How Insects Use Chemicals to Survive
Silkworm moth "flutterdance "
Why are Monarch butterfly populations shrinking?
Monarch butterfly migrations are in decline. Could it possibly be due to the increase of non-native host plants like tropical milkweeds which continue to produce leaves well into the start of the migration season, or are predators becoming immune to the bitter taste of the caterpillars?
In California, our native species of this plant has become rare in urban areas due to land development and agriculture. Butterflies lay eggs on introduced varieties including the most commonly available Asclepias curassavica, a non-native tropical variety with colorful orange and yellow blossoms. Milkweeds produce many seedpods which burst open with hairy winged seeds. They are carried on the wind and easily germinate making them somewhat invasive. The concern with this non-native variety is the year-round leaf production which encourages butterflies to continue laying eggs instead of migrating as nature intends for them. This could have potentially serious consequences. It also harbors a tiny mite which can be fatal to the monarchs. It is important to cut this species to the ground in fall. A good source for information about insect preservation and habitat is the Xerces Society
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 butterfies and moths, have bold markings or shapes which mimic larger, more dangerous stalkers, and some, like house centipedes, have distracting features like detatchable 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 milk weeds 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 insure 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 in an exothermic reaction to produce a burning spray of 210 degrees.
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 imperceptable to humans unless carefully studied in the lab; however, these same chemicals are used by insects as keys to 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 pheromone was discovered in 1956 after a team of German scientists worked for 20 years to isolate it from the abdominal glands of the female silk moth. They observed that a tiny amount would make a male respond with a "flutter dance." 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 farther 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.
The clever bola spider:
A Rattlebox Moth- Utetheisa ornatrix
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 that 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 disasteful 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.
Parasitic Wasps: nature's best garden defense
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 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, " insects won't inherit the earth, they own it . . . now."
© 2011 Catherine Tally
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