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Plant/Arthropod Interactions

Many complex relationships have arisen between plants and animals, in particular, flowering plants and arthropods. Some interactions between plants and arthropods are mutually beneficial, while others are beneficial to one partner but detrimental (and sometimes even lethal) to the other. Plants have therefore developed numerous modifications that help them attract the arthropods that help them and others that fend off arthropods that do them harm. These adaptations may determine whether a plant is able to survive and successfully reproduce. Plants that successfully reproduce will pass advantageous traits on to their offspring.

Some of these attributes are chemical. Plants manufacture a vast array of chemicals that are not directly involved in the day-to-day business of trapping solar energy and converting it into the chemical energy that drives plant growth. These chemicals give plants many of the characteristic colors, scents, and flavors that distinguish one plant species from another. Some pigments and aromatic compounds are instrumental in luring arthropods or other animals to disperse pollen or seeds. Other compounds are primarily defensive in nature. They protect vulnerable plant tissues from predation via repellent smells, contact irritation, or direct toxicity.

Plants also have advantageous mechanical modifications. Flowering plants often provide prospective pollinators with nutritious rewards, including nectar, that represent a significant investment of energy. Flowers are often designed so that the reward is available only to the arthropod or animal that is most likely to deliver its pollen to the stigma of an appropriate flower but won't be wasted on visitors unlikely to aid in pollination. Plants also have mechanical attributes that aid in defense, including spines, dense coverings of hairs, or tough tissues that are difficult to penetrate or digest. Latex (sticky white or colored sap) can be considered both a chemical and a mechanical defense strategy.

POLLINATION

Plants are by nature sedentary. Flowering plants are the dominant form of plant life in many parts of Earth, and their success has been in large part due to modifications that have enabled them to use animals to gain reproductive mobility. Flowers are often characterized by groups of characteristics (pollination syndromes) that enable us to propose a hypothesis about the optimal method of pollination for a plant species. The pollination syndromes provide clues, but careful field observations are always required.

These are some of the questions to ask when you observe flowers: 
  • Is the flower large and showy or small and inconspicuous?
  • Is the flower open during the day or during the night?
  • Is the flower aromatic? If so, what does it smell like, and when is the aroma strongest?
  • What color is the flower?
  • What shape is the flower? Does it look like a cup, an urn, a bell, a funnel, or a tube? Does it have any "lips" or landing platforms for arthropods?
  • Does the flower produce nectar? If so, can you tell where it's produced or where it accumulates? Are there any markings on the flower that might guide an arthropod to the nectar?
  • Are the anthers hidden down within the flower, or are they in an exposed position?
  • Does it look as if the anthers touch the stigma? Could the flower self-pollinate?
  • If you see an arthropod visiting a flower, can you tell if it contacts pollen? Does the pollen stick to a particular part of its body? Can you see where the arthropod goes after it leaves the flower?
Common Pollination Syndromes

Wind: Wind-pollinated flowers often appear before the leaves develop. The flowers are exposed, reduced, and inconspicuous. The anthers and stigmas are exposed. The anthers often dangle from their filaments and produce large amounts of pollen; the stigmas are often long and feathery; the flowers lack an odor.

Beetles: Beetle-pollinated flowers are usually flat or shaped like a shallow bowl (easily accessible). The anthers and stigma are exposed. The pollen or nectar is easily accessible. The flowers have a dull greenish or off-white color and a strong fruity or putrid odor.

Carrion and Dung Flies: The anthers and stigma are hidden. The flowers lack nectar; have a dull greenish, brownish, or purplish color; and have a strong odor of decaying protein. (Note that other flies visit different types of flowers, and several fly families actually mimic bees.)

Bees: Bee-pollinated flowers usually have an intricate shape, and strength and dexterity are often required to enter. The anthers and stigma are usually hidden. The flowers produce moderate amounts of nectar that is hidden (but there may be patterned nectar guides, visible in either ambient or ultraviolet light). The flowers are blue, violet, or yellow, with a weak but pleasant odor.

Butterflies and Moths: Butterfly and moth-pollinated flowers are often tubular or funnel-shaped, with nectar at the base. The length of the tube may be correlated with the length of the arthropods proboscis. The flower often has some sort of landing platform. Butterfly flowers can be yellow, blue, violet, or red and often lack a strong odor. Moth flowers are nocturnal, often with a white or drab color and a sweet scent.

Bats: Bat-pollinated flowers are nocturnal. They have an easily accessible position away from the leaves or may appear before the leaves develop. The flowers may be either large and cup-shaped or "brushlike," with many exposed stamens. They produce large amounts of both pollen and nectar, and they have a dull color and a strong, sometimes unpleasant odor.

Hummingbirds: Hummingbird-pollinated flowers are tubular, and the length of the tube may be precisely correlated with the length of the pollinator bill. They produce large amounts of nectar available at the base of the tube, have a red color, and lack an odor.

PLANT DEFENSE

Most plants maintain some sort of chemical arsenal against any predators or parasites, including arthropods, fungi, and bacteria. The line between toxic and medicinal is often slender (depending on dose, mode of preparation, or mode of ingestion), and about 40 percent of the drugs in our modern pharmacopoeia are based on compounds originally identified in plants. Insects are, however, notorious in their ability to circumnavigate chemical defenses. Those that do develop the capacity to detoxify or sequester plant toxins for their own defense may gain access to both a safe haven and an unexploited food source. They may ultimately require the presence of supposed "defensive" compounds to stimulate feeding or reproductive behavior. These arthropods are often considered specialists, because they feed on a single plant species or a group of related plant species. Other arthropods are considered generalists, because they apparently tolerate a wide range of plant chemicals and feed on many different plant species.

The association between monarchs and their milkweed (Asciepias) host plants is one of the most thoroughly investigated system of arthropod/plant interactions. Many milkweeds produce toxic compounds (cardiac glycosides) with a bitter taste. Monarch larvae are able not only to feed upon these plants, but they sequester the toxins that are passed on to the chrysalids, the adults, and even the eggs of the next generation. The sequestered toxins provide the monarchs with a degree of defense against predators. Blue jays that have not yet encountered monarchs will attempt to eat them. The dose of cardiac glycoside that stimulates vomiting is about half the lethal dose, so the blue jay survives, but it is loath to sample another monarch. Because monarchs are conspicuously colored, the blue jay presumably associates its unfortunate gustatory experience with the characteristic coloration. Thereafter, the bird will avoid not only a monarch but also any arthropod that even resembles a monarch.

These are some questions to ask when you see an arthropod on a plant:
  • Does the arthropod have a beneficial, neutral, or detrimental impact on the plant?
  • Is the arthropod collecting pollen or any other substance from the plant?
  • Is the arthropod eating part of the plant, resting on the plant, or looking for prey on the plant?
  • If the arthropod is eating the plant, what part of the plant is it attacking (flower, leaf, stem, fruit, seed, root, bark)?
  • Do you see any relationship between the mouthparts of the arthropod and how it exploits the plant?
  • Is the arthropod an exposed feeder (on the surface of the plant) or a concealed feeder (within the plant tissues)?
  • Is the arthropod a specialist (found on a single plant species or related plant species) or a generalist (found on many different plant species)?
  • Does the plant have any sort of latex (sticky white or colored sap)?
  • Does the plant have any characteristic odor?
  • Does the plant have any mechanical defenses? If so, how does the arthropod avoid them?
  • How might the arthropod defend itself from predators?
  • Is the arthropod camouflaged, or does it have warning coloration (that might alert a predator to its toxicity or bad flavor)?

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