Department of Mathematics Longnan Teachers College Longnan Gansu 742500 P.R.China;
School of Mathematics and Statistics Lanzhou University Lanzhou Gansu 730000 P.R.China;
School of Mathematics and Statistics Lanzhou University Lanzhou Gansu 730000 P.R.China;
College of Mathematics and System Science Shandong University of Science and Technology Qingdao Shandong 266590 P.R.China;
Chernostat; nonlocal delay; uniform persistence; periodic solution;