Honey Bee Excrement

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The Origin of Mysterious Yellow Globs
Small Yellow Globs Rain Down On Cars At Palomar College

© W.P. Armstrong 2006

Anyone who has parked at Palomar College during recent years has probably noticed small yellow globs on their vehicle. They seem to appear magically during the daytime hours, and apparently fall from the sky. The globs may be circular, oblong or linear, and are usually less than a centimeter in length. Their shape seems to be dependent on the angle of impact and whether your vehicle is moving or stationary. They are especially noticeable on light-colored cars, particularly when you are washing the windows. Governing Board Trustee Mark Evilsizer has asked me about the yellow globs on two occasions, so I decided to investigate this puzzling phenomenon. Is this a form of air pollution that we should be concerned about?

Several hypotheses have been proposed for the origin of the yellow globs, including fall-out from the new Palomar Power Plant in Escondido, local trees on campus, and excrement from high-flying honey bees. According to San Diego Gas & Electric/Sempra Energy, there were yellow droplets in the fallout from the Palomar Power Plant last November because they were testing a new turbine. The prevailing winds blow in the direction of Escondido and not toward Palomar College in San Marcos, except when "Santa Ana" winds blow toward the coast. The white smoke I observed last November appeared to be blowing inland in the direction of Escondido. A brief search on the Internet yielded several articles about "yellow rain" from honey bees, so I decided to test the honey bee hypothesis by examining yellow globs on my car. Since I haven't washed it for about a year, some of these spots certainly resulted while it was parked at Palomar College.

  See Animated Representation Of This Hypothesis  

Our common honey bees (Apis mellifera) were originally imported from Europe. They greatly outnumber other native species of bees and are by far the most common bees in our skies. They excrete minute yellow globs while flying to or from their nests (hives). This is a natural phenomenon in all animals, although the color and composition of excrement varies with different diets. Honey bees derive their protein nutrition from pollen. They carry pollen back to the nest within specially arranged hairs on their legs called "pollen baskets." They also sip flower nectar for their carbohydrate source, a sweet liquid consisting mainly of glucose and fructose sugars. Nectar is converted into honey in the bee's stomach and is regurgitated and stored in hexanogonal wax cells within the hive. Honey is fed to larvae and serves as a supplemental food for adult bees. Honey is essentially "bee vomit," although it is not in good taste to request honey by this name in restaurants.

Worker female honey bees (Apis mellifera) on their wax honeycomb. The hexagonal cells are used to store honey and to incubate larvae. The remarkable geometric structure of the cells provides for maximum utilization of space.

The wax honeycomb of the honey bee (Apis mellifera) is composed of two layers of hexagonal cells. Beeswax, the construction material for the wax cells, is secreted by special glands in the abdomen of worker bees. One layer of cells can be accessed from the front side, and another layer can be accessed from the back side. This ingenious construction of the two layers of cells provides for the maximum utilization of space. The cells are used to store honey and larvae. Larger cells are constructed by the worker bees to accommodate the male drones which develop from unfertilized eggs. Extra large cells are used for larvae of fertilized eggs which are fed "royal jelly." These special females develop into sexually mature queens.

  Parthenogenesis & Sex Determination In The Honey Bee  
Sexual Suicide Of The Male (Drone) Honey Bee

Discussion and Conclusion

I have observed numerous honey bees flying in a path or beeline while standing on Owens Peak (Palomar's "P" Mountain) north of the college. The bees were flying to hives placed near the summit. They were also flying down the slopes of Owens Peak in the direction of Palomar College. Perhaps they were headed toward flowering trees and shrubs on campus, or toward flowering shrubs in the coastal sage scrub plant community north and east of the campus. In fact, the hives were placed on Owens Peak to collect sage honey. On one occasion a honey bee flew into my ear and stung me within my ear canal when I attempted to remove it. The feeling is comparable to a baseball bat striking your ear.

Depending on the number of honey bees flying over Palomar College, it seems very likely that excrement from some bees might land on the body or windows of your car. This is purely a random hit or miss probability, like a random bombing run. I took scrapings from the windshield of my car and placed them under a compound microscope. The scrapings readily dispersed in a drop of water under a cover slip. At 100x magnification the scrapings were composed of thousands of pollen grains, some of them intact and some broken. Some of the grains had different shapes and sizes, with different groove patterns and ornamentation in the outer wall (exine). This variation indicates that the pollen grains came from more than one species of flowering plant. The appearance and consistency of the pollen mass indicated that it was bee excrement, and not merely pollen stored in the bee's pollen baskets.

View of honey bee excrement on right front windshield of Toyota Prias (red arrow). The fresh glob of excrement appeared on a clean windshield within one hour after parking in Twin Oaks Valley. This indicates that bee droppings can occur at any time during daytime hours in San Marcos, even if you are not parked at Palomar College. Since honey bees navigate by the position of the sun, they generally do not fly during the hours of darkness.

Honey bees do not discriminate between the brand of cars. This BMW was parked in the front parking lot at Palomar College.

It is possible that the yellow excrement droplets might be produced by other flying insects, such as various species of butterflies that pass over Palomar College each day during the warmer spring and summer months. Swarms of painted ladies (Vanessa cardui) migrate northward through California, particularly after a wet winter in desert regions of the southwestern United States and northern Mexico. Although they can clear tall buildings, they often fly much lower and hit the windshields of moving vehicles. They feed primarily on nectar, and the yellow splats on your windows and grille are their fat supply for the long journey. These splatches are different in composition from yellow globs of ingested pollen from honey bees.

  See Images Of The Painted Lady Butterfly  

Magnified view of scrapings of bee excrement taken from the windshield of my car. The scrapings consist of a mass of pollen grains in a fluid matrix that dries when exposed to the air. Photos taken at 100x and 400x magnifications with a light microscope.

Left: Magnified view (20x) of assorted clumps of pollen taken from foraging honey bees. The clumps were formed as honey bees compressed pollen in the enlarged joint between the femur and tibia of their hind legs. Fringes of long hairs on the legs serve as miniature baskets to carry pollen back to the hive. The bees pass through a fine mesh screen and the clumps are dislodged from their pollen baskets. Bee pollen is packaged and sold to humans who eat it for numerous health reasons. Right: Microscopic view (400x) of pollen from the clump outlined in red. Pollen from two species of flowers are shown, including a larger grain covered with spiny projections. Other clumps in the left image contain of different types of pollen grains.

  Pollen Grains & Flowering Plant Life Cycle  

The honey bee has specially adapted legs with combs of rigid hairs. The tibia of each hind leg is fringed with long, curved hairs forming a "pollen basket." Using combs of hairs on its other legs, moistened pollen is transferred into the pollen basket where it accumulates in a compact mass. Heavily laden bees often have pollen masses protruding from their pollen baskets. These pollen masses are the source of bee pollen sold in natural food stores.

Pollen grains come in many shapes and sizes, depending on the species of plant. Some pollen is colored by water soluble flavonoids, such as blue and red anthocyanin pigments found in delphiniums and fuchsias. Oak pollen is colored by quercetin, a yellow flavonoid pigment. Fat soluble yellow and orange carotenoids are also present in pollen and beeswax. The proposed health benefits of eating bee pollen may involve the antioxidant properties of bioflavonoids and carotenoids that are found in fresh fruits and vegetables.

A mature lily anther releases thousands of reddish-orange pollen grains. Lily pollen can stain clothing, especially if it gets rubbed into the fabric. This is why the anthers are often removed from lily blossoms in bouquets. The color of pollen grains can definitely affect the color of bee pollen and honey.

Pollen is not always yellow. These lily anthers contain reddish-orange pollen that leaves an indelible stain on light-colored clothing.

Considering the tough, protective wall of pollen grains, they could easily pass through the honey bee's digestive system and still retain the distinctive grooves and ornamentation of the outer exine layer. Its chemical composition is different from the siliceous shells of microscopic diatoms and grass phytoliths found in fossilized dinosaur dung. It also different from the chitinous exoskeletons of arthropods and the calcareous shells of mollusks. The exine coating of pollen grains is composed of a durable biopolymer called sporopollenin. Although it contains only carbon, hydrogen and oxygen, it is different from carbohydrate polymers such as starch and cellulose. In fact, it is one of the most stable organic compounds known. It has been extracted from plant spores using strong acid and alkali solutions that would disintegrate other substances. Although its structure is not completely understood, some scientists describe sporopollenin as "polycarotenoid in character."

  Other Terpenes: Including Types Of Carotenoids  

In conclusion, I am reasonably certain that the bee hypothesis is correct, and the mysterious yellow globs on my vehicle are not fallout from the Palomar Power Plant in Escondido (unless they were cleaning the turbine with bee pollen). The bee excrement probably does not pose any harmful threat to people or their automobiles; however, it might be wise to keep your mouth closed while looking up into the sky for extended periods of time.

Gee Whiz Notes About Honey Bees

Worker honey bees build elaborate wax honeycombs of hexagonal cells. The cells are used to store honey and for the developing bee larvae. Foraging bees drink flower nectar, filling their special stomachs where the nectar is converted into honey. Upon returning to the hive the sweet fluid is regurgitated into the wax cells. If the average distance traveled by a honey bee on a single nectar collecting trip is one mile, one pound of honey represents about 20,000 bee miles. The United States produces about 500 million pounds of honey per year representing 10 trillion honey bee miles. This distance is roughly equivalent to 50,000 round trips to the sun, or 20 million round trips to the moon.

To really appreciate the role of insects in the pollination of flowers, consider the pollination of red clover (Trifolium pratense) by the European honey bee (Apis mellifera). According to D.J. Borror and D.M. DeLong (An Introduction To The Study Of Insects, 1964), one acre of red clover contains up to 216 million individual flowers, and every one of these must be visited by an insect before it will produce seed. In Ohio, 82 percent of the red clover population are pollinated by honey bees, and the remaining 15 percent are pollinated by bumble bees. The average seed yields of red clover are about 1-1.6 bushels per acre. With a dense honey bee population in the clover fields, the yield is increased to 8 bushels per acre, and with maximum insect pollination 12 to 20 bushels per acre. In some fruit orchards without honey bees, fruit set occurs in less than one percent of the blossoms. With honey bees, fruit set is increased to 44 percent of the blossoms. This is one of the reasons bee hives are placed in orchards. Can you estimate the number of tons of excrement produced by all of these bees?

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