The insects are believed to have emerged from a passenger's bag on the flight back from Venice, the station reported.
The plane landed around 2 p.m. and was taken out of service, United said.
"We are concerned by the experience our customer reported on United flight 169 from Venice to Newark," United said in a statement.
"We have been in contact with the crew and they have advised the ants have been isolated from a customer's bag. At this time, the aircraft will continue to its final destination. We will be taking the aircraft out of service when it arrives in Newark."
Charlotte Burns documented the flight in a tweet.
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