Poisoned locusts were used in an attempt to assassinate Daenerys Targaryen by means of a bowl of edible honeyed locusts laced with poison. Honeyed locusts are sweet and tasty and it must have been hoped that they would tempt Daenerys to take a bite. Whether Daenerys would have had to consume just one locust or a number of them in order to successfully kill her is unknown.
While in their box in Daznak's Pit Dany notes that Hizdahr had well stocked it with flagons of chilled wine and sweetwater, figs, dates, melons, pomegranates, pecans, peppers - and a big bowl of honeyed locusts. 
Upon entering the box alongside Daenerys Strong Belwas immediately spotted the bowl of honeyed locusts and began to crunch them by the handful, at the same time Hizdahr tries to encourage Dany to eat the locusts:
These are very tasty … you ought to try a few yourself, my love. They are rolled in spice before honey, so they are sweet and hot at once.
Dany merely replies:
During the fighting Belwas finishes the entire bowl of honeyed locusts on his own. He gives a belch and takes a swig of wine.
After Barsena Blackhair is killed by the boar, Belwas rubs his swollen stomach and remarks that he is sick in his belly. A little later he says that he has eaten too many locusts, and he has a queasy look on his face. When Daenerys’s black dragon Drogon suddenly appears above the pit amidst the uproar Belwas gives a moan, stumbles to his feet and falls to his knees. As Drogon consumes the boar and Barsena Blackhair Strong Belwas is retching nosily – the poison is taking effect.
Sometime thereafter the Shavepate informs Ser Barristan that he has the poisoner – Hizdahr’s confectioner. However the confectioner was just a catspaw, the Sons of the Harpy took the confectioner’s daughter and swore that she would be returned unharmed once the queen was dead. Belwas and the dragon saved Daenerys. No one saved the girl, she was returned to her father in the black of night, in nine pieces. One for every year she lived.
These are very tasty .. sweet and hot .. yet he had never touched so much as one himself.
Ser Barristan decides to question the confectioner himself. He also wonders if the poisoned locusts were actually meant for Hizdahr as his death would have smashed the fragile peace.
- An analysis on who poisoned the locusts on: The Meereenese Blot
Where is liver and onions? Strong Belwas is not so strong as before, he must eat, get big again. They made strong Belwas sick. Someone must die.