If the State Entomologist finds that the existence of the gypsy moth has reached epidemic proportions in any area of the state, he shall so instruct the affected municipalities which shall jointly undertake such spraying as the State Entomologist deems necessary, one-half of the cost of which shall be borne by the state on application of such municipalities to the State Board of Pesticide Control not later than June fifteenth of the succeeding year and one-half of the balance of such cost may be assessed against landowners as provided in section 22-91d.