Does Intensive Herbicide Use in Natural Areas Indirectly Drive Declines in Pollinator Abundance?

EPA Grant Number: FP917147
Title: Does Intensive Herbicide Use in Natural Areas Indirectly Drive Declines in Pollinator Abundance?
Investigators: Palladini, Jennifer Dawn
Institution: University of Montana
EPA Project Officer: Cobbs-Green, Gladys M.
Project Period: September 1, 2010 through August 31, 2013
Project Amount: $111,000
RFA: STAR Graduate Fellowships (2010) RFA Text |  Recipients Lists
Research Category: Academic Fellowships , Fellowship - Ecosystem Services: Terrestrial Systems Animal Ecology


Herbicides are increasingly used in natural areas to suppress invasive plants, yet the consequences of herbicide use in the communities in which they are applied are largely unknown. Although herbicides can have direct non-target effects on plants, they may also have strong indirect effects on other trophic levels, particularly pollinators. Native bee populations have suffered extreme declines in recent years, and though the cause of these declines remains unclear, exposure to chemicals and habitat alteration are two likely drivers. Because many plant species require an animal pollinator for sexual reproduction, reductions in the abundance of pollinators could greatly disrupt the viability of plant populations. This work will examine the influence of herbicides on (1) species richness and abundance of native bees, (2) nest establishment and offspring production for native bees, and (3) pollinator visitation and seed production for native plants.


Herbicides are increasingly used in natural areas to suppress invasive plants, which commonly results in a decrease in the abundance of native flowering plants. Pollinators are likely particularly susceptible to the loss of native flowering plants, though this link has not been explored. The goal of my work is to determine how herbicides affect abundance and diversity of native bees, and to explore whether changes in bee abundance have consequences for the pollination of native plants.


My research takes place at 10 low-elevation and 10 mid-elevation intermountain prairie sites in western Montana. Of the 20 study areas, 10 (5 low-elevation and 5 mid-elevation) have a history of intense broadleaf herbicide use, including aerial application and/or broadcast spraying with Milestone ® (aminopyralid) or Tordon ® (picloram). Herbicides have been used in these areas to suppress three invasive forbs: spotted knapweed, leafy spurge, and Dalmatian toadflax. Vegetation and bee communities will be surveyed at each site. To explore differences in bee reproductive rates, I will place a wooden nesting block in each site and monitor nesting by Osmia lignaria (Megachilidae), a native solitary bee. Finally, pollination of native plants will be examined by observing visits by pollinators to arrays of potted Clarkia pulchella (Onagraceae) placed at each site.

Expected Results:

I predict that vegetation communities in areas with a history of herbicide use will have greater grass cover and lower forb cover. Reductions in forb cover that accompany herbicide use will result in lower bee diversity and abundance, as well as reduced nest establishment and offspring production. Finally, I predict that visitation by pollinators to C. pulchella will be reduced in areas with a history of herbicide use.

Potential to Further Environmental/Human Health Protection:

My research will assess the importance of a proposed mechanism of pollinator declines, chemical usage. Herbicides are widely employed to suppress invasive plants, and nontarget effects are of great concern to land managers. However, data on the indirect effects of herbicides on other trophic levels are lacking even though indirect effects, while more difficult to detect, can be as strong or stronger than direct effects. Declines in pollinator populations in natural areas due to herbicide use will have broad implications for land management. By elucidating mechanisms of native pollinator declines, my work will aid conservation biologists in the maintenance of native pollinator populations in both natural and agricultural settings. Finally, my work has relevance for farmers of insect-pollinated crops. As honeybees continue to decline, biologists predict that services by native pollinators could help ameliorate these losses. If herbicides negatively affect native bees, farmers may benefit from limiting the use of herbicides in uncultivated areas.

Supplemental Keywords:

herbicide, Osmia lignaria, plant invasion, pollinator, solitary bee,