A composition comprising a terpene, an emulsifier, and a copper complex is provided, along with methods for its use as an algaecide. The composition is effective when used as an algaecide to treat algae growth in aquatic environments, even though it contains much less terpene than other commonly-used algaecides and can be used without a conventional surfactant. The composition is also less toxic and less flammable than other commonly-used algaecides.