In a potentially futile bid to stem the spread of an invasive insect that has already wiped out 50 million ash trees in 24 states, New Jersey on Thursday announced plans to treat ash trees on state-owned lands in nine counties, including The Hermitage in Ho-Ho-Kus and Ringwood State Forest in Bergen and Passaic counties.

The tiny emerald ash borer beetle, native to Asia, was first spotted in New Jersey in May 2014 and has now been seen in 14 towns, including Hillsdale.

This week the state Department of Agriculture started placing purple sticky traps in areas where the beetle is active, and also began to release parasites in parts of Morris and Somerset counties that eat the beetle’s eggs and larvae.

The state may also start treating some ash trees on state-owned land with preventive pesticide. But such efforts will be limited because, unlike the widespread aerial spraying for gypsy moths, the process is time-consuming and labor-intensive. One of the methods is boring quarter-inch holes into the tree’s trunk about two inches off the ground, injecting a liquid mixture of the insecticide into the trunk and then sealing the holes with a biodegradable cap, said DEP spokesman Larry Hajna. The insecticide goes into the tree’s vascular system and does not touch the ground or air, he said.

North Jersey is particularly vulnerable to an ash borer beetle invasion, since many of the state’s estimated 25 million ash trees are located in northern forested areas. Ash trees are also frequently used to line North Jersey’s suburban streets and in residential landscaping because they provide good shade and have vibrant fall color.