Thanks for the question. 

There are numerous insect critters that may be feasting upon your leaves.  Without seeing an actual picture of the insect, it is impossible to provide you with a specific identification.  See the following:

https://extension.umn.edu/fruit/growing-strawberries-home-garden#insects-and-other-creatures-989061

A possible candidate might be the four-lined plant bug.  See:

https://extension.umn.edu/yard-and-garden-insects/four-lined-plant-bugs

https://savvygardening.com/four-lined-plant-bugs/

http://entnemdept.ufl.edu/creatures/veg/leaf/Poecilocapsus_lineatus.htm

Usually the damage is only cosmetic and normal fruit development may still occur.  Unless you actually see this insect, broad application of an insecticide may do more harm than good.

Good Luck