TWO known Ross River virus activity hotspots cannot be treated by the Shire of Harvey, despite eight infections being reported to the Department of Health since July.
Two sites in the Leschenault Peninsula remain untreated because they are listed by the State Government as highly protected conservation areas.
One site is at the northern entrance of the park, near Buffalo Rd.
Shire principal environmental health officer Scott Dandridge said mosquitoes from around that area have been tracked migrating into nearby residential areas approximately two kilometres across the estuary.
He said the second site was at the southern end halfway between the camping ground and Belvedere.
Mr Dandridge said spraying in the area would cause minimal damage, as it did not kill the mosquito larvae but stops them progressing to adults and spreading the Ross River virus.
He also said the treatment would not poison birds that ate the larvae.
But the Department of Environment and Conservation will not allow spraying in the park because mosquitoes are an important food source for native birds and a significant component of the protected ecosystem.
Mr Dandridge said the population was at risk because more buildings were being approved closer to the reserve and sealed roads and walkways in the park have made it more accessible to tourists.
The shire and University of WA monitor virus activity to know when it reaches a level needed to be treated.
The DEC said there were signs in the reserve warning visitors of the mosquito risk.
It also said the insects do not travel across the estuary and would not affect Eaton and Australind residents.
The DEC said the shire had not contacted the Department on the issue and a normal course of action would be to work with the shire to do a risk assessment and decide on a course of action.
The shire is working with the DEC to erect new, more visual and easy to understand signs warning park visitors of the mosquito risk.