Question: Is one allowed to soak lettuce on Shabbos to get rid of any bugs?
Answer: The Shulchan Aruch (OC 319:8) forbids soaking karshinim, grain for animals, in water on Shabbos as doing so will separate the dirt and grain which is borer. The Mishna Berura (319:29) writes that this would apply equally to washing dirty potatoes, etc.
Nonetheless, R’ Moshe Feinstein (Igros Moshe OC 1:125) differentiates between soaking karshinim and rinsing fruits and vegetables. Just as one can peel onions and garlics as that is considered derech achila, the normal way of eating them, so too, it is acceptable to rinse fruit before eating them. Additionally, one can’t compare dirty potatoes that everyone would wash, to fruit and vegetables that many would eat without rinsing.
Nonetheless, R’ Moshe writes that while one can rinse them off under a running tap, one shouldn’t soak them in a bowl of water.
R’ Shlomo Zalman Auerbach (quoted in Shemiras Shabbos Kehilchasa 3:n48) also explained that there are a number of differences between soaking karshinim and rinsing fruits and vegetables.
Nonetheless, R’ Shmuel Wosner (Shevet Halevi 1:52:2) and R’ Yehoshua Neuwirth (Shemiras Shabbos Kehilchasa 3:21) write that if the fruit or vegetable is particularly dirty to the extent that most people wouldn’t eat it, then one shouldn’t wash it on Shabbos.
Regular lettuce must be soaked in soapy water and inspected in order to ensure that it is bug free. If the lettuce is dirty, one mustn’t do so on Shabbos because of borer. Even clean lettuce likely has aphids and thrips which will be killed in water and so shouldn’t be washed on Shabbos (Shemiras Shabbos Kehilchasa 3:36). Thus, unless one has lettuce that one knows is unlikely to be infested, one shouldn’t soak it on Shabbos. One may rinse it under a tap on Shabbos, and inspect it under a light, though any small bugs should be removed with part of the leaf.Wherever possible, lettuce should be soaked and inspected before Shabbos.