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Understanding Systemic Pesticides

Pesticide Research Institute *** Bees *** Understanding Systemic Pesticides
Posted on April 20, 2015 · Posted in Bees

Control flower—no food coloring
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 results are illuminating. You can clearly see the dye as it is taken up through the xylem (the part of the stem that transports water-soluble components upward) in the stem of the sunflower. You can see the same phenomenon with celery.
The red food coloring is transported through the xylem of the stem to the flower head

The red food coloring is transported
through the xylem of the stem to the
flower head.
Celery also transports the red food coloring up through the xylem

Celery also transports the red food coloring up through the xylem.

 

Different food colorings are transported differently in the sunflower, with the more water-soluble blue dye (100 mg/L [1,2]) transported to the tips of the petals and the less soluble red dye (20 mg/L [1,2]) adsorbing to the veins of the petals.
The red food coloring adsorbs to the tissue in the veins of the flower petals
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 blue food coloring is transported to
the tips of the petals.

 

Once the flower is cut open, it is easy to see how the food coloring is distributed, with the xylem feeding each nascent sunflower seed. The type of vascularization (xylem vs phloem) that feeds floral and extrafloral nectaries varies across plant species, so the xylem may not directly feed the nectary. For the Asteraceae family to which sunflowers belong, approximately 21% of the species studied have their floral nectaries fed by both phloem and xylem, 27% by phloem alone, and 52% lack vascular tissue [3]. Sunflowers also have extrafloral nectaries [4]. Regardless of whether the xylem feeds the nectaries directly, the influx of water from the roots of the plant is the source of all dissolved substances in the plant.
The food coloring remains concentrated in the xylem at the base of the flower
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

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.

Water Solubility and Toxicity of Common Pesticides

Chemical Water Solubility
(mg/L)
Oral Toxicity to Honey Bees
(LD50 in ug/bee)
FD&C Red Dye #40 20 NA
FD&C Blue Dye #2 100 NA
Dinotefuran 39,800 0.023
Imidacloprid 514 0.0037
Thiamethoxam 4,100 0.0050
Clothianidin 259 0.0038
Sulfoxaflor 1,380 0.29
Propiconazole 100 14
Tebuconazole 32 176
Cyantraniliprole 14.2 0.1055
Azoxystrobin 6 25
Chlorantraniliprole 1 104
Fipronil 22 0.0042
Abamectin (Avermectin) 1.21 0.0090
lambda-Cyhalothrin 0.005 0.0270
Chlorpyrifos 1.18 0.24

References:

[1] Human Metabolome Database, http://www.hmdb.ca/

[2] 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

[3] 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.

[4] 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.

About the Author

Susan Kegley is Principal and CEO of Pesticide Research Institute. She is a PhD Organic chemist with expertise in pesticide chemistry, fate and transport, toxicology, and U.S. pesticide regulation.

 

 

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