Pumpkin and squash crops (Cucurbita spp.) are entirely dependent upon insect pollinators for reproduction. These crops have staminate (male or pollen producing) flowers and pistillate (female or pollen receiving) flowers that each last a single day. Pollen must be transferred by insects from the staminate to the pistillate flowers to produce a marketable fruit. Nectar and pollen production in these crops is copious and they are attractive to many insects for that reason. Our study focused on the pollen produced by squash flowers.
In Ontario, Canada, the most important pollinator of pumpkin and squash crops is the wild hoary squash bee (Eucera pruinosa), a ground-nesting, solitary bee that is also a pollen specialist on plants in the Cucurbita genus. In many parts of its present range, including Ontario, the only source of pollen for this bee species is Cucurbita crops because no wild Cucurbita are present. Although they do visit flowers and provide pollination services, male hoary squash bees do not participate in harvesting pollen to feed to their offspring. Because of this, in this study we focused on female hoary squash bees only.
As such, in this context we have a unique opportunity to study pollen partitioning between a crop plant and a wild pollinator. In this cropping system, to evaluate how pollen is partitioned between the plant’s reproductive needs (to set seed and produce a viable marketable fruit) and the bee’s reproductive needs (to produce viable offspring), we measured pollen production by staminate (pollen producing) squash flowers, the pollen dislodged from the anthers and lost as waste due to the activities of bees in staminate squash flowers, squash pollen loads collected by female squash bees in a single foraging trip, and the number of pollen grains in fully provisioned hoary squash bee nest cells.
We then compared these to the crop’s pollination requirements (2500 pollen grains deposited on a stigma for maximum pollination) as reported in the literature and the flower sex ratio of acorn squash (2 staminate flowers to 1 pistillate flower). Under these circumstances, each staminate flower would contribute 1250 pollen grains to each pistillate flower for the purposes of plant reproduction.
In this pollination system study, which excludes all bees except female hoary squash bees, the plant incurs costs to produce pollen to ensure its own reproduction via insect pollination. These costs could be considered the cost of pollination services by the bee. Overall, we found that about 13% (2,136 pollen grains) of the total pollen produced by the staminate flowers was wasted. Of the remaining 14,160 pollen grains, 1,250 grains (~9%) were needed to maximize pollination, leaving 12,910 pollen grains (~91%) available to hoary squash bee females to provision cells in their underground nests where they raise their offspring. As such, using the services of hoary squash bees, the crop incurs about tenfold cost in pollen grains to achieve maximum pollination of about 2,500 pollen grains per pistillate flower.
Female hoary squash bees need about 63,000 pollen grains, (or all the pollen produced by about 5 staminate flowers after waste and pollination needs have been accounted for) to fully provision a nest cell before they can lay an egg in it. In reality, a single bee likely visits many more than 5 flowers as other females are also competing for the pollen resources and as pollen resources decline over the day. On average, the female bees carry about 3000 pollen grains into their nests from a single foraging trip. This means that a female must make about 21 foraging trips before she can amass enough pollen to produce a single offspring.
By weighing live female squash bees and the pollen loads they carry in a single foraging trip, we also found that the average amount of pollen a female hoary squash bee could carry in a single foraging trip was about 4% of her body mass. This is fairly low compared to many other wild bees who carry 7-64% of their body weight in pollen in a single foraging trip.
The information provided here contributes to the discussion about costs to plants for insect pollination services by providing an example of a concrete pollen budget for crop-wild pollinator system. However, where both male and female hoary squash bees and many other bee species are present, waste may be higher and pollen available to any single species may be lower, depending on the density of pollinator populations with respect to available pollen resources. However, it should be noted that in Ontario, Canada, hoary squash bees are the first species to begin foraging on the squash flowers when they open at dawn, followed by bumble bees, and long before honey bees or many other small solitary bees are present in large numbers, giving them the advantage of first access to pollen on the crop flowers.