differences in nut development between pecan cultivars can be seen early in the growing season. Today, I drove by our Wilson hican tree (a pecan/hickory hybrid) and was surprised by the size of the nuts hanging from the branches. Just for fun, I decided to compare nuts from our hican, a shagbark hickory and a Mohawk pecan (photo above). It is obvious, by looking at nut size, that the hican and hickory are way ahead of the pecan in terms of nut development.
If you have ever collected hickory nuts in the wild you will find that most nuts have been attacked by pecan weevil. In fact, the life cycle of pecan weevil seems to be more in tune with hickory nut development than with pecans. Pecan weevils start emerging from the soil in late July. At this time, hickory nuts are filled with kernel and provide a great spot for female weevils to lay eggs. But in a pecan grove, early emerging weevils often need to wait around until pecans fill their kernels in mid-august to finally start laying eggs. Since female weevils don't like to wait, they will migrate to trees that fill their kernels first. This leads to weevils populations to become clustered around early ripening pecan trees.
Pecan weevil has placed evolutionary pressure on pecan as a species to delay fill kernel later in the season. This happens because the nuts of early kernel filling species (the hickories) or cultivars are often completely destroyed by weevil larvae making it difficult for these trees to produce viable seed to create a new generation of trees. In contrast, late filling pecans avoid weevil attack and are able to spread thousands of viable seed into the environment.
Filling kernel late in the growing season creates another set of problems for pecan trees that is not associated with pecan weevil. Pushed too late into the fall, late kernel filling cultivars often have trouble creating a full, plump kernels and nuts can fail to split their shucks before first fall freeze. From my experience, the best compromise between late kernel fill for weevil avoidance and nut development early enough to avoid frost damage are pecan cultivars that fill their kernel by mid-August and shuck split one week before the average date of first fall freeze.