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Surprisingly, the robbers might have little to no effect on fitweed and might even benefit the plant

As Trail 403 climbs out of Washington Gulch in the Elk Mountains, it winds through many patches of a conspicuous and strikingly beautiful wildflower, Corydalis caseana, also known as Case's fitweed. They display colorful racemes or towers of up to 70 flowers, on stems from 4 to 6 feet tall. 

Each flower is a tall column of white to pink petals fused to form a tube with bright purple at the throat of the tube and a long spur above it. A young plant has a single stem, a hollow tube a half inch in diameter. But as the plants age, they may have up to 20 stems in close proximity, making it difficult to identify individuals. 

Fitweed grows in dense populations, and its geographic distribution includes all of the western states, but the populations are separated by great distances and are sufficiently different to have five or six (opinions differ) subspecies. The subspecies brandegei is in the Elk and San Juan Mountains in Colorado,.    

The common name, fitweed, refers to effects suffered by cattle after browsing the leaves. Fits precede death, but fortunately, most cattle avoid fitweed without tasting it, perhaps detecting a nasty fragrance. Fitweed is protected by at least 10 different alkaloids, and in 1971 it yielded an alkaloid never seen before, now known as caseanadine.

Fitweed grows in dense populations straddling mountain streams. Photos by Jeff Mitton.

The genus Corydalis has about 300 species native to North America, Europe, Asia and the mountains of Africa.

Flowers are adaptations to attract pollinators, presenting bright visual signals and fragrant aromas and offering nectar and pollen. But the world is full of insects, and it is to the benefit of a plant species to limit the number of visiting species. 

Receiving many types of pollen from other species could decrease seed production, for alien pollen can coat and jam the stigma, where pollen is deposited. One of the mechanisms to reduce the number of pollinator species is to place the nectar reward at the end of a long corolla of fused petals, excluding insects with short tongues. 

Fitweed has a long tube of fused petals leading to the nectaries, which produce nectar. The long, nearly vertical tube excludes flies, bugs and beetles and denies butterflies a place to perch. 

The sole insect known to pollinate fitweed is the white—shouldered bumble bee, Bombus appositus, which has a long tongue. It also pollinates  other native wildflowers, such as Nuttall's larkspur, Delphinium nuttallianum, andscarlet gilia, Ipomopsis aggregata, as well as introduced yellow toadflax, Linaria vulgaris, all with long corolla tubes. 

Bumble bees with short tongues can bypass their exclusion by long corollas by biting a hole in the base of the corolla tube, providing easy access to the nectar. The western bumble bee, Bombus occidentalis, is a nectar robber, for this short-tongued bee bites a hole through the base of the corolla tube and extracts most of the nectar without pollinating the flower. 

Joan Maloof and David Inouye studied fitweed and its pollinator and its nectar robber with a series of experiments at the Rocky Mountain Biological Laboratory in Gothic, and they reported some unexpected results.

The proportion of fitweed flowers robbed in natural stands around Gothic varies between 40 and 80%. This sounds like a dire situation for fitweed, assuming that robbed flowers produce few or no seed. 

But when undisturbed and robbed flowers were compared, visitations by white-shouldered bumble bees were the same. Either the pollinator could not detect the flower damage and lower level of nectar, or they were not dissuaded by the robbers. Flowers did not produce seed if they were robbed but not visited by the pollinator. 

But a surprising result was that when robbed and unrobbed flowers were compared after visitation by a pollinator, seed set did not differ between the groups. I was surprised to learn that robbing a fitweed flower of most of its nectar and piercing  its corolla tube did not have any impact on its seed production. 

When western bumble bees had robbed a high proportion of the flowers in an area, the pollinating white-shouldered bumble bees increased their flight distances between plants. Greater flight distances would increase gene flow and outcrossing, which could only be beneficial to the plants.

This was a careful, thorough set of experiments that produced clear results, but reviews of the literature indicate that this case is not general. Studies of other Corydalis species suggest that each has a bumble bee nectar robber—and nectar robbing is common in other groups of wildflowers as well. 

But in some cases, robbing bumble bees inadvertently pollinate flowers as they maneuver to rob their nectar. One report in the literature found that the robber was a more efficient pollinator than the pollinator! As the numbers of experimental studies increased, we have learned that nectar robbers may reduce seed set, or have no impact, or actually have beneficial impacts on seed production.