A New Jersey county has paid Pennsylvania $10,500 to spray for black flies along the Delaware River.
Hunterdon County took the step after New Jersey's Environmental Protection Department decided not to participate in the program this year.
In a statement, Freeholder Board Deputy Director John Lanza said "black flies are not just annoying and intrusive, they have a negative impact on the quality of life for residents" along the river.
Pennsylvania says it uses naturally occurring bacteria that kills larval as they feed. Officials say it is not toxic to fish or other aquatic life.
New Jersey's DEP plans to participate in the program next spring.