Using Mawhin's continuation theorem, sufficient conditions are obtained for the existence of positive periodic solutions for periodic delay-Lotka-Volterra systems.
Bereanu, C. (2005). Periodic solutions for delay competition systems and delay prey-predator systems. Advanced Nonlinear Studies, 5(3), 393-410. https://hdl.handle.net/2078.5/44802 (Original work published 2005)