If the mechanisms of what attracts and retains a predator or parasitoid to plants are understood, they can be developed and enhanced to optimize control possibilities. Scientists can manipulate the system to maximize performance of biological control measures in agriculture.  The application possibilities will prove to be as intriguing as the research itself.

Carnivorous plants with their unusual shapes, and their most bizarre adaptations to low-nutrient environments, make them both fascinating and intriguing. The approximate 500 carnivorous plant species spread around the world have features to attract, trap, kill, digest prey, and absorb nutrients. They are most abundant in soggy soils in or near bogs and swamps, where the insects and similar pests they catch, help alleviate the shortage of nitrogen. Carnivorous plants, also known as insectivorous plants, prey mostly on insects.

Most plants absorb nitrogen from the soil through their roots, but carnivorous plants absorb nitrogen by trapping and digesting various insects, and other similar pests through their leaves which are specially modified as traps. Carnivorous plants do contain chlorophyll, and so they manufacture food like other green plants. They can, in fact, survive without eating animals, but they grow more vigorously and are healthier when they have this dietary supplement.

Carnivorous plants’ leaves are colorful traps, luring insects and other animals accustomed to associating bright colors with sources of nectar, a sweet-smelling fluid. The bad order of decaying victims may attract such insects as flies, which, of course, often find their food by zeroing in on a malodorous source.

The carnivorous pitcher plants catch insects passively, but effectively. Pitfall traps of pitcher plants are leaves folded into deep, slippery pools filled with digestive enzymes. Typically the lip of the pitcher, the outward-bending portion of the leaf, is scarlet, maroon, or purple, often striped with cream or bright yellow. Along the wing-like ridge inside are cells exuding sweet nectar. They attract insects with the odor of this nectar. Below these is a band of stiff hairs that point downward — a bristly barricade that prevents a creature from crawling up and out of the pitcher. Once inside, the insect finds it cannot get a grip on the walls of the pitcher cup and is doomed because a flaky wax on the interior surface peels off as it struggles to climb, eventually, falling into the water. The motion caused by the struggling pest stimulates the plant’s digestive glands to release an acid. This acid is so strong that insects will disappear within hours. Eventually the pest sinks to the bottom and decays, its soft parts absorbed by the plant.

Pitcher plants, also known as hunter’s cups, contain liquid that is drinkable: the fluid at the top of the top of the pitcher is clear, uncontaminated water. Their tubular leaves form an urn-like basin in which rainwater collects. In some, the leaf has a funnel-shaped top that directs water inside; in others, the tip of the leaf is spread over the opening like a hood, limiting the amount of water that can enter and preventing an overflow in heavy rains. At the bottom, of course, are the remains — the indigestible hard parts — of the plant’s “meals”. However, with care these parts can be avoided, and nearly every pitcher offers at least a swallow or two of water — and some much more.

Nearly half of all the species of carnivorous plants belong to the bladderwort family, with a bladder-like catching device where their leaves join the stems. The bladder has a hinged door lined with trigger hairs. The tiny bladders, rarely measuring more than a tenth of an inch across, are filled with air and have a single opening surrounded by bristles. The “door” remains closed until something pushes against it or touches the bristles, and then it pops open. The instant the “door” opens the water literally rushes in, sucking in such small items as tiny crustaceans and larvae. The suction traps are unique to bladderworts. The door closes again and cannot be opened from the inside. Eventually the water moves out of the bladder, but left behind is the “meal” to be digested and absorbed.

The majority of bladderworts are aquatic plants with submerged feathery leaves and no roots. Their flowers are generally small and bright yellow. However, not all bladderworts are aquatic. Nepenthes, a native of Southeast Asia and Australia, form pitcher cups, hanging from trees. It has a most unusual leaf that first looks like a normal leaf; then, develops a tendril at its tip, and finally the tip of the tendril develops an amazing pitcher. The top of the trap has a lid that initially covers the pitcher cup until its growth is complete. As the Nepenthes mature, it suddenly begins to inflate with air. Once the pitcher cups inflate, they begin to fill with liquid; then opens, revealing the enticing interior. When the leaf is fully grown, the lid opens, and the trap is ready. The trap lures its prey into the pitcher cup by a combination of decaying odors, and sometimes a red coloration. As the pitcher develops, it swells and droops due to its weight. It gains support by twining the tendril around another plant. The largest of the Nepenthes plants, the Rajah Pitcher, is able to digest mice.

The carnivorous sundew plant relies on first effectively trapping its prey with its sticky, glandular hairs, before it slowly rolls up the edges of the leaf. The leaves of sundrews are covered with red or bright orange hairs, each tipped with a shiny drop of fluid that looks deceptively like honey. It can enclose small flies with its’ numerous hairs. A fly, an ant, or another hungry insect touching the droplet discovers instantly that the fluid is powerful glue. The hapless creature’s struggle to break free only stimulates the surrounding hairs to bend over the catch, which soon becomes coated with glue-like slime. The insect suffocates. The sundrew than secretes an enzyme that digests the catch, leaving only the wings, the outer skeleton, and other hard parts. If a leaf catches two insects at once, the hairs divide their work and secure both. These digestive enzymes, including protease and phosphatase, increase in production once a prey has been captured, reaching maximum concentration about the fourth day.

The sundews, so named because their glandular leaf hairs glisten like dew in the sun, are not only common in bogs, but can occur on sandy banks and other mineral soils poor in organic nitrogen and phosphorus. Although one sundew is hardly an effective means of eliminating insect pests; it has been estimated that about six million insects can be trapped in a bog of about two acres. Cultivated sundews showed that those that which were fed insects were more vigorous, produced more flowers, and set more seeds than the ones that were denied any prey.

The European flycatcher grows in narrow coastal or maritime regions, a few kilometers from the coast, with regular morning fogs during summer, and is especially common in northern Morocco, Portugal and southwest Spain. Its sticky, grass-like leaves have been used locally as natural flypaper. In the wild, the European flycatcher is relatively rare. It is a shrub-like carnivorous plant, normally growing to be 40-50 cm in height (maximum 150 cm). Stems are 5-15 mm thick and tend to creep along the ground as they become longer. Ten or more slender, needle-like, triangular shaped leaves are generally found per apical rosette. Leaves typically reach lengths of 10-25 cm with a width of 2.5 mm.

Butterworts operate much like the sundrews, trapping insects in a sticky secretion on their long tapered leaves arranged in a rosette at the base of the plant. The leaves are yellowish in color, which is probably the source of the common name butterwort. The upper surface of the plant is continuously sticky, and tiny flies like mosquitoes, gnats, and midges readily adhere to the surface. There are two kinds of glands on the surface of the leaf. The stalked glands are the sticky ones and trap the insect. The short, surface glands provide the digestive enzymes, including phosphatases, proteases, and ribonuclease, and later resorb the digested material. The leaves bear short hairs, which secrete sticky mucilage, digest insects, known as flypaper traps. When an insect becomes stuck on the mucilage, the edge of the leaf slowly rolls inwards; however, they never completely close. When the insect is digested, the leaf edges unroll, becoming nearly flat, and is ready to act again. Most leaves make only two or three catches before they are shed and replaced.

There are 80 different species of butterworts spread throughout the world, including the southeastern U.S., are located in sunny, open, wet areas. They bear one or two flowers at the end of a long stalk. A butterwort when not in flower can be identified by its sticky yellowish leaves and by its habitat–acid areas that are wet throughout the year. Butterwort plants are perennials, living for several years.

Most carnivorous plants don’t depend on speed to make their catch: they drown their victims in pools of water or catch them on sticky hairs or leaves. The Venus’s-flytrap comes closest to being aggressive. The tip of each leaf on the flytrap is hinged down the middle like a clam-shell. Around its edges are stout tooth-like spines, and in the rose-to-pink center of the leaf are three trigger hairs. An insect or other creature attracted to the leaf cannot avoid touching a trigger. The sensitive hairs at the fold of the leaf prevent the leaf from closing every time a drop of rain lands on it, because the leaf requires that two or more of these hairs be triggered in succession. If a leaf misses its catch or is triggered closed by nonliving material, it opens again within half an hour. With its teeth interlocked and the two halves pushed tightly together, the leaf cannot be forced apart. Unable to escape between the hair-like teeth at the edge of the leaf, the helpless insect is slowly digested and absorbed by the leaf. Glands on the leaf surface secrete several digestive enzymes that help to decompose the insect. Once the insect has been digested sufficiently, the leaf re-opens for another victim.

Otherwise the leaf stays shut until it digests its victim, which may take several weeks. Most leaves make only two or three catches before they die and are replaced by new ones.

The Venus’s-flytrap’s mechanism of closing has fascinated biologists for many years. The most widely accepted explanation had been that a rapid change in the water pressure in the cells – the cells of the bottom part of the midrib, that is. This results in the expansion of the outside of the leaf and the “springing” of the trap. All this happens at lightning fast speed to make the leaf close. The cells remain at this larger size, and the cellulose eventually increases to strengthen the walls. This closes the trap, but in a few days, it must re-open. Once the insect is digested, the cells on the upper surface of the midrib will grow, much more slowly, and the leaf will re-open.

Corkscrew plants, also called Genlisea, are from tropical Africa, Madagascar, and South America. In their natural environment they grow as terrestrials or semi-aquatics. They grow as a rosette of spade shaped leaves, from 1 to 3 inches long. The traps of Genlisea grow underground, and are about 2 to 6 inches long which grow on a cylindrical stalk from the plant. A hollow bulb-like digestive chamber extends from the cylindrical stalk. Two pronged corkscrews emerge from the digestive chamber, thus the plant’s nickname for its’ trap of “lobster-pot”. The corkscrew structure has a slit down the length of the spiral where tiny insects enter. The spiral slit has bristly hairs that make the insects go to the digestive chamber. The digestive chamber structure has digestive juices which dissolve the insect, and then the plant reabsorbs the nutrient rich fluid. The corkscrew plants have of twisted tubular channels lined with hairs and glands.

Scientific investigations are being conducted in order to learn about the biological attraction mechanisms of carnivorous plants. Research has discovered that the carnivorous plant’s chemical compounds can be evolved from plants to repel current attacking insects and other pests. Plants seem to be successful in ensuring that chemical cues are discernible to predators and parasitoids. Several studies have been conducted on the liquids secreted by carnivorous plants. Lima bean plants infested with mites and caterpillars were able to produce a certain odor which attracts mites and caterpillars. However, substances found in caterpillars were necessary for the lima beans to begin manufacturing and releasing odors and secreting liquids which attracts the pests. The lima beans were found to be able to produce the defenses, including manufacturing toxic and repellent chemical compounds, necessary to prevent their natural enemies from feeding on them. If the mechanisms of what attracts and retains a predator or parasitoid to plants are understood, they can be developed and enhanced to optimize control possibilities. Scientists can manipulate the system to maximize performance of biological control measures in agriculture. The application possibilities will prove to be as intriguing as the research itself.