According the the National Agroforestry Center over 100 crop species in North America require a visit from a pollinating insect. Many of these crops are the kinds you would find in a kitchen garden. When the crops don't get the visits they need, what you will see are a plethora of blossoms that never turn into fruit (or veg). We are seeing this in our garden, especially with our pumpkin and squash crops.
In an article written by individuals leading up the Forgotten Pollinators Campaign, I read that "there are fewer beehives today in the U.S. than at any time in the past 50 years." I also read that there are over 40,000 species of bees in the world. Pretty amazing, and sad at the same time. It reminds you how the smallest change in our eco-system can have a major impact.
Tamara, one of our squash crop gardeners was talking to her dad - a long-time farmer - about the lack of squash, and he recommended helping the pollination process along by gently twiddling a small paint brush around inside the head of each flower.
For squash, you start with the male blossom (a flower with no swelling or bump where the stem attaches to the plant) - gently touch the center of each flower with the brush. Then move to the female blossoms (the flowers are attached to the end of a small ovule that will grow into a full sized fruit) and dab the paint brush inside the flower. Voila pollination is complete!
Another option - if you are not into painting - that will help draw the bees to your garden is to plant white clover.