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Plant is deadly to animals, but one species of bee has a unique symbiosis with it

“Poison” comes to mind when I think of the outcrop of Niobrara and Pierre shale where Neva Road meets route 36 several miles north of Boulder. The shales are able to exclude some plant species and also support endemic species, such as Bell's twinpod, which is adapted to grow on the shale but does not grow on adjacent soils typical of shortgrass prairie. Death camas, a poisonous plant, also grows on the shale outcrop.

Death camas is the common name applied to a group of 15 species that synthesize the defensive compound zygacene. They were once united in the genus Zigadenus, but the local species were given new names.

Mountain death camas, Anticlea elegans, grows from montane to alpine habitats. Meriwether Lewis first collected it in 1806 near Lewis and Clark Pass in Montana, and I found a population of them near tree line in the San Juan Mountains. Its flowers have cream-colored tepals (look like petals) with large green nectaries and are presented in a raceme, with flowers on short stalks (pedicels) attached to a long, unbranched axis.

Foothills death camas, Toxicoscordion paniculatum, and meadow death camas, Toxicoscordion venenosum, grow at lower elevations, from deserts and foothills to montane habitats. They have cream colored tepals with yellow or light green nectaries at the base of each tepal. Their flowers are presented in a panicle, a loose branching cluster of flowers. T. paniculatum and T. venenosum are very similar and they are reported to hybridize.

Only death camas miner bees can enjoy nectar from these handsome foothills death camas flowers (Photos by Jeff Mitton).

Furthermore, two systematists have written that they are unable to distinguish specimens with certainty and both suggested they may be the same species.  The species of death camas growing on the shale outcrop appears to be foothills death camas, the species involved in most of the cases of poisoning.

Zygacene, the common defense of death camas species, is a deadly alkaloid neurotoxin, with substantial levels in leaves, stems, bulbs, nectar and pollen. Leaves and bulbs of death camas are similar to leaves and bulbs of onions, blue camas and mariposa lilies, all of which are edible. When flowers are present these species are easily identified. Most cases of human poisoning follow accidental consumption of death camas bulbs, tragic cases of misidentification when flowers were not available. A single death camas bulb is lethal.

In humans, zygacene first causes a cluster of symptoms referred to with the acronym SLUDGE, for Salivation, Lacrimation, Urination, Defecation, GI upset, and Emesis. Cardiac toxicity leads to very low blood pressure and neural toxicity results first in pins and needles in the extremities followed by loss of coordinated movement, coma and death. Sheep, cows and horses are also susceptible to zygacene—consumption of about half a pound of leaves and stems is lethal.

Fortunately, most domesticated animals recognize something dire in the bitter alkaloid and stop grazing death camas.

One of the most curious aspects of death camas is that its pollen and nectar are poisoned by the same neurotoxin that protects its leaves. Death camas plants have been found surrounded by a circle of dead insects that came to the attractive flowers for either pollen or nectar and were unable to fly away.

Many other plant species have chemical defenses for their leaves, stems and roots, but they produce sweet, fragrant and nutritious pollen and nectar to attract pollinators. Why would a plant expend energy to develop showy flowers that attract pollinators but then kill them with poisonous nectar and pollen?

More than a century ago, Henry L. Viereck and T. D. A. Cockerell, both at the University of Colorado, described a miner bee, Andrena astragali, which drank nectar and collected pollen from death camas. Since then, a series of field and laboratory studies have demonstrated that this is the only bee that can drink death camas nectar, and it supplies nectar and pollen in a food cache available for each larva as soon as it hatches from its egg.

This is an extreme case of exclusive codependence of pollinator and plant, one of just a few cases in which a flower species relies on just a single insect for pollination. But the incentives for the plant to poison all insects but one, and for the bee to rely on just one plant species for adult and larval food, are not immediately apparent.  If either the plant or its pollinator becomes extinct, its partner would suffer the same plight almost immediately.

Entomologists noted that miner bees are plagued by kleptoparasites in the genus Nomada that lay an egg next to an Andrena egg. The Nomada egg hatches first and the parasite eats the egg or the emerging larval bee. Then it consumes the provision mass intended for the larval bee, but is killed by the provision mass laced with zygacine. After the parasites learned that death camas miner bees are deadly hosts, the bees were free of the parasites.

Death camas and death camas miner bees are committed to an unusual one-on-one codependence of a plant and its pollinator, mediated by a neurotoxin. The plant has gained a dedicated pollinator. The bee gains fewer competitors for pollen and nectar and freedom from parasites that plague other miner bees.