Canada has few toxic plant species with nectar, pollen, honeydew, or sap harmful to bees or humans. However, during droughts or when food options are scarce, these plants may be visited more frequently by bees. Read this week’s blog to learn more about poisonous plants in Canada, and their impact on bees and honey.

Poisonous Plants and their Impact on Bees and Honey

Toxic plants are characterized by the presence of harmful substances in their nectar, pollen, honeydew, or sap, which can negatively impact animals. The prevalence of toxic plants in Canada is relatively low, and issues related to bee health are infrequent^1,2. The majority of these poisonous plants exhibit characteristics that make them less attractive to bees, such as having limited quantities of nectar or pollen compared to non-toxic species. However, in situations of drought or when alternative food sources are scarce, these less appealing toxic plants may experience increased visitation by bees^1,2.

Bees exposed to poisonous nectar and pollen may exhibit symptoms such as loss of coordination, weakness, or impaired flight ability. Poisonous nectar may also kill brood and newly emerged bees. In severe cases, queens may stop laying, lay only drone brood, or eventually be superseded¹. The symptoms of poisonous nectar will last until the bloom period has ceased. On the other hand, the impact of poisonous pollen may persist as long as the stored pollen remains in the colony¹. Distinguishing plant poisoning from pesticide poisoning can be challenging. However, plant poisoning tends to progress more gradually and may recur in the same location and time each year¹.

A couple of examples of plants toxic to honey bees include certain ornamental rhododendrons and timber milkvetch^1,2. While rhododendrons contain grayanotoxins that can adversely affect honey bees. Poisoning from these plants is rare, as they are more attractive to bumble bees, and the concentration of grayanotoxins varies¹. The nectar of rhododendrons is not considered a risk to human health, as worker bees often die before a substantial amount of honey can be produced¹. Timber milkvetch, found in Alberta and British Columbia, contains miserotoxins with lethal effects for honey bees¹.

Some plants pose a potential threat to human health when present in high concentrations in honey. Plants within the Eriacae family produce grayanotoxins, which are harmful to humans. Honey from Eriacae family members, such as sheep laurel found across Canada, can induce numbness or even loss of consciousness if the concentration is high^1,2.

Sheep laurel (Kalmia angustifolia) (Thiffault et al.©2015)³

Plants containing pyrrolizidine alkaloids may also be of concern for human health. If consumed at high concentrations, these compounds can cause acute poisoning and delayed effects such as hepatic cirrhosis^1,2. Pyrrolizidine alkaloids are found in borage, which has a distribution across Canada. Overall, poisoning from honey is highly unlikely, given that little honey is produced from poisonous plants, and any toxins present will be diluted by the larger volume of nectar gathered from other sources^1,2.

While toxic plants in Canada exhibit a relatively low prevalence, their potential impact on the health of bees and humans needs to be considered. The characteristics of these poisonous plants, such as limited nectar or pollen quantities, make them less attractive to bees. However, the dynamics may shift during periods of drought or scarcity of alternative food sources. It is important to understand the interplay between environmental conditions and bee foraging behavior to mitigate potential risks to bee populations and human health.

References

1. Pernal, S. F., and Clay, H., 2013. Honey bee diseases and pests, 3^rd Edition. Canadian Association Professional Apiculturists, Beaverlodge, AB, Canada 68 pp.
2. Yan, S., Wang, K., Al Naggar, Y., Vander Heyden, Y., Zhao, L., Wu, L. and Xue, X., 2022. Natural plant toxins in honey: An ignored threat to human health. Journal of Hazardous Materials, 424, p.127682.
3. Thiffault, N., Grondin, P., Noël, J. and Poirier, V., 2015. Ecological gradients driving the distribution of four Ericaceae in boreal Quebec, Canada. Ecology and evolution, 5(9), pp.1837-1853.

Connecting with ATTTA Specialists

If you’d like to connect with ATTTA specialists or learn more about our program, you can:

visit our website at https://www.perennia.ca/portfolio-items/honey-bees/

Email abyers@perennia.ca