The Farmer Assurance Provision (FAP), a federal law enacted earlier this spring to protect farmers against lawsuits by anti-biotech activists, expired last week. The provision was part of a temporary federal spending law passed in March and was not renewed by the U.S. Senate.

The law’s expiration, biotech advocates worry, could leave farmers unprotected against a determined push by activists using the courts to try to stop the use of genetically modified organisms (GMOs). In several cases, the activists have filed lawsuits to thwart the planting of biotech crops even though the technology has been...