Haleakala National Park is seeing an unusually high number of Western yellowjacket wasps, and park officials are warning visitors to take necessary safety precautions.

Wasp numbers typically peak in the summer months, but "the numbers are going up in places that visitors frequent," said park biologist Raina Kaholoaa. "As with everything else in nature, sometimes you have higher numbers than others. This year, numbers are exceptionally high."

The park will remain open as resource managers try to control wasp populations.

The problem has been reported at the Kapalaoa Cabin, the base of the Sliding Sands trail and at the trail junction to the Halemau'u Trail.

Those who are allergic to wasp and bee stings are being asked to avoid those areas and to carry prescription medications, Kaholoaa said.

"We don't want to discourage visitors from coming, but we want to warn visitors that there is a possibility you can get stung," she said. "There are things we can do such as not wearing scented perfumes, which attract the wasps."

Wasps are also attracted to water, meat, sunscreen and sweets, according to a park news release. They may swarm around people who are stopping for a snack, and may sting repetitively.

Stings may cause redness, swelling and itching. Severe reactions include difficulty breathing, hives, loss of consciousness, anaphylactic shock and respiratory arrest.

Park officials are urging people to avoid problem areas and to remain calm in the event that a wasp attacks. Swatting at the wasps will only make them more aggressive, the release said. Visitors are advised to walk calmly away from the swarm and to notify park staff in the case of a medical emergency. If a serious reaction occurs, visitors should call 911.

Meanwhile, park resource managers are trying to control the pests by setting water traps in areas of high activity. They are also working to locate and eradicate wasp nests throughout park grounds.

The wasps may pose an inconvenience to park visitors, but they may also have wide-ranging impacts on Hawai'i's ecological systems.

The Western yellowjacket is just one of many invasive species that threaten to harm Hawai'i's native plants and animals. According to a study by researchers Daniel S. Gruner and David Foote, the black and yellow wasp "poses a major threat to the continued viability and health of Hawaiian arthropod communities."

"Its ecological impacts may lead to dramatic ecosystem changes and facilitation of invasion by other introduced species."

By preying on native arthropod species, the wasps may cause other problems, such as a decrease in food availability for birds and a reduction of pollinators for plants.

The state has taken various efforts in recent years to control alien species. In 2003, the Hawai'i Invasive Species Council was formed to address the problem of imported pests.

Millions of dollars are spent every year to manage harmful invasive species statewide. In 2006, the invasive species council estimated spending $40.8 million to control alien species, still short of the recommended $50 million per year.