to four citrus varieties (Xuegan, Rugan, Jiaogan, and Fujue) was investigated at citrus garden in Fuzhou suburbs from September 1993 to December 1994. Because the errors in economic threshold could be produced by different damage figures and annual fluctuation of citrus yield, the autumnal treetops were adopted as numerating units, and the determined twigs were rooundly cut. The photosynthesis intensity of leaves damaged by leafminer, the growth of the damaged autumnal treetops in current year and the vernal and autumnal treetops in second year, the fruit quality and quantity in second year, and the relationship between the damage degree of leaves and the density of leafminer population were studied. The results show that after damaged by leafminer, the leaves had certain compensation abilities in photosynthesis and tissue concrescence. According to the production level of citrus in Fuzhou in1994, the economic threshold on Xuegan was set at 1.21 larvae of first instar per tender leaf, which reduced the errors in economic threshold produced by annual fluctuation of citrus yield. Based on several ecological factors, e.g. citrus varieties, hydrothermic conditions in citrus garden, yield and natural enemies, a multi factor economic threshold model was established.

Key words: Citrus leafminer, Economoic threshold, Model