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The Ecosystem of Cultivated Pumpkin

The pumpkin is an interesting plant embedded into North American culture. For thousands of years before arrival of Europeans, people in what is now Mexico cultivated winter squash (Curcurbita sp.). They selected varieties with desirable traits, creating the modern pumpkin we use for everything from decorations to pies.

As people migrated throughout the country, they brought their most valuable crops (including the pumpkin). But with the pumpkin came all of the species that utilize the pumpkin. Perhaps the most famous animal associated with the pumpkin is the squash bee (Peponapis sp. and Xenoglossa sp.). Squash bees specialize on gathering pollen from members of the cucumber family (including squash and pumpkin), and probably had a very small native range until these crops were cultivated and disseminated by indigenous peoples.

A squash bee gathers pollen from the flower of a cultivated pumpkin in an Omaha, Nebraska garden.

The "squash bee" is actually a collection of approximately a dozen species that specialize on squash and cucumber flowers (termed oligolectic because they gather pollen from only a few species; "oligo" means few). These squash bees are also some of the most effective pollinators of their host plants because they spend all of their foraging time visiting these flowers. This increases the likelihood that a bee with transfer pollen from one pumpkin plant to another pumpkin plant as opposed to transferring pollen from a non-compatible milkweed or rose.

The squash cuckoo bee (Triepeolus remigatus) spends the evening sleeping in an Omaha, Nebraska native garden, on the end of a blade of Echinacea purpurea.

With the spread of the squash bees came a fascinating bee that depends on them. The squash cuckoo bee is a kleptoparasite, meaning they lay their eggs in the nest of their host species. The larvae of cuckoo bees rely on the pollen gathered from the host bees. Most cuckoo bees are specialists, and the squash cuckoo be is no exception. As the pumpkin spread, the squash bee spread; as the squash bee spread, the squash cuckoo bee followed.

Some less desirable, albeit equally beautiful, species have benefitted from the broadened distribution of pumpkins. Many native insects eat the vegetation and fruit of pumpkins, potentially becoming pests that can decimate crop yields. Cucumber beetles eat pumpkins, squash, and cucumber, and have the potential to spread the devastating diseases cucumber wilt bacteria and cucumber mosaic virus.

The spotted cucumber beetle (Diabrotica undecimpunctata) is a beautiful insect somewhat resembling a ladybug. However, unlike a ladybug which predates on pest species, cucumber beetles are herbivores of curcubits.

In our yard, the remains of last year's pumpkins were strewn about by squirrels. The result has been several volunteer pumpkins we have decided to allow. The first such volunteer has not met a very favorable outcome. Its vines have twisted up into desiccated gray ropes, leaving behind a single pumpkin. The culprit: a major outbreak of squash bugs (Anasa tristis). Squash bugs are true bugs (Order: Hemiptera), native to North America. These bugs, like all Hemipterans, have piercing mouthparts that have the ability to stab into the pumpkins vasculature (known as xylem). They suck all the fluid from the vasculature, almost like a leech sucks mammalian blood. The result is a pumpkin that wilts from desiccation because it cannot transport water throughout the vine.

Our first pumpkin of the year has suffered from a population explosion of squash bugs. This pumpkin probably won't make it until fall, but there is hope for the other plants.

All is not lost. Several of the other volunteers look healthy so far, and the squash bug population has experienced a substantial decline without the use of pesticides. Although I assume multiple natural predators are likely responsible, the one predator I witnessed was an ant. Colonies of ants marching over the surface of the pumpkin were ripping the limbs and antennae from the nymphal squash bugs, then carrying them away to their nest. Within a couple days, squash bugs were nowhere to be seen. The remaining pumpkin plants in our yard are green and healthy, with big yellow flowers closing from successful pollination. With any luck, our yard can hold a successful harvest of a crop that has been cultivated on this land for thousands of years.

A common house ant immobilizes a tiny squash bug nymph on the surface of our only pumpkin. With the help of these ants, our remaining pumpkin plants might be able to produce a harvest.

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