Answered by Shaykh Hamza Karamali, SunniPath Academy Teacher
Is it permissible for someone in a state of pilgrim sanctity (ihram) to kill an ant?
[This answer was originally posted as a response to a student question in the course, The Reliance Explained — Part 2.]
In the Name of Allah, Most Merciful and Compassionate
Scholars distinguish between harmless large ants (al-naml al-sulaymaniyy) and harmful small ants. The latter are permissible to kill–just as it is permissible for a pilgrim to kill other harmful animals, such as scorpions and mice–but the former are impermissible to kill because (a) they are harmless and (b) the Prophet (Allah bless him and give him peace) forbade the killing of harmless large ants.  
And Allah Most High knows best.
 Sulayman al-Jamal, Hashiyat al-Jamal `ala Fath al-Wahhab, Bab: Ma haruma bi al-ihram
 Abu Dawud narrates that the Messenger of Allah (Allah bless him and give him peace) forbade the killing of four kinds of animals: ants, bees, hoopoes, and surad [h: a kind of bird]. Baghawi, Nawawi, and others understood “ants” here to exclusively mean harmless black ants, not other kinds of ants.