Control flower—no food coloring.
|Many insecticides and fungicides are systemic in nature, meaning that they are readily taken up by the plant through the roots. Last week, I decided to do a quick experiment to explore ways to demonstrate systemic transport in plant tissues. I purchased some cut sunflowers and put one in water containing red food coloring and the other in water containing blue food coloring, leaving a few flowers as controls placed in pure water.|
The red food coloring is transported
through the xylem of the stem to the
Celery also transports the red food coloring up through the xylem.
The red food coloring adsorbs to the
tissue in the veins of the flower petals.
The blue food coloring is transported to
the tips of the petals.
The food coloring remains
concentrated in the xylem at
the base of the flower.
Once the solution with the soluble food-coloring reaches the flower head, it is distributed to each floret in the sunflower.
Whether or not a pesticide will be problematic as a systemic depends on several factors, including water solubility, the pKa of the chemical (i.e., the fraction of the compound that is ionized in solution) and the toxicity of the chemical. The table below provides water solubilities and oral LD50 values for some common pesticides. Click on the column headers to sort by LD50 or water solubility.
A pesticide that is highly toxic can still pose significant problems for pollinators consuming the nectar even if the pesticide is not very water soluble. It only takes a small amount of a toxic substance to have an effect. Compare imidacloprid (LD50 = 0.0037 mg/bee and water solubility of 514 mg/L) to dinotefuran (LD50 = 0.023 mg/bee and water solubility of 39,800 mg/L). Although dinotefuran is 16 times less toxic than imidacloprid, it is 77 times more water soluble, which means that more of the chemical will be dissolved in the nectar, all other things being equal.
|Oral Toxicity to Honey Bees
(LD50 in ug/bee)
|FD&C Red Dye #40||20||NA|
|FD&C Blue Dye #2||100||NA|
 Human Metabolome Database, http://www.hmdb.ca/
 Wishart DS, Jewison T, Guo AC, Wilson M, Knox C, et al., HMDB 3.0 — The Human Metabolome Database in 2013. Nucleic Acids Res. 2013. Jan 1;41(D1):D801-7. 23161693
 Wist TJ. 2005. Floral Nectar Production and Nectary Anatomy and Ultrastructure of Echinacea purpurea (Asteraceae). Annals of Botany 97:177–193; doi:10.1093/aob/mcj027.
 Inouye DW, Taylor OR Jr. 1979. A Temperate Region Plant-Ant-Seed Predator System: Consequences of Extra Floral Nectar Secretion by Helianthella Quinquenervis. Ecology 60:2–7; doi:10.2307/1936460.