Entomologist’s Corner: The Mysteries of Pollinator Preference

By David Cappaert, IAE Entomologist

May 2023

If I were to summarize the basic question of the science of ecology, it might be: why does this organism, this population, or this community occur in this place? Answering this question is foundational to what we – at IAE – do. 

We consider a rare plant – Willamette daisy, or golden paintbrush—and try to decipher what kind of environment it needs. In what kind of soil will its seeds germinate? What other plants hinder or facilitate its spread? How does it respond to drought and flooding? The list of such conditions is long and complex.

For IAE entomologists such as myself, we ask a narrower question: how do pollinating insects determine the success of plants in Willamette Valley prairies? We can think about how to answer this question with the benefit of 5 years of data on every flowering plant species, in half a dozen sites across the Willamette Valley. For each plant, we know the insects who come to visit. For each insect, we know which plants it appears to prefer. Through combining this data we can paint a picture of the whole relational system – what we call a pollinator network.

From our enormous dataset gathered over many years, one might suppose that we could now tell you just what to expect in the prairie ecosystems we study. However, the data defies simple conclusions.

Case in point: 2023 observations of camas at Herbert Farm and Fisher Butte.

Camas at Fisher Butte creates a vast blue blanket that covers 10 acres. Photo credit: David Cappaert

Fisher Butte, a high-quality wet prairie west of Eugene, should be a bonanza for pollinators of camas. We have 5 years of data documenting thousands of individual insects among several dozen bee, fly, and beetle species that visit camas at the site. However, this year’s cool, wet spring appears to have been a challenge for many of the typical pollinators we observed. This April, we spent a few hours on one of the few warm sunny days scouting for pollinators: it was unfortunately a bust. Among a thousand plants, there were only a few bumblebees (Bombus californicus), honeybees, and the occasional fly. Based on this data, you might conclude that camas is a poor host for pollinators, or that 2023 is just an incredibly off year for camas pollination.

Camas at Herbert Farm occurs in small stands of a few dozen flowers. Photo credit: David Cappaert

But if you visited Herbert Farm, an equally beautiful restored prairie just south of Corvallis, you would conclude the opposite. Camas was a pollinator magnet, both in terms of traffic–easily ten times greater than at Fisher Butte–and diversity of pollinators.

A few of the species visiting camas at Herbert Farm. a) mason bees; b) California bumblebee; c) honeybee; d) small carpenter bee; e) formicine ant; f) sinuous ladybeetle; g) greater beefly; h) goldenrod crab spider. Photo credit: David Cappaert

Additional insects visiting camas at Herbert Farm, captured and photographed later. a) sweat bee; b) sedon sweat bee species; c) horn-faced mason bee; d) second mason bee species; e) nomad bee; f) prong-faced slender mason bee; g) dance fly; h) longhorn bee. Photo credit: David Cappaert

Why are pollinators of camas so active at Herbert Farm, but not at Fisher Butte? Of course, questions like these are common, because we know so little about all of the variables that underlie a pollination network, with dozens of plants, and hundreds of insect visitors every month. 

The answer might seem fairly simple: Fisher Butte is exceptionally wet this year, which might suppress pollinator populations and prevent them from visiting their host plants with the same frequency we have observed in the past. However, this is a weak hypothesis. The advantage of having five years of data is that we can look at previous wet years, and find robust pollinator activity.

Is it an issue with the host plants? Are the camas at Herbert Farm stronger plants that produce more pollen of nectar than those at Fisher Butte? Could it be a result of natural enemies, predators, and parasites that are impacting one site more than the other? Is it a question of competition, in which other flowering plant species at Fisher Butte have become more attractive to pollinators? 

These kinds of unresolved questions are what motivate us to do the work we do, as we try to learn how pollination affects an ecosystem (and hopefully use this knowledge to conserve native species and habitats). But there is another motive, true for all scientists: to watch any one insect, or to discover an unexpected plant and appreciate its life history, is an opportunity to partake of the beauty in nature.

IAE would like to thank the Army Corps of Engineers and the City of Corvallis for supporting our work to restore habitats for pollinators at Fisher Butte and Herbert Farm!