Fire ants have found in various places (mostly international ports) in Japan recently. And they were also found in Honmoku Pier, Yokohama.
The Japan News:
Over 500 fire ants found in Yokohama
Press release by the city (PDF in Japanese)
Report on the fire ant investigation at Yokohama port (July 14th, 2017)
(quote and translation from the report)
When you find fire ant, stay away and don't touch or try to break the nest.
When you were stung by fire ants, stay calm and rest for about 20-30 minutes. If it gets worse, go immediately to the doctor's. Tell the doctor that you were stung by fire ants and you might have an anaphylaxis.
Other fire ants news in Japan:
Nikkei Asian Review:
Fire ant scare gives Japanese pesticide stocks a boost
About the fire ant
National Institute for Environmental Studies