Plants, bees work together to keep things growing

A bumble bee feasts on a purple cone-flower. At its next stop it will offload nectar unknowingly. — Photo by Matt Pelikan

You probably know the basic facts about pollination. Without singles bars and online dating services, plants, when it’s time to reproduce, face the problem of getting genetic material from the male parts of one plant to female parts of another.

Many kinds of plants, including most grasses and many trees, opt for the shotgun approach, releasing huge quantities of pollen onto the breeze. A few grains, hopefully enough to perpetuate the species, arrive at their destination. The rest end up stuck to patio furniture and the hoods of cars. But many other kinds of plants hire a delivery service to transport their pollen. Attracting insects or other animals with colorful flowers and a sweet, presumably tasty liquid known as nectar, these plants try to ensure that pollen grains, stuck to the visiting insects, are flown directly to another flower, where some of the pollen rubs off on the female organ. Voila! Happy insects and fertile seeds. (Many insects also eat the pollen itself, but still transport enough to benefit the plant overall.)

Indeed, the insects are more than happy. Studies comparing female butterflies that have access to nectar to females that don’t show that the well-fed butterflies live longer and produce more, and larger, eggs. And the importance of this partnership to the plants is amply illustrated by the enormous effort they put into producing flowers — colorful, scented structures that exist solely to attract insects. If pollination didn’t matter so much, plants would surely put those resources into the growth of stems, roots, or leaves.

Many flowers also feature clever adaptations to enhance pollen transfer. A great example would be the flower of a prickly pear cactus (and yes, we have one species that is native to the Vineyard, though it is rare or perhaps extirpated as a naturally occurring population). Cactus flowers are pollen factories to start with, usually having showy flowers with many stamens (the male organ) producing clouds of pollen. But when a visiting bee touches one of those stamens (and it can hardly avoid doing so), an instantaneous change occurs: cells on the side of the stamen’s stalk that was touched leak water through their cell walls, effectively shortening that side of the stamen. As a result, the stamen actively bends over and touches the bee, swabbing it with pollen and ensuring that the insect flies away with a hefty genetic payload.

And the insect end of the partnership can feature some special talents, too. Bumble bees practice “buzz pollination,” putting their wing beats into overdrive to shake pollen loose from a flower that is otherwise difficult to tap into.

On a recent field outing, entomologist Paul Goldstein introduced me to the case of Lyonia, a shrub that is locally common on the edges of Vineyard wetlands. Like blueberries (a close relative), Lyonia produces clusters of small, white flowers that are bell-shaped, narrowing at the mouth. But unlike a blueberry flower, a Lyonia flower is almost entirely closed at the mouth: the flower puts its stamens out of reach of visiting insects — except for a group of highly specialized bees. Equipped with jaws and suitable instincts, these bees snip away at the flower to enlarge the opening, then enter to claim their reward (and pick up a load of pollen). In this case, pollination has evolved into a marriage between two highly specialized, and therefore highly efficient, partners.

Bees, both our native ones and the honey bee, which is an introduced species, are generally among the best pollinators, due largely to their behavior and their hairy bodies. But wasps, ants, beetles, butterflies, moths, flies, and even hummingbirds and bats can serve as pollinators.

Specialized relationships aside, many pollinators visit a wide variety of flowers. Still, pollinators differ in how effectively they move the genes of any particular plant species (determined in part by how the flower parts interacts with the size, shape, and behavior of the pollinator). Honey bees, for example, are stunningly efficient pollinators of fruit trees and some other agricultural crops. But they do a poor job pollinating some native shrubs and wildflowers. Many native bees may prefer wildflowers to crop plants, but as a group, these insects can contribute substantially to the pollination of food plants.

Like plants, pollinators also vary in the seasonality of their activity and their preferences for habitat. So a high diversity of pollinators benefits both the natural environment and the human food supply.

In short, pollination is a process as complex as it is important. If you’re curious about pollination, Paul Goldstein and I will be giving a presentation called “Nature’s Partners: Plants, Pollinators, and You” tomorrow morning, July 20, at 10 am at Polly Hill Arboretum. Island Grown Initiative is the event’s primary sponsor, and admission is free.