Brown marmorated stink bug. Photo by David R. Lance, USDA APHIS PPQ - This image is Image Number 1460048 at Invasive.org, a source for images of invasive and exotic species operated by The Bugwood Network at the University of Georgia and the USDA APHIS Plant Protection and Quarantine program., CC BY 3.0

TOKYO — Japan is bracing for a swarm of stink bugs that can attack tree fruits, with an unusually large number of prefectures already issuing alerts.

Of Japan's 47 prefectures, 30 from west to east, including Tokyo, had issued stink bug alerts as of Friday, according to Ministry of Agriculture, Forestry and Fisheries data.

The tally is in contrast to the previous 10 years, when the number of prefectures issuing stink bug alerts between January and May averaged around five but never topped 15.

The number of insects has already been rising and is expected to surge further following the summer breeding season.

Warmer temperatures appear to be one of the factors contributing to the insect outbreak.

They benefited from an abundance of food, such as cedar and cypress nuts, in the fall, and a substantial number of them seem to have made it through the warm winter, said Mantaro Hironaka, an associate professor with expertise in entomology at Ishikawa Prefectural University.

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In Japan, three stink bugs — brown-winged green stink bugs, brown marmorated stink bugs, and polished green stink bugs — are considered major pests that cause damage to fruits on the branch, including apples, pears and peaches, by feeding on the fruit flesh.

"We call (on farmers) to frequently make the rounds of their farms," a ministry official said. Protective measures include spraying chemicals and bagging fruit.

So far this year, stink bug alerts have been largely issued in prefectures in warmer western parts of the country, with the earliest one issued by Ehime on March 22.

Chiba Prefecture, east of Tokyo, known as one of the top pear producers in the country, issued an alert this month for the first time in 10 years. Personnel conducting a survey at 10 locations in the prefecture in April captured more than seven times the usual number of stink bugs.

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Experts warned that cooler areas of the country need to be aware of potential outbreaks as well given that alerts have been issued in the past in prefectures in northeastern Japan.

Stink bugs earned their name for exuding a fluid that usually has a disagreeable odor from glands between the legs.