Answer: The Gemara (Pesachim 24a; Makkos 16b) writes that one who eats an insect transgresses multiple aveiros.
While mideoraisa certain prohibited foods in a mixture would be battul berov, annulled against the majority, Tosafos (Chullin 95a) writes that this wouldn’t apply when the forbidden item can be clearly seen.
Additionally, the Shulchan Aruch (YD 100:1) writes that a beriya, whole creature, is not even battul in a ratio against one thousand kosher parts (See Gemara Chullin 99b).
The Rema (YD 84:8) quotes the Rashba (Toras Habayis 3:3; Teshuvos 274) who writes that one only needs to check dates if there is a miut hamatzuy, reasonable chance, that they are infested. There is a machlokes, however, as to what constitutes a miut hamatzuy. R’ Yaakov Bruchin (Mishkenos Yaakov YD:17) writes that it refers to a 10% chance. There is a further debate as to how to calculate this (See Minchas Shlomo 2:61). R’ Shmuel Wosner (Shevet Halevi 4:81; 5:156:4), however, writes that it cannot be measured by such statistics. Rather, it refers to anything considered to be a common occurrence. Different kashrus organizations follow various opinions on this matter.
R’ Ovadia Yosef (Yechave Daas 1:18) stresses the importance of checking one’s lettuce before eating it for maror on seder night, though stresses that there is no need to look for insects with a magnifying glass (See Shevet Halevi 7:122; Igros Moshe YD 4:2).
In conclusion, it is very important to only eat lettuce that has been checked for bugs, though there is no need to use magnifying glasses, etc. to check them.