Insects and their Injury to Plants

The majority of small invertebrate animals belong to Phylum Arthropoda, which consists of organisms with external skeletons and jointed legs. Insects, Class Hexapoda (meaning "six-footed"), has the largest number of species within this phylum. In addition to the characteristics already mentioned, insects also are distinguished by having one pair of antennae, and most have wings and three body regions as adults. Centipedes, millipedes, mites, sowbugs, and spiders are non-insect arthropods: all have external skeletons but adults have more than six legs. Insects are by far the most abundant animals in the world, both in numbers of species and individuals. Fossil remains show that insects inhabited the world long before other animals now living appeared.

Most insects obtain their food from plants. Bees live on nectar and pollen from flowers. The larvae of many beetles, moths, butterflies, and flies live in or on plants. Many bugs suck the sap or cell contents from plants as a source of food. Many other insects, such as the parasitic wasps, feed as parasites, usually in the bodies of host insects. Others, such as the praying mantis, some thrips, bugs, flies, beetles, ants, and wasps, prey on other insects. This publication focuses on the insects more commonly considered to be plant pests.

How Insects Develop

Most insects have four distinct stages of growth: (1) egg, (2) larva, commonly called a caterpillar, grub or maggot, (3) pupa, and (4) adult. This type of development is known as complete metamorphosis. The sucking insects and chewing insects, such as grasshoppers and crickets, have three life stages: (1) egg, (2) nymph, and (3) adult. This type of development is known as incomplete metamorphosis. There are some groups that are considered to have an incomplete metamorphosis, even though an intermediate stage undergoes a great enough transformation to be called a pupa or pseudopupa. Examples include whiteflies and thrips.

The external skeleton of insects is relatively rigid, and cannot be stretched very much. As insects grow, they split the old skeleton, crawl out of it, and harden a new skeleton larger than the old one. This process, called molting, is controlled by hormones. The time required for development of insects from egg to adult varies from a few days for flies to 17 years for a cicada.

How Insects Injure Plants

Insects may feed on leaves, stems, roots, and flowers of plants. The chewing insects actually consume the infested parts. Types of leaf feeding by chewing insects include pit feeding on leaves by leaf beetles, flea beetles, and young caterpillars. Irregular notches along the edges of leaves are typically caused by various weevils, larger caterpillars, grasshoppers and katydids. Perfect semicircular cut portions of leaves indicate the presence of leaf cutter bees. Feeding entirely within leaves is called mining. Leaf miners can be found among beetles, flies, sawflies, and moths.

Stem chewing typically is done by borers, which feed internally as larvae. Important borers include longhorned beetles (roundheaded borers), metallic wood boring beetles (flatheaded borers), engraver beetles, clearwing moths, American plum borer (a moth), and a few less commonly encountered moths.

Root chewing insects include species that subsist entirely on plant tissue for development, such as root weevils and root maggots, and those that feed on a combination of soil organic matter and roots (most white grubs).

Sucking insects remove cell contents (e.g., thrips) or sap (e.g., aphids, leafhoppers, scales, etc...) and thereby weaken the plants. Some of these sucking insects inject salivary fluids into plants. This secretion may (1) kill plants, as evidenced by armored scale feeding, (2) cause galls to form, as in the case of gall aphids, or (3) kill portions of a leaf, as seen in leafhopper "burn." Sucking insects tends to ingest more water and sugars than amino acids. Sucking insects balance their nutrition by excreting the excess sugar-water as honeydew, which is objectionably sticky and supports the growth of sooty mold. Honeydew can also lure nuisance stinging wasps, and also attracts ants, which protect the sucking aphids from predators and parasites. One key to managing populations of honeydew producing insects is to control the ants that protect them.

Insertion of sucking mouthparts into plants increases potential for the transmission of plant disease organisms. Sucking insects, such as leafhoppers moving among plants can transmit mycoplasma-like organisms that cause Peach X-disease and aster yellows. Aphids and leafhoppers transmit viruses to plants. Preventing the transmission of viruses can be the chief reason to control certain insects. An example is the importance of controlling western flower thrips when growing impatiens. Their transmission of tomato spotted wilt virus can devastate a crop being started in a greenhouse.

Some insects cause damage by cutting the plants for egg-laying. Conspicuous among these are cicadas, which during years of peak emergence can cause considerable damage to small branches of trees. Tree crickets also lay eggs in stems, and while doing so, may transmit disease agents.

How to Live with Insects

Many plants offer a suitable breeding place for many kinds of insects. Apples, cabbage, corn, elm, grape, grass, maple, oak, peach, pear, pines, poplar, potatoes, and roses have many serious pests. However, insects seldom destroy these plants. The threat of damage by insects need not prevent an attempt to grow any of these plants. A knowledge of what to expect and how to control the pests when they are destructive enables anyone to tolerate low numbers of insects.

Some insects are perennial pests. Plum curculio, flea beetles, striped cucumber beetles, Colorado potato beetle, plant bugs, white grubs and many others regularly occur in some locations whenever appropriate plant hosts are available. Other insects seem to be influenced easily by the weather. Temperature and rainfall seem to affect the numbers of aphids, mites, gall midges, and some scale insects. The third type is represented by the gypsy moth, canker worms, and tent caterpillars. The abundance of these pests changes over several years. The "boom and bust" phases are apparently caused by interactions with predators, parasites, and diseases.

The key to successfully growing plants and living with insects is to understand management principles. The need to protect plants is based on whether damaging populations of certain pests are present or expected to be present. The decision to treat plants, for example by spraying with insecticides, should be determined by an economic injury level or aesthetic injury level. These levels define the break-even cost for the extent of damage caused by insects or mites compared to the cost of the control measure. For example, if it costs $15 in labor and chemical costs to control imported cabbage worm by spraying, then the effort would only be justified if the value of cabbage saved by this action exceeds $15.

Preventive measures may be warranted for perennially damaging pests, especially when we lack a method to determine whether a damaging pest population is present quickly enough to react in a timely manner to prevent economic injury. The best strategy for these pests is to base the need for treatment on the past history of the site, with treatment timing dictated by crop development. For example, plum curculio is active every year for approximately one month following fruit set when nighttime minimum temperatures exceed 65 F. To protect tree fruits, the decision to spray is based on fruit development and predicted moderate night temperatures. Another example is treating lawns to control white grubs. The most effective insecticides work best when applied at the time adults are active, long before a homeowner can determine whether a damaging population will result. A third example of justifiable preventive insecticide application is when specimen trees are being dug for transplanting. These trees are especially prone to attack from opportunistic borers (beetles and lepidoptera) because the stress induced from root loss causes the trees to emit chemical cues that these borers can detect from a considerable distance. Therefore, these trees can be protected with residual insecticides either immediately prior to or following digging.

For pests that are sporadic due to weather or biotic factors, or that take a long time to build to damaging populations, scouting and reactive treatments are most appropriate. Scouting, or monitoring for the presence of pests, is the key to integrated insect and mite management in most crops. Some pests, like spider mites, only become damaging when reaching high populations. Regular scouting can detect these populations long before damage occurs, giving plenty of time to plan needed treatment. Most shade tree pests, for example, are sporadic and, thus, can simply be managed when the numbers increase to unacceptable levels. By observing the presence and abundance of pests and their response to treatment measures, one can gain experience in fine-tuning pest management to an individual's needs.

How Insect Pests are Controlled on Plants

Many pests, particularly bark beetles and borers attacking shrubs and trees, seem to be attracted to injured or weak plants. Keeping the plants in good growing condition by preventing mechanical injury to the bark, judicious use of fertilizer, particularly early in the growing season, and of irrigation during droughts helps reduce the hazard of infestation by these pests. Increased vigor does not reduce infestation by leaf-feeding insects, but the appearance of plants may be improved. Excessive vigor, however, can promote improved survival and greater egg production in certain pests, including mites and aphids. Succulent growth, typical of over-fertilized plants, also becomes highly susceptible to many diseases. Therefore, observations of plant growth should be coupled with soil tests to adjust fertility for optimal plant health.

The use of resistant plant material provides an ideal method to reduce pest populations, and thereby decreases the need for other management techniques. There are many examples of variation in susceptibility of plants to diseases and insects. Varieties of rhododendrons, strawberries, and yew ('PJM', 'Allstar', and 'Greenwave', respectively) resist adult black vine weevil feeding, and, consequently, may have fewer larvae feeding on their roots. The 'NewLeaf' variety potato genetically engineered to express Bacillus thuringiensis proteins toxic to Colorado potato beetle shouldn’t require additional control measures against this pest, at least until beetles evolve resistance to that toxin.

Natural mechanisms regulate the abundance of most insects. When destructive outbreaks occur, it is necessary to decide whether it is preferable to "let nature take its course" and possibly lose some plants, or to apply insecticides as an immediate and often temporary protection. Conifers can be killed with a single defoliation. For non-conifer shade trees, defoliation caused by chewing insects can occur for 2 - 3 consecutive years without killing the tree. If insecticides are to be used successfully, they should be applied before damage is severe. These chemicals will not revive dead twigs or trees or replace holes eaten in leaves. The use of insecticides to control defoliator pests, such as gypsy moths, may actually prolong an outbreak of cyclical pests by preventing biological control organisms from building up quickly and by preventing the pest larvae from becoming overcrowded, stressed, and consequently highly susceptible to diseases.


Numerous insecticides have been developed. Legislation requires that the labels on all insecticides give full directions for use, including all necessary precautions. These labels are important sources of information and must be followed carefully. Since information for use of each pesticide is available on each package, it is unnecessary to repeat those directions in this handbook.

All of the insecticides mentioned in this publication have been studied thoroughly at this Experiment Station or in other research institutions. They have been used in experiments either on the crops and pests mentioned or on closely allied pests. All are reasonably safe to the gardener or farmer, if used as directed on the package.

Insecticides, especially carbamate and long-residual pyrethroid products, tend to kill beneficial mites, predators, and parasites for several weeks following foliar sprays, thus allowing pests tolerant of these chemicals to increase without restraint. Pests that tend to be troublesome following applications of long-residual insecticides include spider mites, aphids, soft scales, armored scales and mealy bugs. This phenomenon, called secondary resurgence, can be avoided by using highly selective insecticides, systemics, or short-residual products. The products in this guide suggested for controlling certain pests have been chosen for their effectiveness, low toxicity to humans and pets, and to minimize secondary resurgence of pests.

Botanical insecticides and related structures

Pyrethrum is practically non-toxic to mammals as used. It kills insects on contact, quickly degrades on exposure to sunlight, and is most useful on pests of vegetables. Derived from the pyrethrum daisy, this compound acts on an insect’s nervous system. Sodium channels along nerve axons are affected, causing a single signal sent to an affected nerve to trigger repeated responses. Rapid loss of coordination is quickly followed by paralysis and death in insects. Chemists have modified pyrethrin to form semi-synthetic compounds called pyrethroids. These manufactured compounds, such as fluvalinate, have some of the same properties as pyrethrin, but tend to have longer residual activity, and may be effective against insects at remarkably low concentrations. Pyrethrum, and more importantly the more stable pyrethroids, are extremely toxic to fish. Some are lethal to fish at concentrations as low as 0.15 parts per billion. This is equivalent in linear units to 75 feet compared to the distance to the sun! Obviously, as cautioned on the product labels, pyrethrin or pyrethroids should never be allowed to enter surface waters.

Rotenone is also practically non-toxic to mammals, but is highly toxic to many beneficial insects and to fish. Its mode of action is to block the electron transport chain involved in oxidative phosphorylation. Consequently, chemical energy from food cannot be converted to ATP, the basic energy carrying molecule required by cells. It persists longer than pyrethrum, so can be highly disruptive to beneficial insects. It too is a preferred material for vegetables.

Garlic extracts have insecticidal properties. However, even though these are the same ingredients as are used in our cooking, some constituents appear to be toxic to beneficial insects as well as to targeted pests.

Neem oil, containing the active ingredient azadirachtin, has several interesting properties. At low concentrations, azadirachtin affects the endocrine system of insects. Though insects, like whiteflies, may complete development to the adult stage following treatment as immatures, they may not be able to reproduce. This delayed effect on pest populations can make the effectiveness of neem products difficult to evaluate. At higher concentrations, neem can be an effective feeding deterrent to many insects, including certain plant bugs and Japanese beetle adults. This behavioral effect may be the result of more gross disruption of the endocrine system brought into play through exposure to these higher concentrations of active ingredient. Neem oil and azadirachtin have a long history of safety for humans, and have even been used in personal care products like shampoo and toothpastes.

Bacillus thuringiensis products

Bacillus thuringiensis (Bt), a species of soil-dwelling bacteria, consists of several strains that produce various proteins toxic to insects. The general pattern for the mode of action is that an insect ingests the proteins. Digestive enzymes then cut the inactive protein (protoxin) into fragments, some of which then may bind to "receptor" proteins already present in the insect's midgut. When this occurs, a channel opens through the midgut and the uncontrolled flow of substances across the midgut wall causes the insect to sicken and die. Specificity of Bt strains is determined by the sequence of amino acids in the protoxin, which in turn determines how the protein will be cut by digestive enzymes, and whether those fragments may then bind to receptors in the gut. Commercially available Bt products only produce endotoxins, a class of Bt proteins that are virtually non-toxic to mammals. Available varieties of Bt include: kurstaki, active against caterpillars of many moths and butterflies; aizawai, active against additional species of moth caterpillars; san diego, active against leaf beetles (including Colorado potato beetle); and israeliensis, active against larvae of mosquitoes, blackflies, and fungus gnats. Another variety, japonensis, strain buibui, is being developed to control certain white grubs.

Bt products break down very rapidly when exposed to sunlight. Several methods have been adopted to extend the benefits of using these toxins. One company engineers the expression of toxins into a bacterium with a thick cell wall, so that the proteins are protected from sunlight. Another method is to have the toxins expressed by plants. This method is controversial, as it may cause such intensive selective pressure that pests may rapidly develop resistance to Bt. Applying Bt products with ultrafine oil may prolong the toxin residues, and the oil itself can be toxic to the target pest.

Organophosphates and Carbamates

Organophosphate and carbamate insecticides act through a common mechanism. Both classes of insecticides bind to acetylcholinesterase, an enzyme needed for the normal rapid removal of acetylcholine from the nerve synapse. When acetylcholine is not removed from the synapse, nerve fibers send multiple trains of signals. As a group, organophosphates and carbamates vary tremendously in their toxicity to humans, however, following repeated exposure from misuse, even the safest of these insecticides can cause human poisoning.

Among organophosphates, malathion has been widely and effectively used by homeowners for controlling many insects. Its toxicity to humans is very low, and its residues disappear from plants relatively rapidly. It is a contact insecticide, meaning that the chemical stays on the surface of the plant and affects insects through contact or ingestion. It is well adapted for use in both vegetable and flower gardens.

Acephate is an organophosphate especially useful for controlling sucking insects on ornamental plants because it is a systemic, meaning that the active ingredient is absorbed into the plant tissues and sap, where it may be consumed by sap feeders. It has a relatively short residual, and is also useful for control of chewing insects.

Chlorpyrifos is an organophosphate with greater mammalian toxicity than malathion or acephate. The special properties of chlorpyrifos that make it useful are its longer residual toxicity. This makes it particularly useful on ornamental plants when control is needed for an extended period, for example, against bark borer adults and scale crawlers.

Carbaryl is a carbamate insecticide. With low relative toxicity to humans, Carbaryl is widely used by homeowners on certain vegetables, fruits, and ornamentals. It has been implicated in killing honeybees when inappropriately applied to open blossoms and is also highly toxic to earthworms, predatory mites, insect predators and parasites.

Dormant and Ultrafine oils

Horticultural oils used for controlling insects and mites and for suppressing some plant diseases vary considerably in their potential to cause injury to plants. They are very useful in killing scales, aphids, and mites. These oils work by coating the respiratory apparatus of target pests, thus causing their suffocation. Dormant oil sprays, particularly the "superior" type, may be used on dormant deciduous trees and shrubs. True dormant oils are most effective when applied in the early spring after the danger of freezing weather and before buds swell. A more highly refined grade of oil is called "ultrafine oil." Though many of its characteristics are the same as superior oils, ultrafine oils are distilled over a narrow temperature range, producing a purer product optimized for suffocating insects and mites while minimizing the potential to cause injury to plants. Ultrafine oils may be used on a great variety of plants, including conifers, and on foliage during the growing season. Oil is especially useful against spider mites, because all stages are sensitive and, at a 1% concentration or lower, beneficial predatory mites are not adversely affected. The waxes on Colorado blue or Koster spruces are permanently darkened following oil applications, so if the natural color of the foliage is important, oil should not be applied to these species. Agitation of the spray mixture is extremely important to maintain an emulsion. Thorough coverage is essential when using oil, because it only acts after coating the insect or mite.

Insecticidal soap

Commercially labeled insecticidal soap is a potassium salt of a fatty acid, and is little different from liquid dishwashing detergent, except that the insecticide product is chemically purer. For the same reason that the purer ultrafine oils are safer for use on plants than dormant oil, commercial insecticidal soap is also safer for plant treatments. The product has been optimized for insecticidal action, while limiting the potential to cause plant injury. Insecticidal soaps appear to principally work by allowing a film of water to coat the respiratory apparatus of targeted aphids, mites, or soft-bodied insects. When this occurs, the insect or mite drowns. Once the residue has dried, soap becomes non-toxic to insects and mites. Soaps may also enter the body of the insect, disrupting cell membranes in the process. Soaps are especially useful for control of aphids. The ability to control any pest with soap depends on thorough coverage with the spray. Soap solutions also interact with the waxes on plant leaves to determine how well they work to control spider mites. Spider mites on very waxy leaf surfaces are much more difficult to kill with soap than those on leaves that are easily wetted.

Chloronicotinyl Insecticides

Imidacloprid is the first chloronicotinyl insecticide to gain registration. It has a mode of action for insects related to that of nicotine, however, unlike the natural product, it has very low toxicity to mammals. It binds to the nicotinic acetylcholine receptor in the nerve synapse, and consequently disrupts nerve transmission in insects by causing uncontrolled firing of nerves. Imidacloprid has a short residual life when exposed to the sun on leaf surfaces, thereby limiting its usefulness in foliar sprays. However, it has a long residual (100 days or more) when protected from the sun in the soil. It is readily taken up by the roots of plants, and usually provides at least one season of control for aphids, adelgids, soft scales, whiteflies, lace bugs and plant bugs in ornamental plants. Treatments of trees, shrubs, and flowers, require a wettable powder formulation labeled for these uses. One management strategy is to treat in the early spring, then not treat again until the pest reappears (which can be two or more years later). It has low toxicity to earthworms, and because it is a systemic and is contained within plant tissues, it is probably non-toxic to beneficial mites and insects visiting leaf surfaces of systemically treated plants. Imidacloprid is highly effective against white grub larvae in turf, and is applied at a fraction of the amount of active ingredient compared to other insecticides. Granular products specifically labeled for control of white grubs in lawns do not have claims on their labels for control of sucking pests on adjacent trees and shrubs. However, the homeowner should anticipate some benefit to these plants because they may take up the active ingredient through roots overlapping turf areas.

Professionally labeled insecticides

Many insecticides registered for use on landscape ornamentals are not available to homeowners because their sale is restricted to licensed pesticide applicators. These products are not listed in the remainder of this publication, but some of them have properties that are important to consider if the work needs to be done by a professional.

Abamectin is a natural product obtained by fermenting a soil-dwelling fungus. It is effective at extremely low concentrations against spider mites and leaf mining fly larvae. The active ingredient is absorbed into the leaf where it can remain active for two weeks. Spinosad is similarly produced, but from a different fungus. It is also used at exceptionally low rates against leaf beetles and caterpillars. This product acts quickly, but is not likely to cause resurgence of other pests. Halofenozide is an insect growth regulator, causing accelerated molting and death of some white grubs species in turf. It is non-toxic to earthworms and vertebrates. A large number of pyrethroids are available to commercial applicators, including lambda-cyhalothrin, bifenthrin, cyfluthrin, permethrin, and deltamethrin. These are broad-spectrum insecticides, but tend to cause secondary outbreaks of mites, aphids, scales, and mealybugs.