Now we have some experts in pollination on this list and I am not one of
them but here is my 2 cents worth.
 
      Pumpkins are a native crop in the americas in fact one of the three
sisters the indians grew (corn beans and pumpkins or squash)
   If you wipe out the native pollinators by plowing fence to fence or using
 pesticides profusely then you will have to bring in honeybees.
   In my garden which has not seen much in the way of ag chemicals for more
than 15 years there are wild bees I havn't found pictures of in books
pollinating pumpkins and squash.
    Some of these bees  can be seen on sunflowers also.
Tom