Critter Chemistry

September 15, 2003 Volume 81, Number 37

Gee, Your Hair Smells Dangerous

Volatile fragrance chemicals may attract unwanted attention from hornets and bees

Melody Voith

Day-trippers who plan to visit the mountains of Japan should be careful when primping and packing their picnic baskets, according to the latest research. The chemical signal that hornets use to identify and attack invaders can also be found in certain food flavorings and fragrances [Nature, 424, 637 (2003)].

SMELL THE ALARM! Giant hornets rush toward filter paper impregnated with 2-pentanol, 3-methyl-1-butanol, and 1-methylbutyl 3-methylbutanoate, the components of hornet alarm pheromone. ©NATURE

Hornets can be more than a meal-ruining pest: 30 to 40 Japanese die each year after being stung by Hymenopteran insects (hornets, bees, and wasps). The number of fatalities varies; in 1984, there were 73 victims. Japan has more species of Hymenoptera than the U.S. or any European country. In the U.S., wasps and bees cause between 30 and 120 deaths each year.

Masato Ono, professor of insect technology at Tamagawa University, and his team have identified substances that comprise a volatile alarm pheromone in the venom of the world's largest hornet, Vespa mandarinia. The Japanese hornets, which range in length from 27 to 55 mm, secrete the pheromone to identify nest invaders and to mark prey such as honeybees. The presence of this pheromone spurs an attack on the invader or prey by a large group of nest-mates.

Ono's earlier research [Nature, 377, 334 (1995)] examined the complex chemical arms race that has evolved between the giant hornet and its prey species, the Japanese honeybee Apis cerana. Observations and experiments have shown that the hornets' intended victims can interpret the chemical signal and have developed an unusual defense.

When a foraging giant hornet locates a honeybee nest, it hunts individual bees and leaves the pheromone marker behind so nest-mates can find the nest and attack. But like a baseball team that has decoded the other team's batting signs, the honeybees have learned to recognize the signal and take a defensive posture. Worker bees lie in wait, and when a hornet enters the nest, they engulf it in a ball of over 500 bees, raising the temperature to 47 °C, which is lethal to the hornet, but not to the bees.

To find out the ingredients of the powerful attack pheromone, Ono used solid-phase micro-extraction (SPME) to capture volatile components from the venom sacs of three V. mandarinia hornets. He analyzed the compounds by gas chromatography/mass spectrometry, identifying three biologically active chemicals: 2-pentanol, 3-methyl-1-butanol, and 1-methylbutyl 3-methylbutanoate.

Field tests with 2-pentanol triggered mild alarm and defensive behavior among hornets when placed near a nest, but adding 3-methyl-1-butanol and 1-methylbutyl 3-methylbutanoate created a synergistic effect on the hornet reactions. "This kind of multiple-component alarm pheromone had not yet been seen in our research," Ono says. Commercial products such as banana and apple flavorings containing C 5 alcohols and C 10 esters also elicit defense reactions.

The extreme volatility of the alarm chemicals makes them ideal for raising an alarm because the short duration of the signal may allow the hornet colony to return to normal behavior after the threat has passed. Unfortunately, this volatility also makes the compounds useful in the flavors and fragrances industry.

Entomology professor Richard D. Fell of Virginia Polytechnic Institute & State University, Blacksburg, has seen similar chemical make-ups of alarm pheromones in bees, wasps, and ants. "The combinations of the small molecules tend to be species-specific," he says. Usually, it's not just one chemical acting alone, but rather several acting in concert that triggers insect behavior.

Serious incidents of hornet attacks occur only around nesting sites. Two or three hornets may investigate a person who ventures too close. If instead of backing away from the area, which would be the wisest course, the interloper waves at or hits the hornets, they will spray venom containing the volatile alarm pheromone, summoning many more hornets to defend the nest. Unfortunately for the dark-haired Japanese, the hornets are especially aggressive toward the color black, causing victims to be stung many times on the head.

According to Fell, "There are two things that the pheromones trigger--an attraction phase, which draws the insects to a location, and an attack phase." He says it would be hard to tell whether hornets attracted to artificial fragrances would actually sting someone wearing them if the insects were not otherwise threatened.

In his latest Nature paper, Ono suggests that the presence of the volatile compounds might provoke normally peaceful hornets to attack anyone who happens to get too close to a nest. He also suggests that it may make sense to screen products for pheromones that might draw the ire of dangerous insects.

In the meantime, Fell cautions that because many Hymenoptera species--including the often aggressive yellow jacket--use alarm pheromones, outdoors people should think twice before applying products containing fragrances. He warns, "If you don't want to attract hornets and bees, don't try to smell like a flower."