Burrowing Trigger For Zerg

[wc2wc2]

In a map that I have made, I want the zerglings to automatically burrow when they are created. The zerglings are made by activating a trigger, not by using the hive. No luck yet looking under the "Actions" in triggers.
¿Any thoughts?

 

[Hiho-E(A)]

Create units with properties, and select burrowed. Make sure you have the burrowed ability automatically upgraded.