Pheromones:How Insects Use Chemicals to Survive
Silkworm Moth "Flutterdance"
It is said that insects will outlast us all. How can something so tiny survive amid larger and more aggressive predators? First of all, they escape notice because of size, and they are masters of camouflage. Bug behavior is mysterious. Bites are dreaded. A lot of people have phobias. The tiniest of insects can spread life-threatening disease or undermine successful agribusiness.
Insects are able to repeat their life cycles several times per season. Hot, humid weather can increase this cycle even more. Insects also become immune to pesticides as they adapt and evolve. Many species, especially caterpillars, are equipped with irritating hairs or sharp spines to repel predators. Others, like butterflies and moths, have bold markings or shapes which mimic larger, more dangerous stalkers. Some, like house centipedes, have distracting features such as detachable legs which wiggle independently once dropped. The most sophisticated means of survival, however, comes from the use of naturally produced chemicals.
The ever popular Monarch butterfly caterpillar feeds exclusively on the milkweeds of the Asclepias family. The leaves of these plants impart a foul and toxic taste to the caterpillars making them unpalatable to predators and helping to ensure their survival through metamorphosis. Ladybugs also produce an acrid smell and taste.
Thomas Eisner, reverently referred to as the Father of Chemical Ecology, left us a great legacy in his studies of how insects use chemicals. He claimed that each species of insect relies on 100 or more chemicals during its lifetime. His most famous discovery was the weaponry of the bombardier beetle. This insect was found to have two separate internal reservoirs: one for hydrogen peroxide and the other for hydroquinone. When threatened, this beetle sprays both together where they combine with an enzyme in an exothermic reaction to produce a burning spray of 210 degrees.
His fascinating studies of the Bombardier beetle and many other species are covered in detail in his book, For Love of Insects.
Using a similar line of defense, the wood ant also produces an acid from its rear end when its mound is threatened. The acid is so acrid that one can actually smell it!
Chemicals can be used in ways that are imperceptible to humans unless carefully studied in the lab; however, these same chemicals are used by insects for communication. Whereas humans use eyes, ears, and voice, insects use these "messenger chemicals" called pheromones. They are involved in mating, finding food, detecting enemies, protecting off-spring, and escaping predators. The queen bee actually uses pheromones to direct hive activities to her worker bees.
The first of these attractant chemicals was discovered in 1936 by Adolf Butenandt and a team of German scientists. They worked for 20 years to isolate enough fluid from the abdominal glands of female silk moths to conclusively analyze its chemical make-up. The substance was named "bombykol" after the species of moth from which it was derived. They observed that a tiny amount would make a male respond with a "flutter dance." In 1959, Peter Karlson and Martin Luscher named these chemical attractants "pheromones" from the Greek meaning I carry stimulant(ing.)
It was stated by Lewis Thomas in his book, The Lives of a Cell, that "if a female were to release all of her pheromone at once, she could theoretically attract a trillion males in an instant." Now that's some powerful perfume!
Unlike sight or sound, pheromones last longer and extend further distances, permeating in many directions at once where they cast a larger net. A male silk moth has been found to travel over 30 miles to mate!
Sex pheromones can also be used as a mating deterrent for pest control by overwhelming the male with too much scent. In areas of heavy cotton production where the beet armyworms are a serious problem, farmers often spray their fields with a female sex attractant chemical. The poor male is so confused that he is unable to single out a mate. The result is a pretty effective birth control.
Honey bees communicate with members of the hive through the use of pheromones too. There are many types, each specific in its message. They relate information about the health of the queen and the hive in general. The queen can relay messages regarding mating behavior, egg-laying, and swarming to reduce colony size.
The defensive alarm pheromone smells like bananas which is a good reason to avoid tropical scented sunscreens and lotions.
Bees communicate food sources to other hive members through a waggle dance. The movements, in relation to the position of the sun, relay both distance and exact direction. This information, however, is not enough. The bees must also carry the scent of the flowers for the actual pollination source to be found. Communication within a bee's social network is highly complex.
Bola spiders can produce a pheromone that mimics that of a female moth so that a male in search of a mate will find himself in a web instead.
There is also a female rattlebox moth- Utetheisa ornatrix which feeds on plants containing poisonous alkaloids during its larval stage.She retains her poison through metamorphosis into adulthood. During mating, the male passes on more alkaloid toxin which she, in turn, passes to her eggs. This chemical transfer protects the eggs from predators and makes the adult moth distasteful to spiders. Rare is the spider who will free a moth from his web, but this clever moth gets a ticket to freedom. Now that's brilliant strategy!
One species of female firefly that doesn't produce toxic chemicals of her own has learned to fake a mating signal to lure an unsuspecting male. She then kills and eats him in an effort to steal his immunity. How shrewd!
Ladybugs produce a chemical to make it distasteful to birds, and aquatic beetles produce one that makes fish spit it out.
Pheromones are also used to protect eggs in order to perpeptuate a species. An example is the aforementioned rattlebox moth and her foul tasting eggs. Another twist comes from the male mealworm Beetle who will mate with a female then mark her with a pheromone that makes her unattractive to any other potential mate.
Natural Pest Control
Parasitic wasps have evolved to recognize and follow the sex attractant of host insects. Once they locate them, they lay eggs inside the host or parasitize and eat it. They have become one of the best biological controls we have today against aphids, scales, caterpillars, and whiteflies.
Pheromone traps are widely used by the USDA to trap insects for study and population assessment. We also use them to attract ladybugs, lure yellowjackets and fruit flies, and to catch Indian meal moths and citrus leafminers onto sticky strips.
Nature is brilliant in her design, and I am always in awe of the many fine examples of her ingenuity. Take the time to observe. This summer you might see a wasp hovering around your tomato plants. Don't panic and run for the bug spray! Watch instead as it carefully positions itself to lay its eggs into the body of that fat, green, ugly hornworm.
Thomas Eisner once said, "Bugs are not going to inherit the earth. They own it now. So we might as well make peace with the landlord."
Agosta, William C. (1992) Chemical Communication: The Language of Pheromones
Eisner, T, Eisner, M, & Siegler, M, (2005) Secret Weapons: Defenses of Insects, Spiders, Scorpions, and Other Many-Legged Creatures. Harvard University Press.
http://ipm.ucanr.edu- parasitic wasps
http://www.sciencedirect.com/science/article/pii/S096098221000240X- honeybee communication
https://www.si.edu/Encyclopedia_SI/nmnh/buginfo/pheromones.htm- beetworm mating
https://en.wikipedia.org/wiki/Utetheisa_ornatrix- rattle moth behavior
© 2011 Catherine Tally