Answer: Rock
Earlier in the movie, Flik and Dot have a conversation in which Flik gives Dot a rock but tells her to pretend it is a seed, telling Dot that all trees start off as seeds and that Dot will eventually blossom into an adult ant just like seeds blossom into trees. Despite having trouble understanding that she is supposed to pretend that the rock is a seed, Dot likes Flik for this. Later in the movie, after Flik is expelled from Ant Island and Dot overhears that the grasshoppers will eat all the food on the island and kill the queen, Dot flies across the canyon and tells Flik to come back, telling him what she overheard. At first, Flik refuses to come back, telling Dot that he deserved to be expelled because he only makes things worse and telling Dot to go home so she doesn't make the same mistakes he made. The circus bugs and Dot try as hard as they can to cheer him up. What works is when Dot gets a rock and tells him to pretend it is a seed. This is what convinces Flik to come home. The circus bugs, despite being confused about the rock, help Flik return home and decide to help the ants, this time using their circus talents rather than pretending to be warrior bugs.