Section 26D. The county commissioners, subject to appropriation therefor, may grant to any city or town sums of money to aid in the suppression of the pests declared to be public nuisances in section eleven, and Dutch elm disease; provided, that no such payments shall be made unless the chief superintendent certifies that the work has been done under his direction; and said county commissioners may assist in organizing cities and towns in co-operative associations for the suppression of said public nuisances or scouting or sampling for Dutch elm diseased trees or wood.