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.

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.

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.

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.

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.

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."

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.

Return To WAYNE'S WORD Home Page Return To NOTEWORTHY PLANTS Page Go To Biology GEE WHIZ TRIVIA Page Go To The LEMNACEAE ON-LINE Page