For more than 30 years, the National Parks and Wildlife Service (NPWS) has been working to eradicate rhododendron from Killarney National Park.
But MEP Lynn Boylan has claimed re-infestation is occurring at an “alarming pace”.
Arts and Heritage Minister Heather Humphreys, however, countered the claim insisting “significant inroads” had been made in tackling the problem.
The department, she said, had spent €2.7m on eradication work in Killarney in the past decade.
The spread of the rhododendron, which suffocates other plants and prevents the regeneration of trees, is seen as one of the main threats to woodland in Ireland.
The labour-intensive work is being carried out in Killarney where ancient oak woods need to be protected by NPWS staff, in co-operation with private contractors and volunteer groups, including Voluntary Services International.
The native oak woods are in a Special Area of Conservation (SAC), under an EU Habitats Directive.
Groundwork, a voluntary organisation which according to Ms Boylan, helped clear 40% of the woodlands between 1981 and 2009, has lodged a formal complaint with the European Commission that the NPWS was in breach of the directive.
An ecologist by profession who previously worked with Groundwork, the MEP said due to differences between park management and Groundwork, the organisation ceased its involvement in 2009.
“Despite promises from the national park management to continue the maintenance in 2013, Groundwork revisited the woods that had been designated clear and found re-infestation was occurring at an alarming pace,” she said.
Kerry-based Senator Paul Coghlan, meanwhile, strongly criticised Sinn Féin for “incorrect statements” about the work being carried out and the success rate.
The Fine Gael senator said a department report showed “considerable success”.
“When rhododendron clearance programmes were introduced in the early 1980s, about 3,000 hectares of the 10,000 hectares in the park were to some extent affected by rhododendron infestation,” Mr Coghlan noted.
“The management programme has made significant inroads into the problem and now about 2,000 of those hectares involving some 40 different sites are under effective control.”