Help with a fairly basic trigger

I am trying to make a spell based off of locust swarm, but in order for it to perform properly I need the locusts to be added to a unit group and still orbit the hero. This is a problem since it is hard to time the (Last Created Unit) since the locusts don't seem to release like clockwork and if you create a locust without the ablity it just flies in circles. I have come up with a sorta half-***ed solution for the moment. What I do is every 3 seconds of gameplay I do a unit group trigger which picks all units owned by the specific hero of the locust type and add them to the unit group, but still leaves anywhere between 0.01 and 3 seconds for error. And if I have possibly 10 triggers like this one going off any less then every 3 seconds I expect the game will lag a lot. Any help?