You Do Need power plug adapter while traveling from Saint Lucia to Benin.

Here is Why You Need An Adapter

The outlet and plug type used in Benin are C and E and in Saint Lucia is G. This is how they look like.

Saint Lucia

In Saint Lucia, outlets of type G are in use.

Type G

Benin

In Benin, outlets of type C and E are in use.

Type C

Type E