Abstract: Crefton weed (Eupatorium adenophorum) has a great influence to the development of agriculture,forestry,livestock husbandry and sideline in Yunnan province and even in southwest China. To control it effectively,a biological control strategy is put forward. In view of optimal control,the authois made a primaiy study on P. utilis-E. adenophorum system. The status equation of optimal control system can be summarized as: Y=2.140282Y-0.029968U the optimal control trail: U^*(t)=14285.7715 exp(-2.140878t) the optimal status trail:Y^*(t)=100 exp(-2.140878t) here Yis the percentage of unparasitized shoot,Uis the number of P. utilis. It is optimal that one P. utilis holds ten shoots of E. adenophorum for its releasing on field. On basis of these results,the strategy of releasing P.utilis in poly-sites and one site at several times was advanced firstly.

Key words: Optimal control, Bio-control strategy