Q. Is it best to remove all the leaves from the soil after they have fallen or leave them on the soil as mulch for fruit trees? My concern is about insects that might stay there during the winter and cause damage in the spring.
A. In a very general sense I have two concerns regarding leaving undecomposed leaves at the base of plants. You are right, the first one is insect problems and it's very possible some insects will overwinter under loose leaves that have not decomposed.
Remember to pick up any fruit lying on the ground as well. Do not leave dried fruit on fruit trees because of potential insect problems.
One insect in particular is the grape leaf skeletonizer. Don’t leave debris around the base of grape plants because of this insect problem. Make sure leaves are shredded or composted. If you have one, lawnmowers are efficient leaf shredders.
The second potential problem is disease. There are a few diseases that cut overwinter on fallen leaves. It is always better to compost or shred leaves if you're going to leave them around the base of plants. Sanitation in the garden is important.