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

Here is Why You Need An Adapter

The outlet and plug type used in Saint Lucia is G and in Pitcairn Islands is I. This is how they look like.

Pitcairn Islands

In Pitcairn Islands, outlets of type I are in use.

Type I

Saint Lucia

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

Type G