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Attorneys for a group of lawn care companies and homeowners asked a Montgomery County Circuit Court judge Wednesday to invalidate a county law barring the use of certain pesticides from private lawns, arguing that the products are well regulated by state and federal agencies.

The law, passed by a divided County Council in 2015, made Montgomery the first major locality to bar nonessential cosmetic pesticides. It is scheduled to take effect at the beginning of next year.

The measure targets pesticides containing chemicals that may cause cancer and allows only compounds that are organically based or deemed to be of minimal risk under the 1996 Federal Insecticide, Fungicide, and Rodenticide Act. It exempts agricultural land, gardens and golf courses.

The measure does not prohibit the sale of lawn pesticides within the county. A provision of the bill eliminating the use of herbicides and pesticides on certain county properties took effect in July. It is not part of the legal challenge, which was filed in November.

Opponents of the law say that the county is improperly regulating substances closely controlled by state and federal environmental agencies and are safe when used correctly. The court challenge is supported by an organization called Responsible Industry for a Sound Environment, which is a national trade association for pesticide manufacturers and distributors.

Attorney Tim Maloney told Circuit Court Judge Terrence McGann during the two-hour hearing that the sheer volume of Maryland regulations effectively preempts any role for the county.

“The state is comprehensively occupying this field,” said Maloney, who represents the Scotts Co., a major manufacturer of lawn care products, and other corporate and homeowner plaintiffs.

He said the new law, which requires retailers to post signs about the dangers of certain pesticides and the options permissible under the statute, will only cause confusion, especially because there are provisions allowing any of the county’s 19 incorporated municipalities to opt out of the law.

McGann, who askedboth sides pointed questions, took issue with Maloney’s assertion.

“Why is that so confusing?” he asked. “Who cares as long as you know the rules in your area? If you live in Mayberry, all you have to know is what the rules are in Mayberry.”

Supporters of the legislation say that the state and federal government do a poor job of oversight, especially in the area of possible links between childhood pesticide exposure and cancer.

Assistant County Attorney Edward Lattner said there is nothing in state law — which focuses on labeling, registration and licensing of lawn care workers who apply pesticides — that prevents the county from intervening in other areas.

“The county pesticide law does not address any of those things,” Lattner said. “It is a limited ban on application of certain pesticides in certain areas.”

McGann expressed skepticism of the broad exemptions in the bill for athletic fields, trees, hedges and golf. “Some children play golf,” he said.

At one point Lattner said that the challenge to the bill “isn’t a case about the wisdom of the law.”

“You’re saying it lacks wisdom?” McGann asked.

Lattner said he meant that the heart of the legal challenge was the issue of preemption, not the specific merits of the law.

“I won’t challenge the county’s wisdom anymore,” said McGann.

The judge said he would issue a written opinion soon.