Insect Life Cycle

Insects either have a complete or incomplete life cycle. Insects in the complete life cycle group have four distinct stages, the egg, larvae, pupae and adult. Examples of these insects are beetles (വണ്ടുകള്‍) and moths (നിശാശലഭം, കമ്പിളി പ്പുഴു). Beetles lay their eggs either singly or in groups, and they hatch into either grubs (ചെറുപുഴു, മണ്ണട്ട ) or larvae (മുട്ട പൊട്ടിയിറങ്ങിയ ഉടനെയുള്ള പുഴു) that move about freely on the plant feeding on roots, tubers, leaves, or fruits. After reaching maturity, they then pupate (the resting stage) and develop into adults. Adult beetles also may damage plant parts, so two damaging stages may exist. The following figure depicts growth stages for insects having a complete life cycle.

Moths and butterflies also have a complete life cycle similar to beetles except that the damaging stage is the larvae or worm stage which usually feeds on the stems, leaves or fruits. The adult stage, moths and butterflies, feed on nectar or may not feed at all. Insects with a complete life cycle almost always have a chewing mouthpart.

Insects with incomplete life cycles include grasshoppers ( പച്ചക്കുതിര,  വിട്ടില്‍, പച്ചത്തുള്ളന്‍, പുല്‍ച്ചാടി ) and true bugs (stink bug - ദുര്‍ഗന്ധം and squash bugs). Many insects in this category have piercing ( മൂര്‍ച്ചയുള്ള, രൂക്ഷമായ, തുളഞ്ഞു കയറുന്ന ), sucking mouthparts and suck juice from plants. Some, such as the grasshopper, chew on leaves and stems. Regardless, insects with an incomplete life cycle are unique in that they hatch from eggs into tiny nymphs* that resemble the adult stage. They stay in the nymphal stage for several weeks, while growing and molting into larger insects until they reach adulthood. Adults have fully developed wings and can fly great distances. Nymphs either do not have wings or have wings that cannot be used for flight.

Insects with an incomplete life cycle can be controlled at any stage, but are easier to control in the nymphal stage just after they hatch from the eggs. Following figure depicts the developmental stages of insects with incomplete life cycles.

* Nymph : an immature form of an insect that does not change greatly as it grows, e.g. a dragonfly, mayfly, or locust.

Insect Injury

Insects injure plants by chewing leaves, stems and roots, sucking juices, egg laying or transmitting diseases.

Injury by Chewing Insects

Insects take their food in a variety of ways. One method is by chewing off external plant parts. Such insects are called chewing insects. It is easy to see examples of this injury. Perhaps the best way to gain an idea of the prevalence of this type of insect damage is to try to find leaves of plants with no sign of insect chewing injury. Cabbageworms, armyworms, grasshoppers, the Colorado potato beetle and the fall webworm are common examples of insects that cause chewing injury.

Injury by Piercing-Sucking Insects

Another important method which insects use to feed on plants is piercing the epidermis (skin) and sucking sap from cells. In this case, only internal and liquid portions of the plant are swallowed, while the insect feeds externally on the plant. These insects have a slender and sharp pointed part of the mouthpart which is thrust into the plant and through which sap is sucked. This results in a very different but nonetheless severe injury. The hole made in this way is so small that it cannot be seen with the unaided eye, but the withdrawal of the sap results in either minute white, brown or red spotting on leaves, fruits and/or twigs; leaf curling; deformed fruit; or a general wilting, browning and dying of the entire plant. Aphids, scale insects, squash bugs, leafhoppers and plant bugs are examples of piercing-sucking insects.

Injury by Internal Feeders

Many insects feed within plant tissue during a part or all of their destructive stages. They gain entrance to plants either in the egg stage when the female thrust into the tissues with sharp ovipositors (അണ്‌ഡേന്ദ്രിയം, മുട്ടയിടുന്ന അവയവം) and deposit the eggs there, or by eating their way in after they hatch from the eggs. In either case, the hole by which they enter is almost always minute and often invisible. A large hole in a fruit, seed, nut, twig (ചിനപ്പ്‌, ചുള്ളിക്കമ്പ്‌, തളിര്‌) or trunk (തടി, തായ്‌ത്തടി) generally indicates where the insect has come out, and not the point where it entered.

The chief groups of internal feeders are indicated by their common group names: borers; worms or weevils (ധാന്യകീടം, കരിഞ്ചെള്ള്‌) in fruits, nuts or seeds; leaf miners; and gall insects. Each group, except the third, contains some of the foremost insect pests of the world. In nearly all of them, the insect lives inside the plant during only a part of its life and emerging sooner or later as an adult. Control measures for internal feeding insects are most effective if aimed at adults or the immature stages prior to their entrance into the plant.

A number of internal feeders are small enough to find comfortable quarters and an abundance of food between the upper and lower epidermis (പുറംതൊലി) of a leaf. These are known as leaf miners.

Gall insects sting plants and cause them to produce a structure of deformed tissue. The insect then finds shelter and abundant food inside this plant growth. Although the gall is entirely plant tissue, the insect controls and directs the form and shape it takes as it grows.

Injury by Subterranean (ഭൂമിക്കടിയിലുള്ള) Insects

Subterranean insects are those insects that attack plants below the surface of the soil. They include chewers, sap suckers, root borers and gall insects*. The attacks differ from the above ground forms only in their position with reference to the soil surface. Some subterranean insects spend their entire life cycle below ground. In other subterranean insects, there is at least one life stage that occurs above the soil surface; these include wireworm, root maggot, pillbug, strawberry root weevil, and corn rootworm. The larvae are root feeders while the adults live above ground.

Notes : *Gall

• Galls are abnormal plant growths caused by insects, mites, nematodes, fungi, bacteria and viruses.
• Galls can be caused by feeding or egg-laying of insects and mites.
• Insect galls rarely affect plant health and their numbers vary from season to season.
• Control is generally not suggested.
  

Injury by Laying Eggs

Probably 95% or more of insect injury to plants is caused by feeding in the various ways just described. In addition, insects may damage plants by laying eggs in critical plant tissues. As soon as the young hatch, they desert the plant causing no further injury.

Use of Plants for Nest Materials

In addition to laying eggs in plants, insects sometimes remove parts of plants for the construction of nests or for provisioning nests.

Insects as Disseminators of Plant Diseases

In 1892, a plant disease (fireblight of fruit trees) was discovered to be spread by an insect (the honeybee). At present, there is evidence that more than 200 plant diseases are disseminated by insects. The majority of them, about 150, belong to the group known as viruses; 25 or more are due to parasitic fungi; 15 or more are bacterial diseases; and a few are caused by protozoa.

Disseminators  : a person or thing that scatters or spreads something widely.

Insects may spread plant diseases in the following ways:

• By feeding, laying eggs or boring into plants, they create an entrance point for a disease that is not actually transported by them.
• They carry and disseminate the causative agents of the disease on or in their bodies from one plant to a susceptible surface of another plant.
• They carry pathogens on the outside or inside of their bodies and inject plants hypodermically ( ഇന്‍ജക്ഷനുള്ള സൂചി, തൊലിയുടെ അടിയിലുള്ള ) as they feed.
• The insect may serve as an essential host for some part of the pathogens life cycle, and the disease could not complete its life cycle without the insect host.

Examples of insect vectored plant diseases are shown below.

Disease	Vector (രോഗാണുവാഹകം)
Fireblight (bacterial)	Pollinating Insects
Tomato curly top (virus)	Beet leafhopper
Cucumber mosaic (virus)	Aphids

Benefits and Value of Insects

Insects must be studied carefully to distinguish the beneficial from the harmful. Producers have often gone to great trouble and expense to destroy insects, only to learn later that the insect destroyed was not only harmless, but it was actually engaged in saving their crops by eating destructive insects.

Insects are beneficial to the vegetable grower in several ways:

• Insects aid in the production of vegetables by pollinating the blossoms. Melons, squash and many other vegetables require insects to carry their pollen before fruit set.
• Parasitic insects destroy other injurious insects by living on or in their bodies and their eggs. Insects also act as predators, capturing and devouring other insects.
• Insects destroy various weeds in the same ways that they injure crop plants.
• Insects improve the physical condition of the soil and promote its fertility by burrowing throughout the surface layer. Also, the dead bodies and droppings of insects serve as fertilizer.
• Insects perform a valuable service as scavengers (പക്ഷി മതുലായവ, ചീഞ്ഞളിഞ്ഞ മാംസം ഭക്ഷിക്കുന്ന മൃഗം / ജീവി ) by devouring the bodies of dead animals and plants and by burying carcasses and dung.

Many of the benefits from insects enumerated above, although genuine, are insignificant compared with the good that insects do fighting among themselves. There is no doubt that the greatest single factor in keeping plant-feeding insects from overwhelming the rest of the world is that they are fed upon by other insects.

Insects that eat other insects are considered in two groups known as predators (മറ്റുള്ളവയെ ഇരയായി പിടിച്ചു തിന്നുന്ന മൃഗം) and parasites (ഇത്തി(ള്‍)ക്കണ്ണി, പരാന്നഭോജി). Predators are insects (or other animals) that catch and devour (നശിപ്പിക്കുക) other creatures (called the prey), usually killing and consuming them in a single meal. The prey generally is smaller and weaker than the predator. Parasites are forms of living organisms that live on or in the bodies of living organisms (called the hosts) from which they get their food, during at least one stage of their existence. The hosts usually are larger and stronger than the parasites and are not killed promptly but continue to live during a period of close association with the parasite. Predators are typically very active and have long life cycles; parasites are typically sluggish and tend to have very short life cycles.

Insect Control

Insect control is also important to keep the pests from spreading to other crops, and it may help reduce the incidence of disease by killing insect vectors. Insects attacking vegetables can be divided into three categories:

• Soil Insects
• Chewing Insects
• Sucking Insects

Soil Insects

Soil insects include wireworms, white grubs (ചെറുപുഴു, മണ്ണട്ട), fire ants, cutworms, seed maggots (പുഴു, കൃമി) and the sweet potato weevil (കരിഞ്ചെള്ള്, മാണവണ്ട്). These insects can be damaging because they feed on the roots, stems and tubers of plants. Often soil insects, especially cutworms, are common in uncultivated soil sites that have had grass and weeds growing the previous season. These undisturbed areas often harbor (സുരക്ഷിതസ്ഥാനം) high populations of soil insects. Once seeds or transplants are planted, soil insects are difficult to control and may begin feeding immediately on the crop. There is a real need for producers to inspect fields for soil insects prior to planting. One or two soil insects per square foot of soil can cause serious damage.

Oftentimes soil insects are clumped (കൂട്ടമായി കാണുക ) in a field, that is, they may be in one area and not in another. Low areas or those areas with the most vegetation often hold the most insects. Controlling soil insects is much easier if done prior to planting. Most insecticides for the control of soil insects should be applied 6 weeks before planting and incorporated into the top 6 inches of the soil. Liquid or granular materials may be used. These can either be broadcast or banded in the row. Sometimes producers will apply insecticides at planting. Make sure to read the label for proper rates and application techniques. Some insecticides may interfere with seed germination and should not be placed in the furrow in contact with the seed.

Chewing Insects

Many chewing insects have a complete life cycle. Therefore, depending on species, there may be one or two damaging stages. Grasshoppers have a chewing-type mouthpart but have an incomplete life cycle. Chewing insects include all species of beetles, grasshoppers and moths and butterfly larvae (most often called worms).

Chewing insects damage foliage, stems and fruit. They may become as numerous as to completely defoliate plants. Eggs of most insects are laid on the plant, and the larvae upon hatching begin to feed. Others may invade the crop by “marching in” or by flying into the field.

Control of chewing insects is basically twofold. One, the grower must watch for eggs and small larvae that begin to feed; two, he must watch for the adults and control them when necessary. Control of these insects is important in the early infestation of the plant. Often, the insect after hatching may bore into the fruit or stem and be hidden from pesticide applications.

These insects often become numerous because producers do not begin treatment early enough. It is vital that fields be watched and these insects controlled at the earliest possible moment.

Sometimes a single application timed properly will control a generation. However, repeated applications are needed to control others like the corn earworm in sweet corn.

Sucking Insects

Sucking insects include aphids (“plant lice”), stink bugs, squash bugs, leafhoppers and spider mites. Spider mites are not insects but are just as damaging and numerous as are some insects. Sucking insects have an incomplete life cycle. After hatching from the egg, they may begin to feed and move about on the plant.

They are usually attracted to the most succulent part of the plant. Aphids usually are found in the terminal or on flowers. Stink bugs and squash bugs readily feed on the tender fruit. These insects damage the plant by reducing the vigor or by injecting a toxin or disease-causing organisms into the plant. Heavy feeding may cause flowers to abort or the leaves to turn yellow and fall off. Feeding on the fruit may cause catfacing injury, hard spots or twisted and misshapen fruit.

Control is easiest to obtain soon after the insects hatch from eggs. This is when the insects are the smallest and most vulnerable to the pesticide. Look for egg clusters, so that timing of the insecticide can be more accurate. Most true bugs have large eggs that can be seen without the aid of a magnifying glass. They are often on the undersurface of leaves and laid in tight groups and glued together, or in the case of squash bugs, they may be laid singly but in a loose fitting group and not glued together.

Resistant Plant Varieties

Use available plant species or varieties which are resistant to, or at least tolerant of, insect activity. Insect resistance in plants frequently is interpreted as meaning “immune to insect damage.” Actually, it is a term for distinguishing plant varieties which exhibit less insect damage when compared to other varieties under similar growing and pest population conditions. Some varieties may be less “tasty” to insect pests, or may possess certain physical or chemical properties which discourage insect feeding or egg-laying, or may be able to support large insect populations without suffering appreciable damage.

Before buying seeds or plants, check with your local county Extension agent for information on resistant varieties which will grow well in your area. Some varieties may be resistant to insect attack, but may be subject to certain restrictions such as soil pH, drainage or temperature. Your experience with different varieties will indicate the ones best suited for your operation.

Organic Controls

Growers have been using soap to control insects since the early 1800’s. Researchers have not yet determined exactly how soaps work. Some soap simply wash off the outer waxy coating of the insect cuticle, destroying its watertight nature and causing the insect to dry up and die. Other soaps have additional insecticidal properties which may affect the nervous system. These soaps appear to have toxic activity only against plant eating insects, and thus may spare beneficial insects such as lady beetles, honeybees, lacewings and predatory mites. Although a number of soaps tested have insecticidal properties, only Safer’s Insecticidal Soap is currently registered for use on edible crops. It controls such pests as spider mites, aphids, mealybugs, whiteflies, harlequin bugs, stink bugs and thrips.

Organic growers have been using a spray mixture containing onions, garlic and pepper mixed together to control insects for many years. Research indicates that of combination of these materials have been erratic and in many cases ineffective for insect control. Sprays of food-derived substances do not appear to be good choices as a pesticide. Some success may be achieved with them, but it is likely to be sporadic. Spraying several times a week might help to bring infestations under control. Control with one application should not be expected.

Cultural Controls

Many cultural practices can be used to reduce the potential for, or actual damage of plants caused by insects:

• Plowing and cultivating exposes soil insects to adverse weather conditions, birds and other predators. In addition, deep plowing will bury some insects and prevent their emergence.
• Crop rotation can be effective against insects that develop on a narrow range of food plants and also against insects with short migration ranges. Movement of crops to different sites will isolate such pests from their food source. If an alternate site is not available, then change the sequence of plants grown in the field. Do not plant members of the same plant family in the same location in consecutive seasons. For example, do not follow melons with cucumbers or squash.
• Proper use of fertilizers and water will induce healthy plant growth and increase the capability of plants to tolerate insect damage. However, excessive amounts of organic matter or manure can encourage millipedes, pillbugs, white grubs and certain other pests.
• Changes in planting or harvesting time often will reduce plant damage or keep insect pests separated for susceptible stages of the host plant. Delayed planting, until the soil is warm enough for corn and bean seeds to germinate quickly, reduces seed maggot damage. Hot caps or row covers placed over plants used during the early season not only will preserve heat, but also will protect plants from damaging wind, hail and insects. In some situations, a healthy transplant will overcome insect damage more easily than a small plant developing from seed in the field.
• Removing soil residues and disposing of weeds and other volunteer plants eliminates food and shelter for many insect pests such as cutworms, webworms, aphids, white grubs, millipedes and spider mites. When plants stop producing, till them into the soil or take them to the compost pile.
• Companion planting (an orderly mixing of crop plants) is a cultural practice aimed at diversifying insect populations. Numerous claims have been made about the ability of certain plants to protect certain other plants from insect damage. However, no data from scientific studies are available to prove the value of companion plantings.

Mechanical Control Methods

Preventive devices often are easy to use, although their effectiveness varies. Here are examples of such devices:

• Cheesecloth or spun bound polyester row covers for plant beds, hot beds and cold frames to prevent insect egg laying.
• Mesh covers for tomatoes and other plants to keep out large insects and birds.
• Aluminum foil mulch to repel aphids.
• Black light traps are effective tools for monitoring insect species in a given area, but usually provide little protection for the crop.
• Light traps attract both harmful and beneficial insects that ordinarily would not be found in the area.
• Attracted insects may not be caught in the traps, but may remain in the area, and the harmful ones may cause damage later.
• Also, some species such as wingless insects and those insects only active in the daytime are not caught in the traps.

Biological Control Methods

Generally, biological control can be defined as the direct or indirect use of parasites, predators or pathogens (bacteria, viruses, fungi protozoans) to hold pest insect populations at low levels to avoid economic losses. Biological control methods fall into three categories:

• Introduction of natural enemies which are not native to the area (these enemies must then establish and perpetuate themselves).
• Enlarging existing populations of natural enemies by collecting, rearing and then releasing them back into the environment.
• Conservation of beneficial organisms by such means as the judicious use of pesticides and the maintenance of alternate host insects, so parasites and predators can continue to develop.
• Many beneficial organisms occur naturally around crops, but often they are not numerous enough to control a pest before it inflicts severe damage. In fact, parasites and predators appear to be most effective when a pest population has stabilized or is relatively low. Their influence on an increasing pest population usually is minimal since any increase in parasite and predator numbers depends on an even greater increase in pest numbers. Pathogens, however, seem to be most effective when pest populations are large. Consequently, the nature of the host insect-natural enemy relationship makes it impossible to have an insect-free environment and at the same time maintain sizable populations of beneficial insects.

The following is a list of some of the more popular biocontrol agents:

• Bacillus thuringiensis: (Dipel, Thuricide, Biological Worm Killer). This bacterial insecticide provides effective control of the larvae of several moths and butterflies. The bacterial spores are harmless to warm blooded animals and beneficial insects.
• Bacillus popilliae: (Milky Spore, Doom, Japedimic). This bacterial insecticide controls grubs of Japanese beetles in the eastern U.S., and some testing has been done for control of white grubs (Phyllophaga spp. and Cotinis spp.) in Texas. It has not been effective against the principal white grub species in Texas.
• Nosema locustae: A spore (Protozoan) used to control grasshoppers. The material is sprayed on the plants which grasshoppers ingest. The spores germinate inside the grasshopper, causing death. Control is extremely slow and growers may not be satisfied with results. Baits have proven more effective.
• Trichogramma wasp: Adult wasps are available from several sources. The tiny wasps attack the eggs of more than 200 pest species, including cutworms, armyworms, fruit worms and many moth and butterfly eggs deposited in orchards and field crops. Wasps should be released when the moths are first seen, but a sequence of releases throughout the season is preferable to a single, large release. Results will depend on the timing of the releases, selection of Trichogramma species and placement of wasps near host egg masses.
• Green lacewings (Chrysopa): The larvae, known as aphid lions, prey on many vegetable pests including aphids, spider mites, leafhoppers, thrips, moth eggs and small larvae. Adult lacewings feed on honeydew, nectar and pollen. Introduced lacewings must have a readily available supply of food, or they will leave. Eggs are sometimes available.
• Praying Mantis: Egg cases, containing about 200 individual eggs, are available from a number of sources. The mantis is a voracious predator. In addition, it is cannibalistic immediately after hatching, so few nymphs survive the first week of life. However, the mantis is a poor searcher for food and usually waits for prey to come to it. This greatly influences the kinds of insects it captures and kills. Food preferences include grasshoppers, crickets, bees, wasps and flies.
• Lady beetles: Adult beetles are available from several sources. Aphids are the preferred hosts, but lady beetles will eat mealy bugs, spider mites and certain other soft bodied pests and eggs. They do not, however, kill grubs, caterpillars and other beetles. Unless an ample supply of live aphids or other hosts are available at the release point, lady beetles will disperse and leave the area. In some cases, most of the beetles will leave the area regardless of the availability of food. Lady beetles can be encouraged to remain on a plant by using small meshed screen cages (remove cages before they devour all of their food supply). In hot dry weather these beetles enter a nonactive (aestivation) state in which feeding and reproduction cease. Little control from lady beetles can be expected during this time.

Natural Insecticides

Despite all efforts, noninsecticidal methods at times will fail to prevent excessive insect damage. At such times, the use of insecticides may be the only alternative left. Insecticides chosen should have only low toxicity for humans and other warm blooded animals. They should be used only when needed and according to label directions. A better understanding of insecticides will enable you to use these materials more effectively and to realize that they can be an aid without harming you or the environment.

• Pyrethrum: Botanical Insecticide. This slightly toxic insecticide is derived from the flowers of a species of Chrysanthemum imported mainly from Kenya and Ecuador. The material causes rapid paralysis of most insects, but the insects usually recover unless the pyrethrum is used in combination with a synergist or other poison. Pyrethrum, mixed with synergists such as piperonyl butoxide or piperonyl cyclonene to increase toxicity and produce longer residual action, is used extensively in crop sprays and dusts. This chemical is registered for use on most vegetables at any time during the growing season.
• Nicotine: Botanical Insecticide. Pure nicotine is a tobacco extract highly toxic to warm blooded animals. The insecticide usually is marketed as a 40% liquid concentrate of nicotine sulfate, which is diluted in water and applied as a spray. Dusts can irritate the skin. Nicotine is used primarily for piercing-sucking insects such as aphids, whiteflies, leafhoppers and thrips. Nicotine is more effective when applied during warm weather. It degrades quickly, so it can be used on many food plants nearing harvest. It is registered for use on a wide range of vegetable and fruit crops.
• Sabadilla Botanical Insecticide: Sabadilla is obtained from the seeds of a lily-like plant and acts as both a contact and stomach poison for insects. It is not particularly toxic to mammals, but does cause irritation of the eyes and respiratory tract. A mask should be worn when working with this insecticide. This material deteriorates rapidly upon exposure to light and can be used safely on food crops shortly before harvest. Sabadilla generally is used as a 5 to 20% dust or as a spray.
• Rotenone: Botanical Insecticide. Rotenone is extracted from the roots of Derris plants in Asia and cube plants in South America. This general insecticide is harmless to plants, highly toxic to fish and many insects, moderately toxic to mammals, and it leaves no harmful residues on vegetable crops. It acts as both a contact and stomach poison to insects. It is slow acting, and in the presence of sun and air, its effectiveness is lost within a week after application. Wear a mask during application because rotenone can irritate the respiratory tract. Rotenone dusts and sprays have been used for years to control aphids, certain beetles and caterpillars on plants.