Chapter 3   Insect Management

General Principles

The management of insect pests rarely relies on a single control tactic; usually, a variety of tactics are integrated to maintain pests at acceptable levels and minimize the chance that insects will adapt to any one management tactic. The goal is not to eliminate all pests, but rather, to reduce pest populations to tolerable levels. Some pests are tolerable and essential, so that their natural enemies remain in the crop. The management tactics used against insect pests include pest resistant or tolerant varieties, and cultural, physical, mechanical, biological, and chemical controls.

Integrated pest management requires an understanding of the ecology of the cropping system including that of the pests and their environment. For example, temperature is the primary factor determining the rate at which insects develop; higher temperatures increase the rate of development. Temperature can be important when determining the frequency of insecticide applications. In sweet corn for example, the interval between insecticide applications for the European corn borer can be increased when the weather is cool because eggs are not hatching as rapidly and the rate of larval development is slower; with warmer temperatures the situation is reversed. Degree-day models can aid in determining how fast insects are developing and the timing of applications. Weather conditions can also influence insect pest populations. For example, wind-driven rain is a major source of mortality to nymphs of the potato leafhopper on potatoes and thrips on onions and frame leaves of cabbage.

Economic threshold and sampling. The decision to use an insecticide, or take some other action, against an insect infestation requires an understanding of the level of damage or insect infestation a crop can tolerate without an unacceptable economic loss. The level of infestation or damage at which some action must be taken to prevent an economic loss is referred to as the "action threshold." Action thresholds are available for many vegetable crops and should serve as a guide only. Thresholds should be adjusted based on market, environmental conditions, variety, etc. To estimate the severity of pest infestations, the crop must be sampled. Sampling may involve examining plants and recording the number of pests or the amount of damage observed, or traps may be used to capture the pest species to estimate pest abundance. Sampling is conducted at regular intervals throughout the season or during critical stages of crop growth.

Weather and predicting insect occurrence: Some insect pest activity can be predicted by using degree days - a method of summarizing accumulated heat units. Degree day information is collected and summarized by the Northeast Weather Association (NEWA) and made available free of charge on its website.

Pest-resistant Crops
An important management option for the control of insect pests is the use of crop varieties that are resistant or tolerant. A resistant variety may be less preferred by the insect pest, adversely affect its development and survival, or the plant may tolerate the damage without an economic loss in yield or quality. For example, varieties of cucurbits (squash, cucumbers, melons) that have lower concentrations of feeding stimulants (cucurbitacins) are less preferred by cucumber beetles. Sweet corn varieties with tight husks are less likely to be infested by corn earworm, and some varieties are resistant to Stewartıs wilt, a bacterial disease of sweet corn vectored by the corn flea beetle. Bt sweet corn varieties can be considered resistant varieties developed through modern biotechnology. In the case of cabbage, the only reliable method of controlling onion thrips is through the use of tolerant varieties. Advantages of pest-resistant crop varieties include ease of use; compatibility with other integrated pest management tactics; low cost; cumulative impact on the pest (each subsequent generation of the pest is further reduced); and reduced negative impact on the environment.

Cultural Control
There are many agricultural practices that make the environment less favorable to insect pests. Crop rotation, for example, is recommended for management of Colorado potato beetle. The beetles overwinter in or near potato fields, and they require potato or related plants for food when they emerge in the spring. Planting potatoes well away from the previous year's crop prevents access to needed food, and the nonflying beetles will starve. Selection of the planting site may also affect the severity of insect infestations. Cabbage planted near wheat or oats is more likely to be infested by onion thrips that disperse from the maturing grain crops.

Trap crops are planted to attract and hold insect pests where they can be managed more efficiently and prevent or reduce their movement onto crops. Early planted potatoes can act as a trap crop for Colorado potato beetles emerging in the spring. Since the early potatoes are the only food source available, the beetles will congregate on these plants where they can more easily be controlled. Adjusting the timing of planting or harvesting is another cultural control technique. Earlier planted sweet corn is less likely to be infested by corn earworm and fall armyworm, pests that typically arrive mid to late in the season.

Cornell cover crop decision tool

Physical and Mechanical Control
The use of physical barriers such as row covers or trenches prevents insects from reaching the crop. Row covers can help prevent early-season damage to cucurbits by cucumber beetles, and plastic-lined trenches are effective in trapping dispersing Colorado potato beetles in the spring and fall. Other methods include handpicking of pests, sticky boards or tapes for control of flying insects in greenhouses, and various trapping techniques.

Biological Control
Biological control is defined as the reduction of pest populations by natural enemies (predators, parasitoids, and pathogens). Predators, such as lady beetles and lacewings, are free-living species that consume a large number of prey during their lifetime. Parasitoids (certain wasps and flies) are species whose immature stage develops on or within a single insect host, ultimately killing the host. Pathogens are disease-causing organisms including bacteria, fungi, and viruses. Pathogens kill or debilitate their host and are relatively specific to certain insect groups.

Natural enemies can be either generalists, which utilize many different species of insects as food, or specialists, which are fairly selective in their prey. Lady beetles are considered generalists because they feed on many types of prey; whereas, species of Trichogramma are microscopic wasps specialized to parasitize eggs of moths. There are three broad and somewhat overlapping types of biological control: classical, augmentative, and conservational.

Click here for more information on biological control!

The conservation of natural enemies is probably the most important and readily available biological control practice available. Natural enemies occur in all vegetable production systems. They are adapted to the local environment and to the target pest, and their conservation is generally simple and cost-effective. With relatively little effort, the activity of these natural enemies can be observed. Lacewings, lady beetles, hover fly larvae, and parasitized aphid mummies are almost always present in aphid colonies. Fungus-infected adult flies are often common following periods of high humidity. These natural controls are important and need to be conserved when making pest management decisions. In many instances, the importance of natural enemies has not been adequately studied or does not become apparent until insecticide use is stopped or reduced. Often the best we can do is to recognize that these factors are present and minimize negative impacts on them. If an insecticide is needed, every effort should be made to use a selective material in a selective manner.

Natural enemies are generally more adversely affected by chemical insecticides than the target pest. Because predators and parasitoids must search for their prey, they generally are very mobile and spend a considerable amount of time moving across plants. This increases the likelihood that they will contact the insecticide. When an insecticide is applied, ideally only the target pest(s) should be affected. The goal is to maximize pest mortality while minimizing harm to natural enemies.

The following factors influence the impact of insecticide applications on natural enemies:

Spectrum of activity. Broad-spectrum insecticides will more adversely affect natural enemies than materials that are selective for specific insect species or life stages.
Residual (half-life) activity of insecticide. Insecticides that remain toxic to pests for a long time and stay on the treated surface will have a similar effect on natural enemies.
Coverage and formulation of insecticide. Full coverage sprays will generally have a greater impact on natural enemies than directed sprays, systemics, or bait formulations. Spot or edge treatments directed at localized pest infestations will be less detrimental than those applied to the entire field.
Dosage and frequency of application. Higher rates and repeated applications will have a more detrimental impact on natural enemies.
Susceptibility of natural enemy to pesticides. Some natural enemies are inherently more resistant to insecticides than others and some populations of natural enemies have been selected to possess higher levels of resistance. Applications of some fungicides can also reduce the incidence of fungal diseases of insects. Fungicides and herbicides may also have lethal and sublethal effects on natural enemies.

In many instances, the complex of natural enemies associated with an insect pest may be inadequate. This is especially evident when an insect pest is accidentally introduced into a new geographic area without its associated natural enemies. These introduced pests (e.g. European corn borer, imported cabbageworm, carrot rust fly, and diamondback moth) comprise about 40 percent of the insect pests in the United States. Classical biological control is the practice of importing and releasing for establishment (following a quarantine process) natural enemies to control an introduced pest. Follow-up studies are conducted to determine if the natural enemy was successfully established at the release site and to assess its long-term benefit.

Augmentation is the supplemental release of natural enemies. Relatively few natural enemies may be released at a critical time of the season or literally millions may be released. For example, the recommended release rates for Trichogramma ostriniae in sweet corn is 30,000 per acre. In contrast entomopathogenic nematodes are released at rates of millions and even billions per acre for control of certain soil-dwelling insect pests. Augmentation may also involve modifying the cropping habitat to favor natural enemies. Many adult parasitoids and predators benefit from sources of nectar and the protection provided by refuges such as hedgerows, cover crops, and weedy borders. The modification of surrounding habitats must, however, be done carefully. Refuges may benefit some pests, but may also be hosts for certain plant diseases, especially plant viruses that could be vectored by insect pests. For additonal information on how to modify the farmscape to enhance biological control see http://www.attra.org/attra-pub/farmscape.html

Michigan State site "Enhancing Beneficial Insects with Native Plants"

Chemical Control

When other management tactics are not available or fail to keep pest infestations from causing economic damage, insecticides are needed. The decision to use an insecticide should be based on an action threshold. A field should be scouted regularly to determine if it is at the action threshold. When an insecticide is required, the choice of material, formulation, and rate and method of application will vary depending on several factors, including the pest(s) to be controlled and the stage of crop development.

Insecticides fall into several chemical classes including: chlorinated hydrocarbons (methoxychlor), organophosphates (malathion and terbufos), carbamates (methomyl), synthetic pyrethroids (esfenvalerate and permethrin), and microbials (Bacillus thuringiensis).More recently several new insectocodeshave been developed and these include neonicotonoids(imidacloprid), avermectins (emamectin-benzoate), naturalyte(spinosad) and others. In addition to these common classes of insecticides, there are insecticides derived from plants (rotenone) and inorganic materials (cryolite). They are also classified by mode of action: physical poisons, protoplasmic poisons, metabolic inhibitors, and nerve poisons. Physical poisons kill by an action such as suffocation and include certain oils. Protoplasmic poisons kill by destroying proteins. Metabolic inhibitors interfere with processes such as respiration or digestion. Nerve poisons, the largest group of insecticides, interfere with the normal functions of the nervous system.

Insecticides are formulated into emulsifiable liquid concentrates, flowable liquids, and dry concentrates such as wettable powders, soluble powders, dusts, and granules. A sticker-spreader should be added to spray mixtures for use on some cole crops, cucumbers, onions, and tomatoes, so the mixture will spread more evenly over the plant surfaces. Insecticides will be effective only if the insect consumes or comes in contact with them; good coverage is thus essential for efficient pest control. Always use enough water and adequate pressure to cover plants completely, especially if insects are concealed (e.g., thrips on onions). Water requirements may vary from five to 150 gallons per acre, depending on the equipment, chemical, crop, and plant size. Use the correct application equipment to assure adequate coverage. Drop nozzles or airblast sprayers may be essential to get the insecticide into the crop canopy and to the pest. Several factors determine the efficacy of an insecticide application, including use of the correct insecticide; proper timing of the application; and good coverage of the crop. Some insecticides are more effective than others for a particular pest, and certain insecticides must be applied at specific times. For example, to be most effective Bacillus thuringiensis (Bt) products must be applied early in the pest's development when it is more susceptible to the toxin. Coverage is also important. Aphids, for example, are frequently most abundant on the undersides of leaves. If the insecticide is applied only to the tops of leaves, aphid control will not be adequate. Thorough coverage is also important for stomach poisons, such as the Bt products, which must be ingested by the pest.

Table 3.1 Registered insecticides by crop.

Managing Resistance

A major obstacle to the continued, effective use of insecticides is the development of resistance. Insect pest populations develop resistance to insecticides through genetic selection. Some individuals in a population have genetic traits that allow them to survive an insecticide application. Some of the progeny of these surviving individuals will possess these traits. When additional applications of insecticides are made, more of the resistant individuals survive. Over time and in the absence of appropriate resistance management, the pest population may consist mostly of resistant individuals. At this point, control may be difficult or impossible. Increasing the frequency of the applications or amounts of insecticide will not improve control. Switching to different or new insecticides within the same class or even different classes may not help because pests resistant to one insecticide may also be resistant or rapidly develop resistance to others. This is referred to as cross-resistance. For example, some insect pests that are resistant to chlorinated hydrocarbons quickly develop resistance to pyrethroids. Repeated applications to control a target pest may lead other nontarget species subjected to these same applications to develop resistance.

Many insect populations now possess some level of resistance to insecticides. A partial list includes some populations of green peach aphid resistant to endosulfan, and pyrethroids, and some populations of the diamondback moth are resistant to pyrethroids, methomyl, and Bt's. The Colorado potato beetle is resistant to several insecticides. The following steps are recommended to minimize the development of resistance to insecticides: apply insecticides only when needed; follow recommended action thresholds where available; use the minimum recommended rate that provides control; rotate insecticides that are in different chemical classes and have different modes of action during the season. Ideally, this rotation should be done across pest generations, that is, each generation should be treated with a different class of insecticide. Also, use insecticide synergists if available and effective.

An insect may be resistant to one insecticide in a particular class (e.g. pyrethroid) but not be susceptible to another insecticide in a different class (e.g. carbonate). Therefore it is important to know the class of the insecticide you choose.

A classification of insecticides based on their mode of action is available in pdf format. This guide can be used to help avoid or delay the development of insecticide resistance, as well as manage populations that have developed resistance to a particular insecticide. It is a large file - 97 page, 780k pdf file 

Shelton, A.M., W.T. Wilsey, and D. M. Soderlund. 2001. Classification of insecticides and acaracides for resistance management

Protecting Honey Bees from Insecticides

Honey bees, wild bees, and other insects are important for proper pollination of many vegetables. Vine crops, for example, must be pollinated because they have male and female flowers, and pollen must be transferred from the male to female flowers if fruit is to set. Poor pollination results in small or odd-shaped fruit as well as low yields. Each flower must be visited eight or more times for adequate pollination. To avoid harming bees with insecticide treatments, remember these points: do not spray crops in bloom; mow blooming weeds before treatment, or spray when the blossoms are closed; avoid application during the time of day when field bees are most numerous; apply application in the early morning or evening; and always read the label before use.

If pesticides that are highly toxic to bees are used in strict accordance with label directions, little or no harm should be done to bees. Label statements on pesticides that are highly toxic to honey bees may carry a caution statement such as the following, "This product is highly toxic to bees exposed to direct treatment or residues on crops or weeds. Do not apply or allow to drift to weeds in bloom on which an economically significant number of bees are actively foraging. Protective information can be obtained from your Cooperative Extension Service."

World Wide Web Sites:

BIOLOGICAL CONTROL: A Guide to Natural Enemies in North America:http://www.cornell.edu/ent/biocontrol/

Measuring Environmental Impact of Pesiticides:
http://nysipm.cornell.edu/publications/eiq/default.asp

Cornell Vegetable Team
http://www.hort.cornell.edu/extension/commercial/vegetables/vegeteam/index.html

The Northeastern Pest Management Center has links to information on IPM in the northeast and throughout the US

Next chapter Weed Management

Maintained by Curtis Petzoldt, New York State IPM Program. Address comments or questions to vegrec@nysaes.cornell.edu
Last modified March 3, 2008.