David Trinklein, Horticulture State Specialist, Division of Plant Sciences, University of Missouri Extension
Jaime Piñero, State Integrated Pest Management Specialist, Lincoln University Cooperative Extension
James Quinn, Regional Horticulture Specialist, University of Missouri Extension
Insects are the most abundant animals on earth. They exert important effects, both positive and negative, on our lives in ways we might not even think about. Although the vast majority of insects are either beneficial or harmless, we often are most familiar with those insects that cause problems. For example, the mosquito, because of diseases it transmits, has been responsible for more deaths than all of the wars in history.
On the whole, insects are enormously beneficial. Insects pollinate plants and provide food for birds, fish and other animals. Many beneficial insects prey on other insects that are pests. By studying insects, we gain a better understanding of their role in the web of life and as pollinators, indicators of environmental quality, predators of harmful species, and potential threats to crops, homes and health. Through applying what we learn from our study of insects, we help to preserve beneficial species by understanding their behavior patterns and conserving or enhancing their habitats. Our ultimate goal as Master Pollinator Stewards is to reduce the damage caused by insects in a sustainable and eco-friendly way.
Insects are divided into categories using a standard classification system. Each insect is identified by a unique genus and specific epithet combination, which constitutes its species name. This system is known as binomial nomenclature, which means "naming with two names." Common names differ from state to state and country to country. The binomial nomenclature system ensures that the scientific name of an organism is the same throughout the world in whatever language is prevalent. Binomial classification is just a filing system for information about organisms, organizing them within related groups based on shared common attributes such as structure, function and life history. To illustrate this system of classification, the honey bee can be described as follows:
Specific epithet: mellifera
Common name: honey bee
The phylum Arthropoda — the highest level in this classification — includes all organisms with an external skeleton, jointed legs and a ventral nerve cord. The arthropods are then subdivided into smaller groups, each with its own unique features. This additional grouping based upon like features is known as a class, the largest of which is Insecta, which includes all insects.
The class Insecta refers to organisms with three pairs of legs; external mouthparts; three body regions of head, thorax and abdomen; and one pair of antennae (Figure 1). Several other types of arthropods — spider, ticks and mites — are commonly confused with insects. These organisms are in the class Arachnida, primary characterized by four pairs of legs and only two body regions.
Insects, like all other living organisms, are grouped into orders, often by characteristics such as physical appearance and behavior. Each insect order consists of any number of families, within which are genera (the plural of genus) and species.
All insects have three main body segments: head, thorax and abdomen. The head contains the mouth and associated parts for food manipulation; the main sensory organs, including antennae and the compound eye; and the brain. The thorax contains the body parts used for locomotion: legs and wings. The abdomen contains the internal organs. On the abdomen of many females is an ovipositor for placement of eggs.
Three main sections of an insect
The head is the hardened region at the front of an insect's body. It includes the eyes, antennae and mouthparts. Most insects have two types of eyes: simple and compound. Compound eyes are the large eyes found on most adult insects. These eyes contain a few to several thousand eye units. Simple eyes are small eyes with a single structure used to detect light. They are located on top of the head of many adult insects.
Insects have one pair of antennae, which are jointed feelers that grow from the insect's head. Antennae can be short or long, and come in many forms (Figure 2). Knowledge of antennae is useful in insect identification. Antennae primarily function as sensors to detect the odors in the surrounding environment.
Insect mouthparts and implications for control. Insect mouthparts are of two main types: chewing and piercing-sucking (Figure 3). Some insects have modifications of these two basic types. Mouthparts determine how an insect feeds and therefore play a role in the type of insect control that is most effective. Many insects feed on leaves, buds, stems, roots, fruits and seeds, as well as on plant tissue at various stages of decay. They feed on plants either externally on plant parts or internally on plant fluids. Insects that restrict their feeding to one type of plant are referred to as monophagous. Insects that are general feeders with a diversity of plants in their diet are said to be polyphagous. Insects that are moderately discriminating in their tastes are referred to as oligophagous. An example of an oligophagous insect is the Colorado potato beetle, which feeds only on plants in the genus Solanum.
Insects exhibit great variation in their mouthparts, which have evolved to fit the various diets on which insects feed. The most basic mouthparts, from which all other types have evolved, are for chewing. Predators such as the lady beetle have chewing mouthparts whose mandibles, or jaws, have evolved into long, pointed appendages that can also be used to grasp prey.
Chewing mouthparts allow an insect to bite or rasp off and swallow solid food. Signs of feeding damage caused by chewing insects include holes in plant tissue, missing leaves, "windowpane" leaves showing bared veins, and scraped areas. Insects with chewing mouthparts feed externally on the plant parts and have strong mandibles. The Japanese beetle, for example, is able to skeletonize the leaves of its preferred host plants. Caterpillars use their chewing mouthparts to consume many times their own weight in plant tissue during the course of their development. Much fibrous tissue passes through the caterpillar gut undigested and forms a major part of the large fecal pellets caterpillars leave behind. These pellets are a characteristic sign of caterpillar damage and serve to distinguish these pests from those with other kinds of mouthparts.
Many insects feed on liquid food for which chewing mouthparts are not effective. Many of these insects have a beak, called a proboscis, that is modified to suck up liquids. The piercing-sucking mouthparts of such insects have evolved into fine stylets that can pierce plant and animal tissue to extract fluid nutrients.
Insects with piercing-sucking mouthparts feed internally on plant sap by piercing plant tissue and extracting plant fluids. Their delicate stylets can penetrate leaves, fruit, stems and even tree bark, and are flexible enough to pass between fibrous plant elements and probe to find phloem or vascular bundles. Because these insects remove plant fluids, signs of feeding damage include spotting, curling, wilting and ultimately tissue death. Damage results from the removal of plant fluids, the toxicity of insect saliva to the plant, or diseases transmitted by the insect mouthparts. Insects with these mouthparts ingest primarily plant fluids, and their feces are watery and sticky (referred to as honeydew in some insects, such as aphids and scales).
Butterflies and moths have a long, slightly coiled proboscis that serves to siphon plant nectar. House flies have a proboscis that has modified into a kind of sponge. House flies regurgitate salivary enzymes onto their food and then lap up the predigested, liquid food with their sponging mouthpart. Thrips have a short, stout proboscis and three stylets modified for rasping-sucking. It is thought that thrips scrape the surface of a plant to make the outer protective layer, called the epidermal layer, easier to penetrate. The stylets of mosquitoes are slender and needlelike to penetrate the skin of animals and suck blood. The itching sensation associated with mosquito bites is due to an anticoagulant that is injected into the skin to keep the blood flowing while mosquitoes feed.
Identification of mouthparts is key to correct diagnosis of insect damage, which in turn is crucial to selecting effective control tactics. An insect with chewing mouthparts will ingest a pesticide that is present on the surface — or as a systemic, in the tissue — of the plant. However, an insect with piercing-sucking mouthparts feeds internally within plant tissue and will not ingest pesticide that is on the surface of the plant. To kill insects with piercing-sucking mouthparts, an insecticide must either have contact toxicity, meaning it kills by just contacting the insect, or be systemic in nature. (See the Integrated pest management section for more information.)
Insect antennae come in different forms, knowledge of which can aid in insect identification. Insects sense their environment through their antennae. They mainly sense smell, but also sound and vibration, finding food as well as mates by perception of chemicals.
Insect mouthparts are highly variable, depending on how an insect feeds. Chewing mouthparts are the most general type. Piercing-sucking mouthparts have become modified for piercing the skin of animals or plants and sucking liquid food. Other common modifications enable particular insects to collect liquid food with long, coiled tubes or spongelike structures.
The thorax is the second section of an insect's body. It contains the muscles that control the insect's movement. Wings and legs are attached to the thorax. Adult insects have three pairs of legs, but not all adult insects have wings. The legs typically have six segments even though these can be hard to distinguish in many species. Legs come in many forms depending on their function, such as running, jumping, grasping or swimming (Figure 4). Insect wings also vary greatly in shape, size, color, thickness and vein pattern. The wing shape and pattern of veins are widely used in insect identification (Figure 5).
Figure 4 (left)
The thorax of an insect is specialized for locomotion and has three pairs of jointed legs. Each leg typically has six segments. Many insects have a pair of claws at the end of their legs, These claws enable insects to climb and hang upside down from surfaces.
Insect wings are almost always found only on mature insects. Most insects have two pairs of wings. In several groups of insects, such as beetles, the front wings are more hardened and serve as protection for the hind wings. Some insects, such as fleas and lice, have no wings.
The abdomen is the third and final section of an insect's body. It may be visible or hidden under the wings. This body section contains the insect's internal organs, including the stomach and intestines, where food is digested and absorbed. The sexual organs are also in the abdomen. In addition, the abdomen has glands that secrete various fluids to mark the insect's trail, for example, or to drive enemies away, attract mates, or signal to others the location of food supplies. The abdomen may also have a needlelike projection for piercing or stinging.
Most female insects have an added appendage called an ovipositor at the end of their abdomen. The primary function of the ovipositor is for placing eggs in a protected location during egg-laying. Most insects with an ovipositor make a hole with this appendage and then place the egg inside the hole. In some insects, such as many bees and wasps, the ovipositor has been modified into a stinger for protection. Male bees and wasps cannot sting as they do not have an ovipositor. Parasitic wasps that lay their eggs on or in their hosts use the ovipositor to sting the prey, leading to paralysis, before placing the egg.
Insect development occurs through changes not only in size but also in form. The series of changes that insects undergo as they grow and develop to the adult stage is known as metamorphosis (Figure 6). Almost all insects begin as eggs, although in a few cases the eggs hatch within the mother and they are born alive. The young insect develops through a series of distinct stages. As an insect develops, its exoskeleton, or cuticle, does not grow and must be shed and replaced. Insects are especially vulnerable during the period between the shedding of an old cuticle and the hardening of the new one.
Most insects undergo complete metamorphosis, which has four stages: egg, larva, pupa and adult (Figure 7). Adult females lay eggs that hatch into larvae (caterpillars in the case of moths and butterflies; maggots in the case of flies). The larva is the primary feeding stage, and it usually goes through four or five stages of larval development, called instars. The larva then becomes a pupa, which is the sedentary stage in which major transformation takes place, leading to an adult insect. The primary function of the adult stage is reproduction. Insects that exhibit complete metamorphosis include beetles, butterflies, moths, flies, bees and wasps. The life stage of an insect is an important consideration when deciding which control methods to use. For example, if insect damage is observed on the leaves of a tree, but the insect has already pupated, applying a pesticide is ineffective because the damage is done and the insect has moved on.
Incomplete metamorphosis has three stages: egg, nymph and adult (Figure 8). The immature stages are similar to the adult in appearance, feeding habits and habitat. Development is a gradual increase in size of the wing pads until the final molt, after which the wings and reproductive organs are fully formed. Insects that undergo incomplete metamorphosis include aphids, scales, true bugs, grasshoppers, katydids, crickets, cockroaches, mayflies, dragonflies and stoneflies. Having similar feeding habits in their nymph and adult stages, these insects have the potential for causing feeding damage to plants at both stages. For example, the squash bug feeds on plants in the family Cucurbitaceae, so if squash bugs are causing severe damage, control could be considered at any developmental stage.
Insects pass through a series of stages as they develop, and these changes from one life stage to the next are called metamorphosis.
Figure 7 (left)
Most insects undergo complete metamorphosis with four primary stages: egg, larva, pupa and adult. Caterpillars pass through several stages, shedding their skin between them, as they feed and develop. Caterpillars of some insects, such as moths, spin silken cocoons to protect the pupa as it develops into an adult.
Immature insects that undergo incomplete metamorphosis are called nymphs, which resemble the adults. Growth from one nymph stage to the next occurs by molting. A nymph's wings get larger with each successive shedding of skin, and by maturity as an adult, the wings are fully developed.
The vast majority of insects visiting gardens are either neutral or beneficial. Many people spend a great deal of time and effort to destroy insects only to find out later that the insects they destroyed were beneficial. Hence, correct insect identification is important.
Insects as pollinators
Insects provide a huge benefit in the pollination of plants. Many plants require an insect to transfer the pollen needed to fertilize the flower and set fruit. Apple crops are not self-pollinated and rely almost exclusively on insects for pollination. Orchards now commonly have honey bee hives among the trees to promote pollination. The pollination activity of bees is suggested to annually provide benefits worth tens of millions of dollars to regional agriculture. Many native plants depend on insect pollination for their survival. Insects that pollinate plants include honey bees, flies, butterflies, moths, bumble bees and other native bees.
Insects and the natural recycling of nutrients
Insects play a key role as scavengers in recycling nutrients by feeding on decaying plant or animal material, thus hastening decomposition. Insects help break down organic matter they scavenge, and add organic matter to the soil as they live and breed. Dung beetles, for example, are usually found beneath cow dung, horse manure or carrion. Insects such as dung beetles help the manure of cattle and other animals break down more quickly. Fly maggots, which are the larvae of flies, play a vital role in the decomposition of animal material, especially carcasses. Termites, although considered pests when they destroy wooden structures, are extremely valuable scavengers that enhance the decomposition of wood materials in the environment.
Insects in the food webs of wildlife
Insects play a valuable role in the food chain of life as food for other species, including fish, birds and small mammals such as bats. Many game fish, such as trout, depend on insects as a major component of their diet, as do many songbirds. Mayflies, which are primarily aquatic insects, are reliable environmental indicators of water quality. If a water source has robust mayfly populations, the water quality can be assumed to be good. Declining populations of mayflies could indicate the water quality is declining.
Insects as biological control of pests
Biological control is any activity of one species that reduces the adverse effects of another species. Virtually every pest, whether weed or insect, has natural enemies that reduce its population under certain circumstances. The most important factor keeping the populations of many pest insects in check is the activity of other insects. There are two categories of insects used for biological control: phytophagous insects that feed on plants and act as biocontrol agents by destroying or limiting plant growth, and entomophagous insects that feed on other insects and keep pest populations in check.
For example, the Klamathweed beetle is one of at least 268 species of insects — mostly flies, beetles and moths — that have been introduced in attempts to control various noxious weeds. Klamathweed beetles have virtually eliminated Klamathweed (also called common St. Johnswort) from millions of acres of rangeland in the western United States, Australia, Canada, Chile, New Zealand and South Africa. Introduced insects have significantly reduced puncturevine, musk thistle, tansy ragwort and waterlettuce. There is reason to hope that insects can also help control leafy spurge and purple loosestrife.
Insects that feed on plant-feeding insects are the single most important factor keeping these pest populations in check. Insects that feed on other insects are considered either predators or parasites. Predators actively hunt for insect prey. The prey is typically smaller than the predator, but not always. Lady beetles can consume up to several hundred aphids during the two or three weeks that they are growing and many more after becoming adults. Parasites live on or in the bodies of their hosts and get their food from them. Parasitic wasps or flies lay eggs on certain kinds of insects, and their young feed inside the host, killing it and emerging in adult form. Hosts are typically larger than the parasites and are not killed immediately but continue to live in close association with the parasite.
Knowledge of some ecological concepts can greatly enhance your understanding of insects and their management. Simply put, insect ecology is the scientific study of how insects — individually or as a community — interact with the surrounding environment, or ecosystem. Of the vast amount of information covered by this definition, some of the key issues are whether the insect overwinters in Missouri or migrates, and the effects of temperature on the insect's development and reproduction. Also important is knowing if the pest in considered invasive.
Insect overwintering. Some insects overwinter in Missouri in a resting state called diapause. Insects can enter diapause in any stage but most commonly do so in the adult and larval stages. Lady beetles, mourning cloak butterflies and fertilized queen bumble bees are examples of insects that overwinter as adults. Insects that overwinter as larvae include woolly bear (Pyrrharctia isabella) caterpillars, dragonflies and cicada killer wasps. Some insects cannot overwinter in Missouri. Populations of some insects seen in Missouri in the summer actually originate in the south and move north each year.
Insect migration. Insect migration is the seasonal long-distance movement of some species of dragonflies, beetles, butterflies and moths. The distance can vary with species, and in most cases, these migrations involve large numbers of individuals. In some cases, the individuals that migrate in one direction may not return, but instead, the next generation will migrate in the opposite direction. One example of a pest that exhibits long-distance, one-way migration is the tomato fruitworm. Because this pest can feed on many different plants, it has been given many different common names, including cotton bollworm and corn earworm. Most populations enter Missouri as migrating swarms of moths from the southern states.
Effects of temperature. Insects cannot regulate their body temperature; they rely on external sources of heat. Temperature has a direct effect on the growth and development of insects. Each species has a temperature range in which it develops. Within this range, the higher the temperature, the faster the insect develops and grows. But at very low or very high temperatures, insect development stops. The insect may die at such times, or it may resume development when the temperature returns to its normal range. Knowing the temperature ranges of an insect can help you to predict pest development and thus to determine the best time to scout and to use control measures.
Invasive insects. Insects, plants, animals and other life forms can be considered invasive species. An invasive species is one that is not native to a specific location, but has been introduced. Invasive species have a tendency to spread to a degree believed to cause damage to the environment, human economy or human health. Invasive insects are generally difficult to control because the ecology in their new location lacks the ability to control their population or limit their spread. In their native environment, they are often much less of a threat. Pesticides might be more effective against invasive insects because naturally occurring biological controls, such as parasites, predators or fungal pathogens, aren't present to help control the population. Eventually, the ecology may rebalance, which is what seems to have occurred with Japanese beetles (Popillia japonica) when they have been in an area long enough (see the Invasive pests box). In other instances, devastating damage may occur, as with the emerald ash borer (Agrilus planipennis). This pest has decimated ash trees where it is active, and no biological control is yet effective for widespread control. It originated from eastern Russia, northern China and Korea, and arrived in Missouri in 2008 from states to the east. Another well-known invasive insect from Eurasia is the gypsy moth (Lymantria dispar), which is currently established in the northeastern U.S. states and parts of the Upper Midwest. In 2013, an Asian fruit fly called the spotted wing drosophila (Drosophila suzukii) swept through Missouri. It is more difficult than other fruit flies to control because it can insert its eggs through the skin of berries, whereas other fruit flies need a skin break or split. Two other pests of concern are the brown marmorated stink bug (Halyomorpha halys) and Asian longhorned beetle (Anoplophora glabripennis). The former has been found in Missouri, but as of 2016, the latter had not been.
The term pest refers to any organism that adversely affects humans, crops, livestock or anything considered to be of value. By this definition, not only insects but also diseases, weeds, nematodes (small unsegmented worms that live in the soil), arthropods other than insects, and vertebrates can be considered pests. Less than 1 percent of insects are pests, but that small fraction can do expensive damage. Insects cause damage in a variety of ways.
Some insects feed by biting off and chewing the external parts of a plant. This type of injury can weaken the plant and cause it to be less productive or to die. Many pests, such as grasshoppers, feed on many different plants, whereas other insects, such as cabbageworm and Colorado potato beetle, feed on only a few types of plants.
Insects such as aphids and scales feed on plant sap by piercing the epidermal layer of the plant and sucking the sap from the cells. In this process, they damage the plant cells and remove valuable nutrients. Plant damage may include distorted leaves; curled leaves; chlorotic-looking, or yellow, tissue; deformed fruit; and even death of the entire plant. Some insects inject a salivary toxin into the plant that does additional damage. Common insects with piercing-sucking mouthparts include aphids, scales, leafhoppers, squash bugs and plant bugs.
In addition to insects that feed internally by way of a proboscis, many insects actually make their way into a plant to feed internally on plant tissue, which makes control difficult or impossible. These insects enter the host plant either in the egg stage when the female adult lays eggs within the plant tissue, or they hatch and immediately eat their way into the plant tissue. Internal feeders include wood borers, codling moth in apples, leaf miners, seed weevils and gall insects. Perhaps the most widely noticed internal feeders are those that cause galls. These unusual growths on plants can be caused by many different insects, including wasps and flies. Most internal feeding insects emerge from their feeding structure as adults. Control must be aimed at the immature stage before it enters the plant or at the emerging adult to reduce the population in later generations by reducing the number of females available to lay eggs.
Insects as disease vectors
Many insects pick up plant viruses and bacteria as they feed, and then later transmit diseases to healthy plants. Insects that transmit diseases are known as vectors. They may carry a disease from plant to plant on their body, or transmit the disease through plant injection as they feed. The diseases they carry can result in severe damage or death to plants. Insects can spread plant diseases either directly, via active feeding such as in thrips, aphids and cucumber beetles; or indirectly, when their feeding or boring into plants creates openings through which pathogens — bacteria, fungal spores or viruses — enter the plant. Insects that transmit plant diseases include the bark beetles that move fungi that cause Dutch elm disease, and the aster leafhopper that injects the bacteria that cause aster yellows. Another example is cucumber beetles, which can carry a bacterial disease called bacterial wilt. Cucurbit plants infected with these bacteria are destined to die. In addition to spreading disease to plants, insects can spread diseases to humans. For example, mosquitoes can transmit malaria, dengue and other diseases to humans. To control such diseases, one needs to control the insect vector.
Types of insecticides
Keep in mind that the way an insect pest feeds, which is determined by the type of mouthparts it has, will dictate which insecticide option will be most effective at controlling it. For example, Sevin, a well-known garden pesticide with the active ingredient carbaryl, is formulated as both a dust to sprinkle or spread and a liquid to spray. The dust is a stomach poison that must be ingested to be effective and therefore provides good control against insects with chewing mouthparts. It does not, however, provide control of insects with piercing-sucking mouthparts because they do not chew external plant material and therefore would not ingest the dust. The spray is a contact poison with some residual activity. It kills most insects on contact and does not need to be ingested to be effective. As a spray, Sevin kills insects with piercing-sucking mouthparts as well as others. Another common insecticide is Dipel, which is a microbial insecticide with the active ingredient Bacillus thuringiensis, or Bt. It also comes in both dust and liquid formulations, but it is not a contact pesticide and must be ingested to provide control in either form. Some contact insecticides work with a physical action, instead of causing a chemical disruption as does Sevin. Insecticides that contain oil or soaps are examples. They can smother an insect by plugging its air supply. Air enters an insect's body through valve-like openings in the exoskeleton. These tiny openings, called spiracles, are located laterally along the thorax and abdomen of most insects — usually one pair of spiracles per body segment. Soaps might also cause the skin of soft-bodied insects to deteriorate, thus causing the insect to dry out. Yet other insecticides are systemic, meaning they can be absorbed by plants and move around in plant tissues. These are sometimes applied as a drench to the soil and then taken up by the roots. Systemic insecticides applied as a drench need to be ingested and can be highly effective on insects with piercing-sucking mouthparts as the insecticide is moving in the plant sap.
Integrated pest management (IPM) is a science-based decision-making process that identifies and reduces risks from pests and pest management-related strategies. IPM coordinates the use of pest biology, environmental information and available technology to prevent unacceptable levels of pest damage by the most economical means, while minimizing risk to people, property and the environment. IPM provides an effective strategy for managing pests in all arenas, including agricultural, residential and natural areas. Although the concept of IPM was originally developed to combat insect pests, it was subsequently expanded to include all pests.
The pioneers of IPM used the term "integrated" because they recognized that biological control as a stand-alone strategy was rarely economically practical — but when combined with ecologically based tactics, such as cultural control, it could be. IPM assumes a good understanding of the life cycle and behavior of the pest, and the ecological interactions between pests, the ecosystem (for example, crops and livestock in agricultural settings), natural enemies and human decision-making.
The need for IPM was recognized shortly after World War II, when it became apparent that the indiscriminate use of the new synthetic insecticides that were developed by that time, including DDT, would create serious problems. Some of those problems actually became a reality due to the overuse of pesticides:
- Pest resistance
The pesticide no longer kills the pest
- Target pest resurgence
The target pest becomes more abundant than before because pesticides killed its natural enemies
- Secondary pest outbreaks
A secondary pest (that is, a pest that was not targeted) reaches damaging levels after pesticides applied against the target pest kill the secondary pest's natural enemies
- Environmental contamination
Chemicals accumulate to levels that can cause damage to a variety of life forms, possibly even humans
Many people now recognize the importance of IPM because it focuses on long-term prevention of pests or their damage through a combination of techniques. These could include biological control, habitat manipulation, modification of cultural practices, and use of resistant crop varieties, leaving the use of pesticides as a last resort. A combination of controls is often more effective than one method by itself. The goal of IPM is to minimize the abundance of insect pests and their damage, not to kill every insect in the treated area. The mere presence of a few insect pests in a garden does not justify the application of insecticides, as they might kill natural enemies that otherwise would control pest populations.
Identifying and monitoring pest insects
Before deciding whether control of an insect is necessary, the pest needs to be correctly identified and its population monitored. As detailed earlier in this chapter, a variety of characteristics can be used to identify an insect, including the length and form of its antennae, and the shape and vein pattern of its wings. The type of plants affected and the damage caused also provide clues to the pest's identity.
Because insects can reproduce rapidly, prompt detection of infestations is crucial. Insect populations can be monitored through scouting or with traps. Both methods provide early warning of pest presence and hot spots. Scouting involves looking closely at your plants for pests and signs of pest damage. When you notice a problem, factors to consider include which plants are affected, how quickly they were affected, their age, and what area of the garden or field they are located in. You also can use traps to detect the present of a pest and monitor its population. Many types of insect traps are available, but yellow sticky cards can be used to monitor many insect pests. Sticky cards provide a relative measure of insect density and indications of whether the pest density is changing or remaining relatively constant over the long term.
IPM control measures
Cultural controls are preventive and comprise the first line of defense when controlling insects in the garden. The range of cultural controls is wide.
Resistant varieties. Resistant crop varieties have innate resistance or tolerance to attack by certain pests. The degree of resistance can vary from slight to almost complete. Unfortunately, fewer plants are resistant to insects than to diseases. When present, insect resistance often is based on plant morphology. The suppression of leaf hoppers and aphids on beans because of fuzzy or hairy leaf surfaces is an example of natural resistance.
Keeping plants healthy. Proper watering and fertilization regimes and good general care will provide plants the extra edge they need to overcome insect feeding. Healthy trees, for example, are not as easily attacked by wood-boring insects.
Sanitation. Sanitation helps to decrease pest pressure by removing or preventing access to sources of infection or sources of food and shelter. Practices include removing infected crop material from the garden, turning over or burning crop residues, and eliminating unharvested plant material. For example, twigs with insects inside them may fall from an infested tree; these insects can be controlled by simply removing the infected material. Pruning tent caterpillar egg cases from tree limbs removes the insect from the area. Removing unharvested squash eliminates an overwintering site for squash bugs from the garden or farm.
Crop rotation. Growing a single crop year after year in the same area of a garden or farm gives insect pest populations time to become established and build up to damaging levels. Rotating the crop to a different location each year can break this cycle by starving pests that cannot adapt to a different host plant. This practice was originally developed to produce higher yields by replenishing soil nutrients and breaking disease and insect pest cycles. In a garden, for example, crops in the potato family, which includes tomatoes and peppers, should be planted in one area; crops in the cucumber family, which includes squash, zucchini, watermelon, cantaloupe, etc., in another area; and crops in the carrot family in yet another area. Each year, change the area in which you plant each of these crop families. Many gardening books contain specific examples of crop rotations.
Trap cropping. Insects have a preference for certain types of foods just as most animals do. Given a choice, insects will likely select their preferred plant. But if they cannot find their favorite plant in the area, they will feed on the plants that are available. Trap cropping is done by growing plants that are attractive to the insect pest along the edges of a garden or cucurbit field. These attractive plants pull the pest away from the cash crop. Insects congregated on trap crop plants can be more easily killed with insecticides or by other means. For example, Blue Hubbard squash can be used as trap crop to pull squash bugs and cucumber beetles away from the cash crop. Two-week-old Blue Hubbard squash seedlings can be transplanted to the field at the same time you sow the seeds of your cucurbit cash crop. If you grow your cash crop from transplants, then you will need to transplant the Blue Hubbard seedlings at least two weeks before you transplant the cash crop.
Mechanical and physical controls
Mechanical and physical controls either directly remove or kill a pest, make the environment unsuitable for it, or physically block it from reaching its host. Handpicking insects and spraying them off plants with a water hose are two effective mechanical controls. Soil cultivation is a physical control method that kills some pests by burying them and others by exposing them to the soil surface where they face drying conditions and greater potential to become food for birds or other predators. Soil cultivation also buries potential food sources for insect pests and kills weed seedlings. Other methods include trapping insects and subjecting insects to freezing temperatures, for example, by placing food items infested with grain insects in a freezer.
The populations of pests' natural enemies can be enhanced by either attracting them to the garden using insectary plants, or buying and releasing predators or parasitoids. The use of insectary plants is much cheaper and sustainable than the second approach. Establishing flowering plants in and around a garden to provide pollen and nectar resources for natural enemies can enhance biological control of insect pests. Beyond providing effective natural pest control, the friendly insects also assist in pollination. For example, the larvae of hover flies are predators of small insects such as aphids, and the adults assist with pollination. Female hover flies need to consume pollen to develop their eggs; therefore, a garden that does not provide pollen due to a lack of flowering plants may have fewer hover flies. Lady beetles can feed on plant nectar if prey is scarce and therefore also can be attracted to gardens with insectary plants. Garden plants that are highly attractive to beneficial insects include sweet alyssum, buckwheat, dwarf sunflower, basil, anise and dill. Perhaps the most important single step toward improving the survival of beneficial insects in a garden is to reduce the use of insecticides.
The use of chemicals to control insects is necessary in many situations. For example, termites can infest structures, fleas want to live on our pets, mosquitoes may transmit deadly diseases, grain pests (such as weevils, beetles and moths) can cause vast spoilage, and a range of agricultural plant pests can cause crop failure or yield losses. In antiquity (around 1000 BC), people used naturally occurring compounds in the struggle against pest insects. Inorganic sulfur, via fumigation, was one of these natural chemicals that helped get rid of lice. The use of arsenics, cryolite and borax followed. In the mid-19th century, Colorado potato beetle was controlled with Paris green, which is the common name for copper acetoarsenite. This chemical's use was expanded to control mosquitoes to suppress the spread of malaria. The modern era of synthetic insecticides began with the synthesis of DDT in 1874 and the understanding of its lethality to insects in 1939. While an excellent insecticide, DDT was also dangerous for many other groups of organisms, such as mammals, birds and reptiles. Other chemicals were discovered to be effective insecticides, and by the mid-20th century, three chemical classes were in wide use: organochlorines, carbamates and organophosphates.
Environmental problems developed for several reasons. First, insects developed resistance to several of these insecticides, and a common initial response was to apply increasingly greater amounts. Second, several of the insecticides were extremely stable (that is, they didn't break down) and would accumulate in the environment, including in animal tissues. Fish, reptiles or birds would eat treated insects and then be eaten by predators themselves. These predators, including bald eagles, would further concentrate the active ingredient and suffer devastating consequences, such as thin egg shells. This problem was recognized and brought to light in 1962 by Rachel Carson's impactful book Silent Spring, the title of which refers to the silence resulting from the presence of fewer songbirds. This book essentially ushered in the modern environmental movement.
Newer chemical classes that would be less harmful to the environment were sought. The first was the synthetic pyrethroid group, which was based on a naturally occurring compound in chrysanthemums called pyrethrin. By the 1970s, these were in widespread use and several other chemical classes were either developed or being discovered. By 2015, more than 25 chemical classes had been defined as registered insecticides. These newer chemistries are often environmentally safer. Several have been modeled after naturally occurring compounds. The best example is the neonicotinoid group, whose chemistry is based on nicotine, the same compound as in tobacco, which was understood as being harmful to many insects.
Rotating between different chemical classes was determined to be an excellent way to prevent insects from developing resistance to an insecticide. Using different insecticides from the same chemical class is less effective because the insects have often developed genetic resistance to a chemical's mode of action — the way it kills. All chemicals within a class have the same mode of action. Gardeners and others who commonly use insecticides (for example, pet owners) are encouraged to rotate between chemical classes, especially if using the insecticide repeatedly to control the same pest.
Insecticides for home garden use as of 2015
|Chemical class||Active ingredients commonly available to gardeners|
|Pyrethroids||Bifenthrin, cyhalothrin, permethrin, pyrethrins|
- Carroll, Steven B., and Steven D. Salt. 2004. Ecology for Gardeners. Portland, Ore.: Timber Press.
- Cranshaw, Whitney. 2004. Garden Insects of North America: The Ultimate Guide to Backyard Bugs. Princeton, N.J.: Princeton University Press.
- Cranshaw, Whitney. 1998. Pests of the West. Golden, Colo.: Fulcrum Publishing.
- Dreistadt, Steve H. 2004. Pests of Landscape Trees and Shrubs. Davis: Statewide Integrated Pest Management Project, University of California, Division of Agriculture and Natural Resources.
- Salsbury, Glenn A., and Stephan C. White. 2000. Insects of Kansas. Manhattan: Kansas Department of Agriculture.