Across much of the Vineyard, on sandy soils both moist and dry, the most obvious late spring flowers may be those of sheep laurel, Kalmia angustifolia. Patchily distributed but locally abundant, this low shrub favors sunny settings and acidic soils.
When not in bloom, sheep laurel can easily be overlooked. It rarely exceeds two feet in height on the Vineyard’s lean soils. And its leaves, elongated and often rather ratty-looking, are nothing special. But in flower, the plant is striking, with bountiful clumps of flat, pink flowers about a half-inch in diameter.
The flowers have a curious design. When they open, their anthers — the pollen-bearing structure of the flower — are tucked into pockets on the petals. When the flower is disturbed by a visiting insect, the anthers pop loose, tossing their pollen loads into the air and, sometimes at least, onto the insect visitor.
But surprisingly few insects visit the colorful flowers of this shrub, and I’ve spent a good deal of time trying to figure out why. One partial explanation is probably that Kalmia angustifolia flowers produce very little nectar, the sweet liquid that attracts bees and other insects to many flowers. It’s also possible that the pollen of sheep laurel is, for some reason, difficult to collect; perhaps the grains are not very sticky, and require specially designed hairs to hold them.
The Vineyard has one bee species that forages with great success on sheep laurel. The Kalmia miner bee, Andrena kalmiae, confines its flower visitation almost totally to this plant. (It would visit some close relatives, if they occurred here in decent quantities, but they don’t.) A small bee, roughly half an inch long, Andrena kalmiae somehow acquires a heavy pollen load from its preferred flower; most of the 40 or so females of this species that I’ve seen have had pollen caked not just on the pollen-carrying hairs on their legs, but on the hairs of their head and thorax, as well. But if specially structured hairs make this possible, you can’t prove it by me: The scopa (pollen-carrying hairs) of A. kalmiae look perfectly typical to my eye under a microscope. So the ability of this bee to forage on kalmia remains a mystery to me.
A few other bees visit Kalmia angustifolia regularly. Another miner bee, the neighborly miner bee, Andrena vicina, forages diligently on sheep laurel. And this species is listed in some sources as an important pollinator of this plant. It may be, but the experience must be rather frustrating for the bee, because most of the Andrena vicina females I’ve observed had only a light pollen load, or no load at all.
At least some of our bumble bee species (three that I’ve seen) visit sheep laurel, as well, sometimes making prolonged stops on the flowers. But the pollen carried by those bumble bees has been yellow, not the off-white that kalmia pollen shows on Andrena kalmiae, and must therefore have come mostly from some other flower. So once again, I suspect that bumble bee visits may work out well for the plant, but do the bee itself little good. Most other bees I’ve seen on kalmia have made only brief visits, evidently not liking what they find, and leaving almost immediately.
Such specialized plant/pollinator relationships are actually not rare. This plant has simply evolved the strategy of depending for its pollination needs on a small number of highly reliable partners, rather than inviting everybody in the neighborhood over for dinner. Perhaps kalmia blossoms make the most of small quantities of transferred pollen, needing only a few grains for successful fertilization.
Moreover, sheep laurel supplements insect pollination with a couple of tricks. For one thing, it holds its flowers for a long time, giving the few bees that effectively pollinate it a lot of time to work. And kalmia shrubs spread clonally, sending up shoots from an expanding root system — a form of self-perpetuation that does away with the need for pollination and seed production entirely.
The non-native honey bee must collect the pollen of this plant successfully, though I personally can’t recall seeing a honey bee on sheep laurel. But toxins in kalmia pollen have been known since Classical times to sometimes accumulate in the honey produced by bees that forage heavily on kalmia flowers, causing symptoms including vomiting, impaired consciousness, and cardiac problems in anyone who eats the honey in quantity. Yikes!
Indeed, the toxic nature of sheep laurel may be the most studied aspect of this plant. All parts of Kalmia angustifolia contain a mix of unpleasant chemicals. At low levels of exposure, grazing animals tolerate sheep laurel with little or no problem. But if larger amounts are consumed, most grazers exhibit distress, salivation, cardiac irregularities, and even convulsion and death. Even goats, famous for happily digesting anything short of boilerplate, are said to be susceptible. So enjoy kalmia flowers, but don’t eat the stuff!
Getting acquainted with sheep laurel has been a bright spot of my field season to date. The flowers are beautiful — and also the focus of some amazing biology.