The cow says moo, the bird says tweet, and the bee says buzz. But there’s much more to the buzz than meets the ears! It’s also a sound that some bees produce while performing a special skill called “buzz pollination,” or sonication.
Over half of our native bee species can perform this skill. And even though only a small portion of plants (between 6 to 8%) rely on this type of pollination, that’s an estimated 20,000 species of flowering plants in 64 plant families. It’s also important for several agricultural products including tomatoes and blueberries.
Buzz pollination has strangely evolved independently many times over in both bees and plants. And yet honeybees, our key agricultural pollinators, don’t have this skill.
So, how does “buzz pollination” work? After a female bee carefully selects the right flower, she’ll grab an anther with her mandibles. For flowers like rhododendrons, larger bees like bumblebees may also wrap their legs around the stamens. After the grab, she creates thoracic vibrations with indirect flight muscles that result in an audible soft buzz. As she reaches a high enough frequency and amplitude, if the plant is ready, pollen will release in an explosion from pores in the anthers. The bee is rewarded with a nice quantity of pollen to make her job of gathering pollen a little easier.
Why would a plant do this? Quite simply, these plants expect pollen gatherers to actually pollinate them! Flowering plants that rely on wildlife put a lot of energy into creating nectar and pollen. When plants provide these treats free for the taking, as most plants do, many visitors consume the goodies without contributing to their pollination. Buzz pollinated plants don’t support the free handout strategy. Instead, they restrict access to pollen, typically through a tubular anther. Only pollinators that can vibrate the anthers to “explode” the pollen out get the reward. In addition, many buzz pollinated plants don’t offer ANY nectar either! This strategy makes these plants some of the most restrictive and even stingiest pollen producers.
Native plants that apply this strategy are primarily in the pea (Fabaceae), nightshade (Solanaceae), and heath (Ericaceae) families. Heath examples include blueberries, cranberries, azaleas and rhododendrons.
Our local native pea family species Senna and Partridge Pea are examples of nectar-less buzz pollinated plants. Because most adult pollinators are at flowers for nectar, these plants have low pollinator activity. While adult bees get their nourishment from nectar too, female solitary and worker social bees’ lives are devoted to gathering pollen to take back to their nest to feed larvae. If a plant makes your job easier and you’re not competing with lots of other pollinators, you just might visit it. Even though butterflies don’t visit Senna and Partridge Peas for nectar, these two plants are obligatory hosts to several of our uncommon migrant butterflies, including Sleepy Orange and Little Yellow. Wild cranberry plants are host to the specialist butterfly the Bog Copper.
Agricultural crops that are buzz-pollinated include major foods such as tomatoes and eggplant (nightshade family) and kiwis, blueberries, and cranberries (heath family). Some of these plants, like tomatoes, are also hermaphroditic and self-compatible. That means the plants can self-pollinate with methods like the plant’s being shaken (in greenhouses or by animals) or relying on wind to achieve the vibration for pollen release. But such fruits are typically inferior and often abort early compared to fruits produced on cross-pollinated plants. Studies have shown that supplementing with buzz pollinating bees can significantly increase the quality and yield of such crops. For those growers who embrace wild bees, fruit yields and quality can increase significantly.
Blueberries and cranberries require cross-pollination. These plants are a little different in that they don’t exclusively require buzz pollination. Honeybees can release a small amount of pollen by drumming or stroking the anthers. But whereas a bumblebee can pollinate a blueberry flower in a single visit, a honeybee needs to make 4 trips. When a blueberry farmer brings in honey bees as pollinators, the farmer usually cuts down any competing vegetation in the area as the honey bees are easily distracted by plants that reward them better. This unfortunately reduces local native bee populations.
Farmers who embrace our native bumblebees to provide agricultural services get an extra bonus. Bumblebees not only start earlier in the day and work later, but are also willing to stay on the job in wetter weather than honeybees.
Listen for this special buzzing of bees as you observe pollinators on flowers. Now you’ll know what they’re up to!
Karen Campbell, a member of Lehigh Valley Audubon and FocusOnNatives.com provided the content and images for this article.