Abstract: 【Objective】The objective of this study is to determine how the density of Liriomyza trifolii eggs and larvae affect its growth and development, explore its density effect of intraspecific competition. In addition, the competition ability for food between L. trifolii and L. sativae larvae under different densities of 1st instar larvae were preliminarily described, and the replacing mechanism about interspecific competition between the two leafminer species was explored.【Method】The extra eggs and larvae of leafminers which were raised in laboratory were killed with needle, so as to form the different egg and 1st instar larvae density gradients, then the remaining leafminers were raised. According to the method of judging the lethal factor of insect whether existed density dependence, N₁ was used as individual numbers before the lethal factor acting on, N₂ was used as individual numbers after the lethal factor acting on. Moreover, lgN₁ as a dependent variable and lgN₂ as an independent variable, then the regression coefficient was calculated, while lgN₁ as an independent variable, lgN₂ as a dependent variable, the regression coefficient was calculated again. If this two straight lines were located in the one side of bisectrix b=1, the factors acted on N₁→N₂ were density dependent factors. Whereas if this two straight lines were located in two sides of the bisectrix b=1, this factor was not considered as a density related factor. By selecting one pair, two pairs and three pairs of adults of L. trifolii and L. sativae, and they were put on the fixed blade for 24 hours for spawn, in order to form different egg and larvae densities. Then the growth status of two leafminer species offspring was estimated, and the effect of different egg and 1st instar larvae densities on interspecific competition replacement was also analyzed. 【Result】 There was no density effect in L. trifolii during its egg stage, and the survival, growth and development rates were not significantly affected by egg density. Moreover, the survival rate of L. trifolii was above 91% under laboratory conditions. However, there was an obvious density effect at larval stage, in particular, the larvae density had significantly impacts on the survival rate of larvae, average weight of pupae, pupation rate, eclosion rate, lifetime of adults and egg laying amount of single female. When the 1st instar larvae density was larger than 0.87/cm² on the host plants of dwarf bean, larvae mortality was increased, the average weight of pupae, pupation and eclosion rates were reduced, the lifetime of adults and reproductive ability were also reduced. While the density of 1st instar larvae had no effect on the development rate of larvae and pupae, the female proportion of eclosion adults. In addition, the density during egg stage had no effect on the competitiveness of the two kinds of leafminers, the general hatching rate of the two kinds of leafminers under different densities were all above 94.25%. The higher larvae density led to the drastic interspecific competition. When the density of the two insects was low, their offspring numbers were similar. While the offspring numbers of L. trifolii was remarkably larger than the L. sativae when the density was high. 【Conclusion】 The egg density of L. trifolii had no significant effect on their population amounts. However, the higher 1st instar larvae density led to strong intraspecific competition, while it would refrain the growth of L. trifolii population. Compared with L. sativae, the interspecific competition ability of L. trifolii was much stronger, because the L. trifolii larvae have variable abilities to resist crowd, adapt for the aggravated environment and occupy food resources.

Key words: Liriomyza trifolii, Liriomyza sativae;density restriction, interspecific competition