Case: Pollinator Ecologies

A Case Study in Reciprocal Thinking: Humans, Cucurbita Crops & Squash Bees

In this case, we examine the reciprocity that has existed for millenia between humans, cucurbita crops (pumpkins, squash, and gourds), and squash bees, and discuss how that reciprocity can be threatened or strengthened by the actions of humans.

Beginning 10,000 years ago in Meso-America and again 5,000 years ago in what is now the American mid-west, Indigenous peoples began domesticating wild cucurbita, especially the wild buffalo gourd (Cucurbita foetidissima)^[1]. Buffalo gourds are perennial vines that produce thick-walled, inedible bitter fruit that were initially used as containers for water, storage, and floats^[2]. Over time, domestication resulted in five species of pumpkin or squash crops grown for their large, nutrient-rich, edible fruit, and which continue to have cultural importance among Indigenous peoples^[3].

Cucurbita domestication can be considered a co-evolutionary process between plants and humans that resulted in crop plants that are incapable of thriving outside of human influence and humans that are dependent upon crops for food^[4]. Modern agriculture enjoys access to hundreds of cultivars of pumpkin or squash crops with different sizes, shapes, colours, growth habits, maturity times, and uses^[5]. The crops are growing in importance globally and are used for food (e.g., fresh vegetables, processing, oil production, snack food), cultural expression (e.g., Thanksgiving, Hallowe’en, artisanal crafts, musical instruments), and medicinal purposes^[6].

A group of wild pollinators known as squash bees^[7] coevolved with wild cucurbita species and are the most important pollinators of these plants across the Americas.

What is the relationship between squash bees and humans?

Unlike most other bee species, the hoary squash bee (Eucera pruinosa) is a truly agricultural bee that has formed a strong, successful, and enduring relationship with humans through their food production systems. In fact, the expansion of the hoary squash bee’s range across North America is the direct product of the human movement and trade of pumpkin and squash seeds, and subsequent cultivation of the plants by both Indigenous and more recently by settler peoples.

Humans, pumpkin/squash crops, and the hoary squash bee thus provide a fascinating example of an intimate, three-way, mutually beneficial exchange known as reciprocity. Over history, humans moved pumpkin and squash seeds across the continent. Initially this was done by planting and tending them in Indigenous growing systems such as the Three Sisters. These seeds are also planted in home and community gardens and, more recently, in large monocultural agricultural systems, all of which have expanded the crops’ range enormously. However, seed set and fruit production in pumpkin/squash crops is entirely dependent upon bees to move pollen from the male flowers to the female flowers that produce fruit (Figure 1).

Movement of pollen

Pollination happens when pollen grains are transferred from the male anther to the female stigma of flowers. On the stigma, pollen grains grow a pollen tube that allows the genetic material in the pollen to be transferred to the ovules in the ovary, thereby producing fertilized seeds that ensure survival of the species. In some crops, the seeds then stimulate the growth of a fruit. Plants are either self-pollinated, or they may need cross-pollination. Cross pollination requires the assistance of a vector (usually wind or an insect) to move the pollen from the male anther of one flower to the female stigma of another flower. The movement of pollen grains is unintentional. In fact, bees are manipulated by flowers in this process. For example, the hoary squash bee has a hairy body that picks up pollen as it unintentionally contacts the anthers of a male pumpkin flower while feeding on the nectar in the flower. When the bee visits a female flower for nectar, it lands first on the stigma of that flower and then walks down to the nectaries in the base of the flower. As it does this, pollen is unintentionally transferred from its body to the sticky surface of the stigma, resulting in pollination.

Cucurbita crops attract bees by providing copious amounts of nutrient-rich nectar and pollen. As the range of cucurbita crops expanded, hoary squash bees followed, in pursuit of those nectar and pollen resources. As they foraged, hoary squash bees pollinated the cucurbita crops, allowing the plant to produce seeds and develop fruit (squash or pumpkins), which humans then harvested as a crop (Figure 2).

What do squash bees use pollen for?

Like people, squash bees require a healthy diet containing carbohydrates, protein or amino acids, fatty acids, and micronutrients. Nectar produced in both male and female cucurbita flowers provides the carbohydrates and micronutrients^[8]. The pollen, produced only in the male flowers, provides fatty acids, protein, or amino acids^[9]. The flowers of cucurbita are vital for the survival of squash bees because unlike most bees that forage on a wide range of plants, squash bees are strict dietary specialists, using only cucurbita pollen to provision the nest cells where they raise their young^[10] (Figure 3).

Squash bees are found in North, Central, and South America, and one species, the hoary squash bee^[11], is found across the continental United States and in Canada as far north as southern Ontario and Quebec (Figure 2). Interestingly, across most of the present range of the hoary squash bee, there are no wild cucurbita occurring naturally, making the hoary squash bee entirely dependent upon human cultivation of pumpkin and squash crops for its pollen supply.

The importance of hoary squash bees in pumpkin and squash production

Because they have separate male and female flowers, pumpkin and squash crops are entirely dependent on insects for pollination and ultimately seed and fruit production. As such, hoary squash bees are a vital part of our agricultural landscape. They are the most abundant flower visitors to cucurbita crops and are ubiquitous on farms growing pumpkin and squash in eastern North America. Their populations are reliably abundant from year to year, and the timing of their foraging activity corresponds well to the crop’s pollination window (6:00 a.m. to 8:00 a.m. daily, from mid–July to mid–August). Hoary squash bees are active on the crop flowers as soon as they open at dawn, when pollen supplies are greatest, and they remain active until the crop flowers wilt at noon. Seasonally, the bees emerge from their natal ground nests around the time that pumpkin and squash crops begin to bloom and begin to forage on the flowers immediately (Figure 4).

The bees’ unique behaviour and biology also contribute to the tightness of their relationship with cucurbita crop flowers. Besides foraging on the crop flowers, hoary squash bees also mate on the flowers, and males and unmated females sleep in the wilted flowers during the afternoon and evening after the nectar resources are exhausted. Other aspects of the biology of the hoary squash bee are linked to pumpkin and squash crops. To effectively collect cucurbita pollen, which is spiny and oily and not favoured by other bees such as the Western honey bee or bumble bees, hoary squash bees have evolved specialized hairs (scopa) on their hind legs.

Unlike the familiar Western honey bee, which is a social insect that lives in a colony with tens of thousands of other bees, squash bees are solitary. This means that each mated female builds her own nest in the ground (often within or close to pumpkin or squash crops) and raises her own offspring within cells in that nest (Figures 3 and 4). However, like people in an urban neighbourhood, many of these bees will live close to each other, creating dense, expanding nesting aggregations, sometimes with thousands of individual nests in a small area. (Watch this video of a hoary squash bee nesting aggregation.)

If large, stable populations of wild bees, such as the hoary squash bee, are maintained on farms, they will provide reliable, free pollination services to crops, reducing an overreliance on honey bees, which are under increasing pressure from pests, diseases, and overwintering losses.

What is the threat to hoary squash bees?

Known threats to bees generally include habitat loss, a changing climate, pests and diseases, pesticide use, and the interaction among these threats. For the squash bee, we will focus on pesticide use.

The hoary squash bee, because of its close association with pumpkin and squash crops across most of its range, may be at risk of exposure to insecticide residues in agricultural soil if those crops are treated with insecticides to control insect pests. Indeed, exposure to crops treated with a common soil-applied neonicotinoid insecticide causes hoary squash bee populations to construct fewer nests and harvest less pollen, resulting in greatly reduced offspring production. This puts cucurbita crop pollination at risk over the long term. This is ironic because it is precisely the hoary squash bee’s enduring relationship with human crop cultivation that has supported the bee’s large population and range expansion^[12]. In fact, by applying this neonicotinoid insecticide to soil, we are causing harm to the very pollinators that help with successful food crop production, thereby harming ourselves.

An Ethic of Reciprocity

Reciprocity describes a mutually beneficial relationship among organisms or groups. The reciprocity of the human–cucurbita crop–hoary squash bee relationship is obvious, and a salient example of what the best (and worst) outcomes can be for all relational partners in human-grown, pollinator-dependent crops. As such, it is a reminder of the ways in which human actions can affect less-studied or less-understood relationships among bees that pollinate food crops for humans and other living things. Reciprocity implies that the choices humans make that have a positive or negative impact on pollinators also have a positive or negative impact on human well-being. Although bees and crops may have little agency to protect themselves, humans can act to protect bees and their own crop yields in several simple but effective ways. This can be illustrated within cucurbita growing systems.

First, growers can acquaint themselves with hoary squash bees. These bees are easy to identify on pumpkin and squash flowers, rarely sting, and are considered to be quite endearing. Information about the bee is readily available online, including photos, videos, and diagrams. A greater awareness of this bee, and familiarity with its behaviour, biology, and ecology, can help strengthen this important relationship through respect, empathy, and understanding.

A real-life example of this enduring and endearing relationship is found on Strom’s Farm & Bakery near Guelph, Ontario. The Stroms have understood the importance of hoary squash bees for their pumpkin growing enterprise. They have educated themselves about the nesting behaviour of hoary squash bees and have allowed hoary squash bees to build nesting aggregations in lawns outside their cropping areas. In these lawns, the nests and the female bees who build them are protected from tillage and exposure to agricultural pesticides used on the crops. As a result, the nesting aggregations on that farm are expanding, ensuring pollination services from the bees into the future.

Next, growers can choose not to apply systemic pesticides to soil to avoid the demonstrated ill effects on hoary squash bees. On Stellmar Farm near Lindsay, Ontario, the farm owners have learned about the effects of neonicotinoids on hoary squash bees by supporting research activities on their farm. As a result, they have stopped using these pesticides in their production practices because they were shown to reduce hoary squash bee nesting and foraging behaviour, resulting in declining populations.

Other growers such as Lunar Rhythm Gardens near Janetville, Ontario have adopted alternative approaches to pesticide use including physical barriers against pests, such as row covers. The row covers are applied after planting, and are removed once flowering begins. This gives the hoary squash bees access to the flowers. Alternately, “trap crops” can be used to draw insect pests away from the cash crop, or less–harmful pesticides can be used, which are applied only in reaction to pest pressure using integrated pest management (IPM) principles.

Conclusion

Clearly, maintaining a healthy reciprocal relationship among humans, cucurbita crops, and hoary squash bees is advantageous to all partners. Humans gain reliable access to cucurbita fruit (pumpkin or squash) or seeds for food, medicine, or cultural expression. Cucurbita crops are planted widely and tended by humans and receive the pollination services that they need to set seed and reproduce. Hoary squash bee populations have reliable sources of the only pollen that they feed to their young, as well as mating and sleeping sites. As the members of this relationship with the most agency, humans hold the responsibility to recognize, value, and protect the integrity of this reciprocal relationship for the benefit of all.

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