Setting Variable - (Correct)

Hello, I'm trying do the following function:

CREATE OR REPLACE FUNCTION lost_hours_temp(date)
RETURNS text AS
$BODY$
DECLARE
    START_DATE date;
    END_DATE date;
    QUERY text;
BEGIN
    START_DATE := $1;
    END_DATE := START_DATE - interval '3 year';

    WHILE EXTRACT(YEAR FROM START_DATE) = EXTRACT(YEAR FROM END_DATE)+3 LOOP
        QUERY := 'SELECT    CAST(EXTRACT(YEAR FROM A.production_date) ||
'/' || EXTRACT(MONTH FROM A.production_date) AS TEXT) as date,
                SUM(production_hours) AS production_hours,
                B.id_production_area
            FROM     production A, product B
            WHERE     EXTRACT(MONTH FROM production_date) =
EXTRACT(MONTH FROM ' || START_DATE || ')
                AND EXTRACT(YEAR FROM A.production_date) = EXTRACT(YEAR
FROM ' || START_DATE || ')
                AND lost_hours = ' || 'S' ||'
                AND A.id_product = B.id_product
            GROUP BY id_production_area, date';

        START_DATE := START_DATE - interval '1 month';
    END LOOP;

    RETURN QUERY;
END;
$BODY$
LANGUAGE 'plpgsql';

My problem is into WHILE, I'm trying to concatenate variables with the
string, but I guess that it's generating an error.
What's the correct form to concatenate strings with query in my case?

Thanks