The Squash Project
Once upon a time, the continents were together, fused in a single mass of land that seemed as though it might stay together forever. Dinosaurs still walked the Earth. And the first ancestor of wild gourd plants evolved. We don’t know for sure, but it seems likely that this first wild gourd plant already looked like the pumpkin plants you are growing now. It would have been a climbing plant, with tendrils. It would have had big, sweetly scented flowers to attract insects. It would have also, we think, had big fruits with hard shells, dispersed from place to place, perhaps by dinosaurs. Eventually, the continents began to break apart at their seams. The dinosaurs went extinct (or all extinct except for the lineages that became birds, that is) and the gourds — well, they began to diverge, to face different fates on different continents. On each continent, at least some gourds evolved large flowers that were pollinated by specialized insects that only visit gourd plants. On each continent, at least some of the gourds produced large bitter fruits. These fruits were eaten by giant mammals that cannot taste bitter (unlike small animals, including humans!) that then pooped out their seeds in some new place, that was good for the seeds to grow into new plants. It went on like this for tens of millions of years. In each place new species of gourds evolved, each one different. Very hungry insect herbivores also evolved to eat roots, leaves, flowers, and fruits of each gourd species. Specialized pollinators evolved, too. Millions of years, tens of millions of years with gourds, insects, and microbes evolving together in different places all around the world. Then, humans evolved, and humans moved from Africa to all continents (except Antarctica) around the world.
At first, humans had little effect on gourds. The gourds were toxic, protected by a bitter substance that only the giant mammals could safely eat — giant sloths used to live in Ohio, and mastadons used to live in Mexico. They don’t anymore. Hungry humans began to kill off the giant mammal megafauna. Once the megafauna — dozens of species of giant mammals — were killed off, the wild gourds started to become rare. Nothing ate them. Nothing moved them. They sat, toxic, on the ground. And, if that weren’t enough, climate began to change too, making even those gourds that were able to disperse from one place to another less likely to survive. But, as giant mammals were disappearing, humans began to use the wild gourds they found around them. The hard shelled gourd fruits could be dried, and were very useful as water containers and for helping fishing nets float, and the non-bitter seeds (the only part of the plant that did not contain bitter toxins) were very oily and nutritious.
Then, around ten thousand years ago, humans began to intentionally plant wild gourd seeds instead of just using the hard shelled gourds they found naturally growing around them. It happened separately in the Americas (where gourds were the first plant domesticated for agriculture, becoming squash and pumpkins, at the same time wheat was domesticated in the Middle East), and in Africa, and in Asia. In many parts of the Middle East and Asia, the compounds in the wild gourds that became cucumbers and cantaloupes were used in traditional medicine, to treat illness such as [illness] much more often than gourds in the Americas. Eventually, these very bitter gourds around the world were bred by humans to not produce these bitter toxins, and to taste sweet and delicious. Gourds would be domesticated around the world no fewer than a dozen times, and perhaps as many as twenty times. Many kinds of gourd appear to have floated from Asia to Africa, Australia, and the Americas on their own, and then been domesticated separately in different regions. People liked gourds. They could be dried and made into instruments, vessels for drinking and eating, and much more. They grew easily, and eventually, people were able to breed gourd fruits without a hard shell and without toxic chemicals, so that the fruits and flowers could be eaten. In the Americas, people made wild gourds into pumpkin and squash. In Africa, people changed gourds into watermelon. In the Americas, gourds were domesticated into squash and pumpkin. In Asia, people domesticated other wild gourds into the sweet fruits we enjoy as cucumbers, bitter melons, honeydew, and cantaloupes.
Thousands of years ago, people began trading different seeds of their crop plants, and carrying them around continents. As for the insect pests and partners of gourds, the beetles and the bees and the moths and the true bugs, some insects moved as people moved individual kinds of gourds (be they squash, pumpkins, cucumbers, gourds, melons, chayote, or even luffa) and other insects didn’t. When Native Americans purposefully moved cultivated pumpkins north from Mexico into North America thousands of years ago, for instance, squash bee pollinators followed with the pumpkins (link). The wild gourds in Asia that became cucumbers and cantaloupes have their own types of insects and microbes in their original homes in Asia.
Starting with the voyages of Christopher Columbus 500 years ago, the movements of cucumber, melon, pumpkin, and squash crop plants began to be global, between continents, with the ships of conquistadors and their latter day equivalents. Most basic types of cucumber and melon were carried to the Americas, and pumpkins and squash were carried away from the Americas. These crop plants were all brought nearly everywhere in the world they could grow (even as the vast diversity of individual varieties were left behind). On Christopher Columbus’s second voyage, he brought cantaloupe plants to the Americas, and cucumber plants arrived soon after. In their new home, cucumber and cantaloupe were missing many partners – their microbes, insect pollinators, and insect herbivores. Instead of these old partners from Asia, cucumbers and cantaloupes encountered brand new partners in the Americas – the different insects and microbes that have been co-evolving with pumpkin and squash type gourds for millions of years. Initially, when pumpkins and squash were introduced to the Old World from North America, they quickly adapted to their new environment and grew better, having escaped their harmful herbivores. But they also suffered, or we suppose they did (few studies were done), from a lack of pollinators. Now, some herbivores have caught up, some haven’t, and new and unusual mixes of herbivores, pathogens, pollinators and gourd plants have been formed in different places around the world – mixes no one has yet studied. This creates one of our great mysteries: What happens to the partners of our most important crop plants, as we move those plants around the world?
Fast forward to 2017. A new bacterial pathogen of pumpkin, squash, cucumber, and cantaloupe has emerged. It lives, it seems, only in the Northeast and Midwestern United States (though we don’t really know). It rides to the pumpkins, squash, and cucumbers in beetles that only eat these closely related plants. When the beetles eat the leaves or flowers, they poop. The bacteria moves from the poop into the floral nectaries or xylem of the damaged leaves of the plant. Once in the xylem (plants veins), it grows rapidly, clogs the veins and triggers something akin to fatal plant heart failure. We can’t be sure where the bacterial pathogen originally came from, but we think the introduction of cucumbers to North America may have played an important role. Cucumbers are, after all, a kind of gourd, like pumpkin and squash.
To make sense of all this, of the moment at which we find ourselves in the gourds story, we really need to know a great deal more about cultivated gourds (like pumpkin, squash, cucumber, cantaloupe, watermelon, and all the other related gourd family crops). We need to understand the biology of each variety and the insect and microbial species associated with them. We need to know if those insects and microbes moved (or not) when people moved crop plants, and we need to know when these insects might be carrying pathogens with them and when not. All of this, from giant sloths to gourds and pumpkins and cucumbers and all the rest, is fascinating – but it is also important. Last year, many farmers and home gardeners lost their pumpkin and cucumber crops in Midwestern and Northeastern North America. Will this pathogen spread? Will it not? We don’t really know. We are going to have to depend on the public to help, because the truth is that very few people study pumpkins, squash, cucumbers, and other gourds. Just a handful, and they are mostly our friends. Some study bees and pollination. Some study beetles and herbivory. Some study the fossilized feces of ancient giant sloths and mastadons. Some study the ancient and modern art on ceramics in Peru and Mexico (depicting pumpkins and gourds). It takes all kinds of researchers to understand gourds and their different insects and microbes, just like it takes all kinds of species to turn a bitter gourd from a seed into a sweet, soft fruit. All kinds of species, and a fascinating history that begins with the splitting of the continents, and ends, well, in your soup.
So what are the mysteries we’re trying to solve?
The Mystery of the Sick Pumpkins — Like animals, plants are vulnerable to diseases (microbial pathogens) and parasites (herbivores). And like animals, plants have immune systems to try and prevent infection by microbes and damage by herbivores. Over time, the wild gourd ancestors of cultivated pumpkin and squash plants evolved to produce defenses against insect pests (such as herbivorous beetles that eat roots, leaves, and flowers) and pathogens (such as some bacteria, fungi, and viruses). Such defenses included physical structures that make it difficult for insects to eat leaves and fruits (some fruits even have spines). Wild gourd plants also have the ability to produce toxic compounds to poison mammals and insects that do eat roots, leaves, flowers, or fruits. The beetle herbivores and plant pathogens, in turn, evolved ever better ways to avoid or even overcome the physical and chemical defenses of wild gourd plants. This evolutionary race, between wild gourds and those organisms hoping to eat or infect it, has led different lineages of wild gourd plants to have slightly different defenses. This same process occurred in the Americas with squash and pumpkin, Africa with watermelon, and in Asia with cucumbers and melons. It has also led insect pests and microbial pathogens that eat different gourd plants to become different beetle and pathogen species. This race went on, generation to generation, for many millions of years. But recently, in Northeastern North America (and perhaps elsewhere, we can’t yet be sure) another player entered this story – a pathogenic bacterial species (Erwinia tracheiphila). This bacteria uses adult beetles that feed on pumpkin, squash, and cucumber to move from plant to plant, and sneaks into plant ‘veins’ (xylem) when beetles poop on flowers or the leaf wounds that the beetles make on the plants when they eat leaves. This bacteria clogs the veins (xylem) of the plants and causes them to have an illness akin to heart failure.
But we don’t know, yet, whether this bacteria is spreading to new geographic areas (like the Southeast or Southwestern United States or Mexico), where it came from, or even how likely it might be to ruin next year’s squash, pumpkin, and cucumber supply. We need your help to document where the different species of beetle herbivores that can move this bacteria, and the bacterial plant pathogen itself live. You can help by photographing beetles on the leaves and in the flowers of squash, pumpkin, and cucumber plants, collecting all the beetles to send to us, and looking for fungal, viral, and bacterial disease symptoms on your garden plants. If you plants have bacterial disease caused by Erwinia tracheiphila, we would like you to contact us and possibly send the infected plant for further analysis!
The Mystery of the Missing Bees — Even as the co-evolutionary arms race between wild gourd plants and their harmful beetle herbivores continued, wild gourd plants were also co-evolving with beneficial bee pollinators at the same time. Millions of years ago, plants in the family of the squash, pumpkins, gourds, and melons (Cucurbitaceae) were likely all pollinated by beetles (perhaps even some of the same kinds of beetles that now eat pumpkin roots, flowers, and leaves). Over millions of years, different branches of this plant family, growing in different geographic regions, came to depend on different kinds of pollinators. Gourds in Africa, for example, depend on moths that pollinate at night (most of them not yet studied at all). Squash and pumpkin plants, on the other hand, depend on bees that pollinate very early in the morning.
Squash plants produce many large, yellow, highly scented flowers that only open for several hours, from sunrise until late morning. These flowers produce copious amounts of pollen and nectar, and are therefore a very nutritious food source for bee pollinators (and beetle herbivores!). A group of solitary bees has evolved to specialize on squash and pumpkin flowers: these bees wake up at sunrise as soon as squash flowers open, and use the special scents emitted by squash flowers to locate them, then enjoy drinking nectar and collect pollen (and in the process pollinate flowers). When the flowers close in the late morning, the bees retire for the day as well – the females return to their underground burrows, and males can often be found sleeping in closed blossoms. Different squash bee species seem to be important in different regions of the United States, and other parts of North, Central, and South America. In some regions, squash and pumpkin may fail to be pollinated because of the lack of squash bees, especially if people do not keep honey bees colonies where squash bees are naturally absent. But just where different species of squash bees live and why different species have different geographic ranges – well, we don’t yet know. That is where you can help us.