Abstract: Mating behavior of Monochamus alternatus Hope, a major vector of Bursaphelenchuh xylophilus (Steiner & Buhrer) Nickle was observed both in the laboratory and field. The results showed that a complete mating of M. alternatus involved three stages：encountering pair-bonding, ejaculation and post-copulatory guarding. There were many ejaculations in the whole mating process. Of the 123 matings observed in the experiment, the average lasting time for one copulation of M. alternatus was 66.27 min, the time for pair-bonding before ejaculation was 3.51 min with each ejaculation of 57.6 s, and the time of post copulatoryguarding was 15.18 min. M. alternatus made 5.15 matings in four days after the beginning of reproductive stage, and the mating chance by different male individuals were quite different from each other. Mating behavior of M. alternatus presented drastic male competition. Males could adjust their investments of mating according to females and their own mating experience. When a female or male is virgin,  total time of ejaculations is significantly longer than the one which has been mated, and the frequency of ejaculations is significantly greater. Mating behavior of M. alternatus observed in the field was basically consistent with the results observed in the laboratory.

Key words: Monochamus alternatus, mating behavior, male competition