About 100 more venomous fire ants have been found at the Port of Tokyo's Oi Pier. It's thought they came from a shipping container.
Thursday's discovery of the pests comes after a single fire ant was found in the same container, which arrived from China, during an inspection on Monday of last week.
A subsequent investigation by the Tokyo Metropolitan Government and the Environment Ministry led to the discovery of over 100 fire ants.
Officials continued their search, peeling off plywood floor panels in the container.
As a result, another 100 fire ants, as well as eggs, larvae and pupae were found inside the wood.
The container in question arrived from China's Guangdong Province via Hong Kong.
Officials suspect the insects might have gotten into the container either in Guangdong or Hong Kong and bred inside the floor panels.
That makes it difficult to rule out the possible existence of a queen fire ant, even though it has not yet been found.
Stings by fire ants, native to South America, can be fatal.
Since May, the ants have been found in four prefectures in Japan, including Tokyo.