Comparison of buckwheat, red clover, and purple tansy as potential surrogate plants for use in semi-field pesticide risk assessments with Bombus impatiens

Abstract

Background. Bumble bees (Bombus spp.) are important wild and managed pollinators. There is increased interest in incorporating data on bumble bees into risk assessments for pesticides, but standardized methods for assessing hazards of pesticides in semi-field and field settings have not yet been established for bumble bees. During semi-field studies, colonies are caged with pesticide-treated flowering surrogate plants, which must be attractive to foragers to ensure colony exposure to the test compound, and must produce an ample nectar and pollen to sustain colonies during testing. However, it is not known which plant(s) are suitable for use in semi-field studies with bumble bees. Materials and Methods. We compared B. impatiens foraging activity and colony development on small plots of flowering buckwheat (Fagopyrum esculentum, var. common), red clover (Trifolium pratense), and purple tansy (Phacelia tanacetifolia) under semi-field conditions to assess their suitability as surrogate plants for pesticide risk assessment studies with bumble bees. We also compared the growth characteristics and input requirements of each plant type. Results. All three plant types generally established and grew well. Red clover and purple tansy experienced significant weed pressure and/or insect pest damage. In contrast, pest pressure was extremely low in buckwheat. Overall, B. impatiens foraging activity was significantly greater on buckwheat plots than red clover or purple tansy, but plant type had no effect on number of individuals produced per colony or colony weight. Discussion. Because of the consistently high foraging activity and successful colony development observed on buckwheat plots, combined with its favourable growth characteristics and low maintenance requirements, we recommend buckwheat as a surrogate plant for use in semi-field pesticide toxicity assessments with B. impatiens.

Keywords: Bumble bee; Colony development; Foraging; Method development; Pesticide risk assessment; Semi-field.

Figure 1. Bombus impatiens foraging activity over time by plant type.

Mean (±SE) number of Bombus impatiens workers (A) actively foraging and (B) entering or exiting colonies on plots of flowering buckwheat (n = 10), red clover (n = 9), or purple tansy (n = 10) per assessment period on each observation day (n = 7).

Figure 2. Effect display of the relationship between temperature and Bombus impatiens foraging activity.

Effect display of the relationship between temperature (° C) and the mean number of Bombus impatiens workers entering or exiting colonies on small plots of flowering buckwheat (n = 10), red clover (n = 9), or purple tansy (n = 10) plots per assessment period over 7 observation days. Shaded areas correspond to 95% confidence intervals.

Figure 3. Bombus impatiens foraging activity by plant type.

Mean (± SE) number of Bombus impatiens workers (A) actively foraging and (B) entering or exiting colonies on plots of flowering buckwheat (n = 10), red clover (n = 9), or purple tansy (n = 10) plots per assessment period over all observation days (n = 7). Columns with the same letter are not significantly different at α = 0.05.

Figure 4. Mean weight (g) (± SE) of Bombus impatiens colonies by observation day (days on which colonies were weighed).

Colonies were initially confined to flowering field plots of buckwheat (n = 10), red clover (n = 9), or purple tansy (n = 10) in the field. After 16 days in the field, which included four observation days, colonies were brought to the lab and maintained in a growth cabinet until two weeks after the first emergence of a new queen.