Abstract: [Objective] This study was intended to reveal the biological characteristics of Mycodiplosis sp. (Diptera:Cecidomyiidae), a mycophagus insect of the Dalbergia tonkinensis rust, and its relationship with the rust disease, which can provide a foundation for utilizing the insect as a biological control agent for the plant rust.[Method] Biological characteristics of different life stages were studied using lab breeding populations, and its relationship with the uredium of rust was studied using field investigation and lab breeding populations.[Result] The results showed that the Mycodiplosis sp. is an insect with complete metamorphosis from egg, larva, pupa to adult. The egg is oval, the surface of egg is smooth and transparent. Egg dorsal is slightly uplift like arc, and the ventral is straight. Apodous larva has no feet, it is fusiform with an orange yellow center and white fat bodies on both sides, its white head is small and often retracts into prothorax. The pupa has three developmental stages including initial stage pupa, medium stage pupa and late stage pupa. The female's moniliform antenna has 2+12 segments, and the male's has 2+9. The flagellomere of the male is longer than the female, so is its antenna. The base of genital of flagellomere of the male is thick, the front end is tapered, and other features are similar to the female. Larvae ate urediospore, 0 to 50 larvae could be found from one piece of leaf. After fed by the insect, the yellow urediospore heap turns into gray, under microscope the urediospores could be found empty, shrunk or ruptured. In Chengmai County, the overwintered larvae stop dormancy in early September, directly pupate in a cocoon, then pupae into adults, and finish eclosion in late September. October to November is the outbreak period of the larvae. The Mycodiplosis sp. could occur three generations a year(in Chengmai)with generation overlapping. D. tonkinensis rust almost occurs throughout the year, with two obvious outbreak periods. Mycodiplosis sp. occurs in the second period of the rust outbreak, the correlation coefficient of the disease index of rust and the number of larvae was 0.82.[Conclusion] Mycodiplosis sp. is a new mycophagous insect found in planted forest of D. tonkinensis. Under natural conditions, Mycodiplosis sp. shows a significant control effect on D. tonkinensis rust, which suggests a certain potential in biological control of rust disease.

Key words: Mycodiplosis sp, Dalbergia tonkinensis, rust, Hainan