In Manhattan, cockroaches are spreading the deadly "Strickler's disease" that is claiming hundreds of the city's children. Entomologist Susan Tyler uses genetic engineering to create what her colleague (and husband) Peter Mann and she call the Judas breed, a large insect that releases an enzyme which causes the roaches metabolism to speed up and starve themselves to death. It successfully kills off the disease. The released population was all-female and designed with a lifespan of only a few months, so that it would only last one generation.
Three years later, a reverend is chased and dragged underground by a strange assailant. The only witness is Chuy, the "special" ward of an immigrant subway shoe-shiner named Manny. Two kids later sell a "weird bug" from the subway to Susan, which she performs test on, and realises is part of the Judas breed. Looking for more valuable specimens, the kids go down the tracks where they find a large egg sac and are then killed. Chuy also enters the church to find "Mr. Funny Shoes" and is abducted. Peter, CDC officer Josh and subway cop Leonard enter the maintenance tunnels to investigate but Peter and Leonard get separated from Josh, who is then killed trying to find his way back up. Susan is taken from the train platform by one of the creatures and brought into the tunnels. Manny also enters the tunnels in search of Chuy and comes across Susan, whom he rescues along with Peter and Leonard, before they barricade themselves in a train car.
Susan surmises that the Judas' accelerated metabolism has allowed it to reproduce and that they have evolved to mimic their human prey. The group formulates a plan to get the car moving: Peter will switch the power on, and Manny will switch the tracks. Susan projects that the Judas will spread throughout the tunnels unless they are able to kill the single fertile male. Manny finds Chuy but is killed by the male Judas, so Susan goes in search of him but finds only Chuy. Leonard's injured leg starts bleeding heavily, so he causes a distraction from the others, and is killed. Peter finds a dumbwaiter and puts Susan and Chuy in it, but stays behind to destroy the breed for good. He gets chased into a room where hundreds are nesting, and blows them all up by setting fire to a loose gas pipe, before diving underwater to safety.
The male Judas escapes the blast and goes after Chuy but is distracted by Susan, who has it chase her into the path of an oncoming train. The two make it to the surface, where they are reunited with Peter, who Susan had assumed died in the blast.

Question: Which roach did Susan realize needs to be killed?
Answer: The only fertile male roach.
Alternative answer: the single fertile male Judas