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Senin, 26 Januari 2009

Insect Biology and Ecology

Insects are the dominant life-form on earth. Millions may exist in a single acre of land. About one million species have been described, and there may be as many as ten times that many yet to be identified. Of all creatures on earth, insects are the main consumers of plants. They also play a major role in the breakdown of plant and animal material and constitute a major food source for many other animals.

Insects are extraordinarily adaptable creatures, having evolved to live successfully in most environments on earth, including deserts and the Antarctic. The only place where insects are not commonly found is the oceans. If they are not physically equipped to live in a stressful environment, insects have adopted behaviors to avoid such stresses. Insects possess an amazing diversity in size, form, and behavior.

It is believed that insects are so successful because they have a protective shell or exoskeleton, they are small, and they can fly. Their small size and ability to fly permits escape from enemies and dispersal to new environments. Because they are small they require only small amounts of food and can exist in very small niches or spaces. In addition, insects can produce large numbers of offspring relatively quickly. Insect populations also possess considerable genetic diversity and a great potential for adaptation to different or changing environments. This makes them an especially formidable pest of crops, able to adapt to new plant varieties as they are developed or rapidly becoming resistant to insecticides.

Insects are directly beneficial to humans by producing honey, silk, wax, and other products. Indirectly, they are important as pollinators of crops, natural enemies of pests, scavengers, and food for other creatures. At the same time, insects are major pests of humans and domesticated animals because they destroy crops and vector diseases. In reality, less than one percent of insect species are pests, and only a few hundred of these are consistently a problem. In the context of agriculture, an insect is a pest if its presence or damage results in an economically important loss.

The adage "know your enemy" is especially appropriate when it comes to insect pests. The more we know about their biology and behavior, including their natural enemies, the more likely we will be able to manage them effectively.

Left: Hippodamia glacialis, a predator of aphids. J.Ogrodnick
Center: Cotesia congregata, a parasitoid of caterpillars. K.Kester
Right: The larvae of Sphenoptera jugoslavica feed on the roots of the plant pest diffuse knapweed. R.Richard

Insect Anatomy

Insects and closely related organisms have a lightweight, but strong exterior skeleton (exoskeleton) or integument. Their muscles and organs are on the inside. This multi-layered exoskeleton protects the insect from the environment and natural enemies. The exoskeleton also has many sense organs for detecting light, pressure, sound, temperature, wind, and odor. Sense organs may be located almost anywhere on the insect body, not just on the head.

Insects have three body regions: head, thorax, and abdomen. The head functions mainly for food and sensory intake and information processing. Insect mouthparts have evolved for chewing (beetles, caterpillars), piercing-sucking (aphids, bugs), sponging (flies), siphoning (moths), rasping-sucking (thrips), cutting-sponging (biting flies), and chewing-lapping (wasps). The thorax provides structural support for the legs (three pairs) and, if present, for one or two pairs of wings. The legs may be adapted for running, grasping, digging, or swimming. The abdomen functions in digestion and reproduction.

The internal anatomy of insects is characterized by an open circulatory system, a multitude of breathing tubes, and a three-chambered digestive system. With the exception of a heart and an aorta, there are few blood vessels; insect blood simply flows around inside the body cavity. Air enters the insect through a few openings (spiracles) in the exoskeleton, and makes its way to all areas of need by way of branching tubes, which permeate the body. The insect digestive system is long and tube-like, often divided into three sections, each with a different function. The insect nervous system transports and processes information received from the sense organs (sight, smell, taste, hearing, and touch). The brain, located in the head, processes information, but some information is also processed at nerve centers elsewhere in the body.

Knowledge about the structure and function of the insect exoskeleton has proven critical in developing insecticide formulations that are able to penetrate this multi-layered protective covering. Studies of insect communication have led to the discovery of chemical compounds used by insects to locate each other or host plants, and many of these have now been identified and produced synthetically. For example, pheromones are very specific compounds released by insects to attract others of the same species, such as for mating. Synthetic pheromones are now widely used to bait insect traps for detecting the presence of a pest, to determine its abundance, or for control. Control may involve the use of many traps to "trap out" the pest or the pheromones can be dispersed throughout the crop to "confuse" insects, making it more difficult for them to find a mate.

As simple as it may seem, knowing what type of mouthparts an insect has can be very important in deciding on a management tactic. For example, insects with chewing mouthparts can be selectively controlled by some insecticides that are applied directly to plant surfaces and are only effective if ingested; contact alone will not result in death of the insect. Consequently, natural enemies that feed on other insects, but not the crop plant, will not be harmed.

Since insects obtain oxygen through their spiracles, plugging these openings causes death. That is how insecticidal oils control insects. Components of the microbial insecticide Bacillus thuringiensis enter the digestive system and break down the gut lining. Knowledge of the nervous system of insects has led to the development of several types of insecticides designed to disrupt normal nerve function. Some of these are effective simply by contacting the insect.

Insect Reproduction

Most species of insects have males and females that mate and reproduce sexually. In some cases, males are rare or present only at certain times of the year. In the absence of males, females of some species may still reproduce. This is common, particularly among aphids. In many species of wasps, unfertilized eggs become males while fertilized eggs become females. In a few species, females produce only females.

A single embryo typically develops within each egg, except in the case of polyembryony, where hundreds of embryos may develop per egg. Insects may reproduce by laying eggs or, in some species, the eggs may hatch within the female which shortly thereafter deposits young. In another strategy common to aphids, the eggs hatch within the female and the immatures remain within the female for some time before birth.

Insect Growth and Development (Metamorphosis)

Insects typically pass through four distinct life stages: egg, larva or nymph, pupa, and adult. Eggs are laid singly or in masses, in or on plant tissue or another insect. The embryo within the egg develops, and eventually a larva or nymph emerges from the egg. There are generally several larval or nymphal stages (instars), each progressively larger and requiring a molt, or shed of the outer skin, between each stage. Most weight gain (sometimes > 90%) occurs during the last one or two instars. In general, neither eggs, pupae, nor adults grow in size; all growth occurs during the larval or nymphal stages.

The two types of metamorphosis typical of insect pests and natural enemies are gradual (egg > nymph > adult) and complete (egg > larva > pupa > adult). In gradual metamorphosis, the nymphal stages resemble the adult except that they lack wings and the nymphs may be colored differently than the adults. Nymphs and adults usually occupy similar habitats and have similar hosts. Gradual metamorphosis is typical of true bugs and grasshoppers; complete metamorphosis is typical of beetles, flies, moths, and wasps. The immatures of these latter species do not resemble the adults, may occupy different habitats, and feed on different hosts. Some moth and wasp larvae weave a silken shell (cocoon) to protect the pupal stage; in flies, the last larval skin becomes a puparium that protects the pupal stage.

Insects are cold-blooded, so that the rate at which they develop is mostly dependent on the temperature of their environment. Cooler temperatures result in slowed growth; higher temperatures speed up the growth process. If a season is hot, more generations may occur than during a cool season.

A better understanding of how insects grow and develop has contributed greatly to their management. For example, knowledge of the hormonal control of insect metamorphosis led to the development of a new class of insecticides called insect growth regulators (IGR). The insect growth regulators are very selective in the insects they affect. Based on information about insect growth rates relative to temperature, computer models can be used to predict when insects will be most abundant during the growing season and, consequently, when crops are most at risk.
Insect Classification and Identification

It is necessary to classify insects so that we can organize what we know about them and determine their relationships with other insects. For example, all members of a particular species will feed on similar foods, have similar developmental characteristics, and exist in similar environments. Most often, insect species are classified based on similarities in appearance (morphology). The flies, for example, can be distinguished and classified separately from all other winged insects because they have only one pair of wings. The hierarchy used to classify the diamondback moth, a worldwide pest of crucifers, is as follows:
  • Phylum - Arthropoda
  • Class - Insecta
  • Order - Lepidoptera
  • Family - Plutellidae
  • Genus - Plutella
  • species - Plutella xylostella

This universal method is used to prevent confusion among geographic regions of the world. Consequently, Plutella xylostella refers to the same insect species in the United States as it does in Asia or anywhere else in the world. Common names, however, can vary from one location to another.

Ecology is the study of the interrelationships between organisms and their environment. An insect's environment may be described by physical factors such as temperature, wind, humidity, light, and biological factors such as other members of the species, food sources, natural enemies, and competitors (organisms using the same space or food source). An understanding or at least an appreciation of these physical and biological (ecological) factors and how they relate to insect diversity, activity (timing of insect appearance or phenology), and abundance is critical for successful pest management.

Some insect species have a single generation per season (univoltine), while others may have several (multivoltine). The striped cucumber beetle, for example, overwinters as an adult, emerges in the spring, and lays eggs near the roots of young cucurbit plants. The eggs hatch, producing larvae that emerge as adults later in the summer. These adults overwinter to start the cycle again the next year. In contrast, egg parasitoids like Trichogramma overwinter as immatures within the egg of their host. During the summer they may have several generations.
Insects adapt to many types of environmental conditions during their seasonal cycle. To survive the harsh winters, cucumber beetles enter a dormant state. While in this dormant state, metabolic activity is minimal and no reproduction or growth occurs. Dormancy can also occur at other times of the year when conditions may be stressful for the insect.

It is often better to consider insects as populations rather than individuals, especially within the context of an agroecosystem. Populations have attributes such as density (number per unit area), age distribution (proportion in each life stage), and birth and death rates. Understanding the attributes of a pest population is important for good management. Knowing the age distribution of a pest population may indicate the potential for crop damage. For example, if most of the striped cucumber beetles are immatures, direct damage to the above ground portions of the plant is unlikely. Similarly, if the density of a pest is known and can be related to the potential for damage, an action may be required to protect the crop. Information about death rates due to natural enemies can be very important. Natural enemies do nothing but reduce pest populations and understanding and quantifying their impact is important to effective pest management. This is all the more reason to conserve their numbers.

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