pesticides pollinator threats

Water droplets sometimes carry insecticides

Guttation is a natural process seen in many vascular plants whereby drops of xylem sap exude from leaf tips or margins. Honey bees are known to drink this water, especially in the early spring before large numbers of nectar-containing flowers are available to foragers.

A problem with this type of water collection occurs in agricultural areas where plants are treated with systemic insecticides. Bees collecting guttation drops can be poisoned by the pesticides flowing through the xylem. Worse, sublethal doses of pesticide can be carried back to the hive and fed to the developing larvae by way of the nurses. Researchers are currently trying to determine the type and frequency of damage this may cause to honey bee colonies.

Tests were performed in Italy to see if the guttation drops of corn treated with seed dressings of neonicotinoid pesticides contained enough pesticide to damage bees. The researchers used corn seed treated with three different systemic neonicotinoids (imidacloprid, thiamethoxam, and clothianidan) and one moderately systemic pesticide (fipronil). The seeds were purchased from the manufacturer with the pesticide already applied and ready for commercial distribution.

Guttation drops were collected from seedling emergence through the first three weeks of growth. For each individual seedling, the drops were collected and held at 2? C. After 2-3 days half the liquid was sent away for analysis and half was used for the bee experiments.

Bees held in captivity were chosen at random and caged with the drops. The bees were monitored constantly and timed from the beginning of drinking until they began to show symptoms. The two symptoms monitored were arching of the abdomen and paralysis of the wings. Previous research has shown that wing paralysis is not reversible, so once that happened it was assumed those bees would die.

The results showed that there was easily enough of all three of the systemic pesticides in the drops to cause wing paralysis and death. The moderately systemic fipronil was not found in the guttation drops.

However, not all the drops containing the insecticides contained a lethal dose. In a way, this is even worse because doses low enough that they don’t kill the forager may be carried home and fed to the developing young–a result that could have serious effects on the survival of the colony. Doses of poison that don’t kill an adult bee may have developmental effects on the young larvae.

If you would like to read the whole article, the information is below.


Girolami, V., L. Mazzon, A. Squartini, N. Mori, M. Marzaro, A. Di Bernardo, M. Greatti, C. Giorio, A. Tapparo. 2009. Translocation of neonicotinoid insecticides from coated seeds to seedling guttation drops: A novel way of intoxication for bees. Journal of Economic Entomology 102(5): 1808-1815.

Guttation drops on a leaf margin. Wikimedia photo by Noah Elhardt

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  • The cited study is laughable in its ignorance of how bees collect water.

    The problem with that study is that it force-fed guttation water by depriving the bees of any other source of water. Caged-bee experiments are fraught with such nonsense, as they subject bees to conditions never found in nature.

    The study is invalidated by nothing more than an understanding of bee behavior.

    Bees select reliable sources of water, and that will never been leaf guttation, as guttation happens only in high humidity, low evaporative conditions, and it dries up quickly after sunrise as the sun heats the air. Corn plants (like the ones used in the study) would have to be over-irrigated to produce guttation during daylight hours when bees can forage, so the corn plant rots due to soggy root systems, and the seedling dies.
    Sounds like a self-correcting problem to me.

    But most important, if one had high humidity and low evaporative conditions, there is certain to be no shortage of available surface water (puddles, ponds, streams, saturated soil) from which bees can obtain water in a much more efficient manner than collecting guttation water from corn (or any other) plants.

    The cited study was not science, but activism, where the “researchers” were looking for any way to show that plans treated with systemic pesticides were “bad for bees”. In fact, these systemics are the best thing to happen to bees, as they are not sprayed, and pesticides that are not sprayed are not going to drift onto my bees, and are not going to be sprayed when bees are foraging, and are never going to be applied at high doses in excess of the amount allowed by the EPA label.

    • The study you dismiss so lightly was written by nine research agronomists and entomologists in Italy and was published in a well-respected peer reviewed journal, The Journal of Economic Entomology. I doubt the authors are nearly as ignorant about honey bee water collection, guttation in plants, and the scientific method as you think they are. If, however, you believe you can improve upon the work they are doing, I urge you to contact them directly. Scientific researchers tend to be extremely open to constructive criticism and more than willing to discuss the strong and weak points in the experiments they design and perform. The lead author of this paper, Vincenzo Girolami, a professor at the University of Padua can be reached at

  • No matter how many learned scientists were involved, the fact remains that the Girolami Study does not reflect natural or normal field conditions. The published chemical analysis data showed that the bees, starved for two hours of food and water before then being drip fed a concoction deliberately infused with sucrose to ensure the bees consumed the dose, showed that levels of active substances were 254 times higher than the LD50 – totally unrealistic.

    It is widely accepted and reported by beekeepers, a Swiss study and a UK research study, that only 1% of the colony’s bees actually go out foraging for water, and then from puddles, damp soil and other standing water, and then not for sucrose, but because they need solutes such as sodium ions.

    It can be reasonably opined that the Girolami Study is not realistic and should be treated with extreme caution.

  • It has to be noted that in the cited paper is not written that bees drink guttation drops on corn plants. This study doesn’t want to give the solution to bee death in spring but underlines an important ecotoxicological problem: the presence in the field for a long period (at least 3 weeks) of poisoned drops that are a source of water for many insects. If you go on a corn field and observe the young plants you will see many aphids, leafhoppers and other insects that use these drops to drink. Nobody has demonstrated that bees drink those drops, but the study tries to dig deeper a problem that could concern bee death in a period in which corn sowing and bee death seem to be strongly linked.