Do I need a mosquito net in Sri Lankan guesthouses?

You generally do not need to bring your own mosquito net, because most reputable guesthouses in Sri Lanka provide them, or they offer fully air-conditioned rooms, which means mosquitoes are less of a problem overall.

Diseases spread by bites, like dengue, do exist in Sri Lanka, so bite prevention matters a lot. If a budget guesthouse uses natural ventilation or relies heavily on fans, then a bed net becomes crucial, and the host will usually provide one as well. In practice, it is easier to pack a good DEET or Picaridin-based insect repellent and also use plug-in mosquito vaporisers instead of hauling around a bulky travel net in your luggage.