Normally, pumpkins should be left on the vine until they mature; when the color turns a rich, dark orange (depending on the variety) and the skin does not puncture when it is pressed with your thumbnail. If you harvest them early, they will not keep.
It sounds like you have pickleworms boring into your pumpkins. Pickleworms normally begin feeding on the flower buds and then move to the fruit when they get larger.
When the buds first form and the flowers begin to open, spray with Bonide Bt Thuricide or Bonide Captain Jack's Deadbug Brew which contains spinosad. These products are normally less harmful to beneficial insects.
- Spray late in the early evening or early in the morning when the bees are less active.
Pickleworms can also be controlled with Bonide Eight sprayed according to the label directions.
- Again be sure to spray any insecticides when the bees are less active.