The perfect tomato, bright red, fully round, exploding with flavor when bitten or sliced, is the poster child of the vegetable garden, the favorite subject of summer chats over the garden fence. The veteran gardener assumes the role of teacher, defining her success in terms of choosing the best variety or building the perfect soil or applying a special fertilizer at just the right time. The novice holds his misshapen cat-faced tomato at his side and pretends to listen, but he is distracted by a bumblebee crawling across the face of a sunflower.
Focus your attention on the bumblebee. She is the only character that understands the anatomy of the tomato flower. Only she knows what it really takes to make a perfect tomato.
Tomatoes have perfect flowers, the term “perfect” used by botanists to describe flowers that have both stamens, the pollen-producing organs, and one or more pistils, the ovule-producing organs. The top portion of each stamen is called the anther and at maturity it contains the pollen grains. The yellow anthers of the tomato flower are collectively fused together to form a ring around a greenish pistil.
In the basal portion of the pistil is the ovary and it contains the ovules, or egg cells, that will eventually become tomato seeds if two sequential processes, pollination and fertilization, are successful. The botanist will say that when you eat a tomato fruit you are eating a ripened ovary filled with mature seeds, each containing an embryo surrounded by a bit of stored nourishment to see the seedling out of darkness and into the light.
Pollination is the transfer of pollen grains from anthers to the topmost portion of the pistil, the stigma. Nothing more, yet pollination is essential for fertilization which begins with germination of the pollen grains on the surface of the stigma.
A slender tube, the “pollen tube”, grows out of the germinating pollen grain, passing downward through the stigma and the style, a thread of connecting tissue between stigma and ovary. Eventually, if all goes well (a big “if”), the pollen tubes meets one of several multicellular embryo sacs in the tomato ovary. Swimming down the elongating pollen tube are two sperms cells that were formed in the pollen grain. One of these sperms will fuse with an ovule in an embryo sac to form the first cell of an embryo. The other sperm will fuse with another cell in the embryo sac to produce the first cell of a nutrient-rich material, the endosperm, that surrounds the mature embryo. Together, the mature embryo and endosperm constitute a seed, an infant plant carrying its own picnic basket.
No pollination means no fertilization, no ultimate union of sperm cells with egg cells in the ovary. No seeds. No fruit. That fertilization succeeds at all is nothing short of miraculous, a tale worth telling at some later time, but fertilization never gets a chance if pollination, the focus of this story, does not occur.
At this point in the story, it should be emphasized that for every seed found in the mature tomato fruit, a pollen grain must be deposited on the pistil’s stigma. Thus fully successful pollination requires deposition of many pollen grains. This is analogous to pollination in corn where every kernel on the cob requires a separate pollen grain sown by the wind on one of the long silk-like stigmas that protrude from the end of an ear shoot. With corn, inadequate pollination means unfilled ears. In tomatoes, inadequate pollination means misshapen, often lobed, fruits.
If you look closely at the fused ring of pollen-producing anthers in the tomato flower, you notice that the only avenue for pollen escape is through a tiny pore at each anther’s tip. The mature pollen grains are smooth-grained and sticky, so they are not likely to simply fall out through this pore. They need to be forced out by agitation of the anthers.
Enter the bumblebee.
Because they have perfect flowers and because some pollen will be shed with moderate agitation of the anthers by wind, tomatoes are often described as self fertile. This is true to the extent that some self fertilization, some union of sperm and egg produced in the same flower, will occur, depending on the quantity of pollen released.
Agitation of the anthers by bumblebees, however, results in a rain of tomato pollen grains. Imagine the pollination dance of the bumblebee. She grasps a blossom with her legs, pulling the flower down into a vertical position and placing the stigma against her fuzzy abdomen. She then vibrates her wing muscles at just the right frequency to release the flower’s pollen, a process called sonication, or “buzz pollination”. The released pollen falls onto her abdomen while pollen obtained from previously visited flowers is being transferred to the flower’s stigma, resulting in cross pollination.
Coevolution of wild tomatoes with sonicating bees was the choreographer of this beautiful dance. In South America, where wild ancestors of our garden tomatoes still grow, there is a bee that does this dance. North American bumblebees were buzzing wild blueberry flowers, which also benefit from sonication, before there were garden tomatoes.
Our native bumblebees are twice as effective in pollinating many plants, including our food plants, as non-native honeybees. This should not come as a surprise, since the honeybee never learned the dance.
Tomato plants pollinated by bumblebees are likely to bear larger fruits and fewer misshapen fruits. Cross pollination between varieties will not affect fruit characteristics in the current year, but seeds from these cross pollinated plants should not be saved as they will likely produce hybrid plants with a wide range of fruit characteristics.
In addition to tomatoes, bumblebees also buzz pollinate eggplant, blueberry and cranberry blossoms. In the garden at harvest time, when I hear familiar buzzing nearby I stop to watch bumblebees at work. Unlike a honeybee, which prefers to forage in a large field of a single plant species, a bumblebee will move from one type of plant to another. It might start with a sunflower head, crawling over each tiny flower until it has filled its hairy hind-leg sacs with bright orange pollen, then dive into the throat of a male squash blossom, dusting its bristly body with bright yellow pollen, and then move on to a cluster of tomato flowers or catmint or campanulas.
What can the gardener do to ensure that these versatile pollinators will be in the garden when tomatoes and other plants that require their services are blooming? Nesting sites and season-long sources of nectar and pollen are the key.
Providing homes for bumblebees is more a matter of keeping a portion of your garden out of cultivation. While several styles of bumblebee nest boxes are sold, they are rarely used if there is an old mouse or vole hole nearby, or an underground cavity in a clump of grass.
If you want to attract bumblebees, be less tidy in the garden. In Marjorie’s garden where the catmint grows, an old tree stump slowly rots, each year sinking a little deeper into the earth, the holes left by decayed roots providing perfect nests for the bumblebees. They live where they work, a perfect arrangement that reminds me of my graduate school days.
Leave part of the garden wild. About a hundred feet from the vegetable garden, we leave a patch of tall grasses and wildflowers for the insects. We never mow this patch and, as a result, there is a season-long supply of nectar and pollen for the bees, including violets and dandelions in spring, goldenrod and Queen Anne’s lace (wild carrot) in summer, asters in autumn. In addition to the pollinators, hoverflies and other beneficial insects thrive in this spot, completing their life cycles undisturbed while frequently visiting the vegetable garden.
Native herbaceous perennials that flower in July and August, including goldenrods, milkweeds, and campanulas, will help sustain bumblebee colonies as vegetable flowers fade. On the early end of the season, before any garden plant is in bloom, dandelions are an essential source of pollen for starting the new colony.
Weed selectively. What a gardener might consider a weed, a bumblebee considers food, either nectar or pollen. Volunteer calendulas and other flowering weeds should be left to flower at the feet of pea plants and in the sunny spaces between squash leaves.
The surest sign that spring has finally come to the garden is the sighting of the first queen bumblebee. She is the sole surviver of the former colony, mating in autumn and then spending the long winter hibernating underground, alone. She emerges in early spring both hungry and frantic to establish a new colony. Other than sipping a little nectar, she pays little attention to food until she finds an old mouse hole or other suitable nesting site. Once this primary objective is met, she searches earnestly for pollen. You can bet than any queen found carrying pollen has already found her nest.
The nest secured, the queen mixes wax secreted from her body with pollen to form a mound on which she lays her first brood of eggs. She also collects nectar which she stores in a waxen “honey pot” located near the brood. Sipping from this nectar pot provides her with enough energy in incubate the eggs for several days until the first batch of larvae emerge.
For the next two weeks, the queen forages nearby flowers for pollen and nectar to feed the growing larvae. These larva eventually spin cocoons in which they pupate, emerging as adult workers ready to take on the task of foraging and caring for subsequent broods. From then on, her majesty remains in the nest, laying more eggs and giving orders.
Queen bumblebees are the first bees of spring. Unlike honeybees, they are willing to forage on the cold damp days of early May. I was in the garden one early spring morning and found a queen bumblebee lying motionless on a dandelion blossom. Taking her for dead, I softly brushed her bristly back with my finger and was surprised to see a leg move. As a shaft of sunlight broke through the trees and washed over her, she began to crawl over the face of the flower, resuming the task she had abandoned at dusk, too tired to return to her nest.