INSECT AND RELATED PESTS OF VEGETABLES

AG-295

Introduction

Vegetable crops account for less than 2.4 percent of the agricultural acreage in North Carolina but sales exceed 200 million dollars. Vegetable crop production requires one of the highest dollar-per-acre investments of all agricultural endeavors, which makes it extremely important to prevent insects from reducing yields, lowering quality, and increasing production costs. The lowering of one market grade can be economically devastating for the producer. Probably the greatest investment with the slimmest margin of profit is found in greenhouse vegetable production, a two million dollar industry in North Carolina.

For the home gardener, interest in growing vegetables is at its highest level since the Victory Garden era of World War II. This current trend owes much to increased food and energy prices. Preference for the freshness and quality of homegrown vegetables as well as personal satisfaction of growing them oneself also pays a large part in the revival of the backyard garden. Since many gardens are urban and limited to small areas, minimizing losses to insects is an important goal.

In commercial field, greenhouse, and home garden situations, the first step toward alleviation of insect damage is a timely diagnosis of the problem, which requires an accurate identification of the pest. Minor insect damage may be easily mistaken for disease, drought, or fertilizer injury. The symptoms can be strikingly similar: chlorosis, wilting, and die-back. Checking closely for insects when plants appear unthrifty is always a good idea, but care must be taken to avoid mistaking insect damage for injuries caused by other factors. Since not all insects and mites threaten plants, a suspected pest may prove to be harmless or even beneficial.

The keys, pest descriptions, illustrations, and indices in this volume provide necessary information to identify the important, common insect or mite pests of vegetables grown in North Carolina. Identification can be made in a number of ways. The field keys to damage, which appear at the beginning of each crop section, are a good place to start if damage is apparent but cannot be linked to a particular insect specimen. If specimens are available, the key to orders and groups of pests may be helpful. The index to host plants (at the rear of this volume) may help narrow the choices. Since all plant disorders are not caused by insects, an additional useful reference is North Carolina Agricultural Extension publication AG-07, Diagnosing Common Disorders of Vegetable Crops.

MANAGEMENT AND CONTROL

To help minimize losses to insects and other pests, growers should have an awareness of the types of pests which attack crops in their locality and a knowledge of pertinent pest biology. Both are equally important to the proper implementation of management and control practices. Scouting methods, equipment selection, timing of controls, and other pest management practices are derived from accurate knowledge of the pest.

CULTURAL CONTROL

Many safe, practical methods of pest control exist. These practices, however, are not always alternatives to pesticide use. Some are preventive measures which may eliminate the necessity to spray; still others are most beneficial when used in conjunction with pesticides.

The importance of cultural, mechanical, physical, and biological controls in a particular pest situation depends largely on the economics of the crop, goals of the grower, and plant environment. Commercial vegetable production practically always requires the use of pesticides, whereas the home gardener with fewer plants to manage may rely strongly on nonchemical methods. This does not imply that nonchemical controls are only for backyard gardeners and organic farmers. Some can be used beneficially in greenhouses or under commercial field conditions, but must be carefully coordinated with pesticide application.

The grower who does choose the "organic" method of control (no synthetic chemicals) should keep in mind several factors. Vegetables produced organically may be slightly to heavily damaged. More labor in controlling insects is involved with organic gardening. Damage from soil pests is to be expected. In order to compensate for anticipated losses, larger gardens should be planted so there will be enough food for both gardener and insects. Susceptible crops should be avoided.

The brief survey of the following nonchemical controls provides a basis for choosing control methods best suited to a particular situation.

Resistant Varieties, Certified Seed, and Pest-Free Transplants - This is the primary nonchemical defense against pests. Resistant varieties are those which repel, are unattractive to, or otherwise are unsuitable as food for certain insects or which withstand feeding by certain pests with little reduction in yield of quality. Use of resistant varieties fits well into just about any production schedule. A factor to consider, however, is whether the produce will be as desirable (large or palatable) as those of comparable nonresistant varieties. In making this decision, the grower should weigh the probable impact of a pest against the desired quality or type of produce. Varieties adapted to weedy conditions should also be considered.

The names of recommended resistant varieties, if available, appear in the control section of each pest description. For a more thorough listing, consult North Carolina Agricultural Extension publication AG-25, Control Vegetable Insects Using Cultural Methods.

Certified seeds are guaranteed to have a certain percent germination and to be virtually free of weed seed. The use of certified seed or pest-free plants, when available, enables the grower to start with healthy, vigorous plants. Such plants -- though not necessarily insect resistant -- have a good chance of withstanding insect attack.

If a grower is producing his own transplants, they should be raised under good conditions. Insects in the plant bed can be effectively controlled by proper management practices. Sanitation, adequate seed bed preparation, isolation of plant beds from infested areas, frequent and thorough examination of plants for initial infestations and prompt treatment will favor plant establishment and minimize insect problems. Growers who purchase transplants should inspect them carefully for pests such as pepper weevil, tomato pinworm and sweet potato weevil before planting.

Cutworms, aphids, crickets, slugs, flea beetles, whiteflies, leafminers, and lepidopterous larvae overwinter in North Carolina and can be troublesome in the plant bed. Other insect pests overwinter further south but may move into North Carolina with spring and summer. Tomato pinworm and pepper weevil are examples of serious pests which are brought into North Carolina, primarily on transplants from Florida.

Good Management Practices - Many vegetable crops are closely related and have the same pests (see table). Insects that become severe problems on cabbage are likely to infest nearby crucifers such as mustard, turnips, broccoli, collards, or cauliflower. The same relationship exists among many crops of the same family. Therefore, it is often advisable to plant closely related crops as far from each other as possible and to avoid planting them in the same place year after year. Consider, for example, a field planted in tomatoes one year, potatoes the second year, and eggplants the third year. Such a field is essentially not being rotated at all.

Another type of rotation is one between root (or tuber) crops and nonroot crops. Certain soil pests are common to carrots, radishes, Irish potatoes, sweet potatoes, turnips, onions, and other crops grown for their edible roots or tubers. However, in a "leaf crop-root crop" rotation, it is possible to alternate crops within a related group, for instance, solanaceous plants like peppers and potatoes. However, since foliage pests of peppers also attack the foliage of potatoes and thereby affect tuber development, rotations within families should be avoided.

Cultivation, Weed Control, and Sanitation - These three practices are interrelated. Cultivation can be used to control weeds, to bury or destroy crop residue, or to expose soil pests to the elements. Regardless of the intended purpose, soil cultivation has the potential to disturb overwintering sites, eliminate alternate hosts plants, bury residue in which insects might feed and hide, and often outright kill soil-inhabiting pests.

Of course, weed control and sanitation can be accomplished without cultivation. Pulling weeds in the home garden or using herbicides in commercial production achieves the same goal. Sanitation also includes the removal and destruction of dead or diseased plants. Since some insects hasten the spread of bacterial and viral plant diseases, the sooner these plants are removed, the better.

Mechanical Barriers - The greenhouse is an obvious example of a mechanical barrier in vegetable production. It also illustrates how ineffective this method can be in excluding all insect or mite pests. Even if a grower pays close attention to screening vents and other openings, some pests, especially smaller ones, eventually gain entry. Mechanical barriers are most effective when aimed at excluding one or a few pests rather than all pests.

Collars made of cardboard, tin cans (with both ends cut out), or aluminum foil are effective barriers against cutworms. When placed around seedlings and inserted about halfway into the soil, collars prevent cutworms from moving through the upper soil to seedling stems. This method is most useful in small garden situations.

Other specific examples of mechanical barriers include: 1) cheesecloth wrapped over collard plants to exclude egg-laying diamondback moths, 2) screened potato storage areas to exclude potato tuberworm moths, and 3) mounds of soil around grape vines to prevent emergence of grape root borer moths.

Handpicking - Removal and destruction of insects by hand is particularly valuable in small gardens. If contact with pests is repugnant to the gardener, pests can be cut in two with scissors, knocked into soapy water, or to the ground and stepped on. Removal of egg masses or leaves with egg masses can also be beneficial. Before waging a vendetta against all insects and mites, however, keep in mind the beneficial insects and their relatives. These arthropods are working for you!

Interplanting - Popular gardening literature suggests planting of certain pest-attracting or repelling plants in and around the garden to reduce pest population. Marigolds and garlic are two plants whose merits are frequently advocated. Unfortunately, most of these claims are unfounded. Unless a gardener particularly likes to spruce up the vegetable patch with flowers or eat garlic, garden space can be used more wisely.

Traps - Insect traps are for survey, detection and management purposes. They may be useful in large-scale commercial production as indicators of potential infestations. Certain traps, however, do have a place in small gardens. Some insects, like squash bugs, will congregate under strategically placed boards. By regularly checking beneath the boards, gardeners can kill a number of pests at once. Japanese beetle traps attract and trap adult beetles. As a final example, shallow tins of beer partially embedded in the soil make effective traps for slugs. The slugs crawl in and drown.

Parasites, Predators, and Diseases - Beneficial organisms are often natural inhabitants of gardens and vegetable fields. Although several companies advertise the sale and shipment of insect parasites and predators, successful establishment of these organisms in new environments is difficult. Parasites of the greenhouse whitefly are available commercially and are effective. However, introduced arthropods often leave if there are not sufficient numbers of pests for them to attack. Therefore, in most cases, it is best to learn to recognize the naturally occurring beneficial insects already present and to work to maintain their populations. The use of synthetic organic pesticides is detrimental to many beneficial insects and mites.

Insect disease organisms also lower pest populations. Although many viral, fungal, bacterial, and nematode pathogens exist, the Bacillus popilliae and Bacillus thuringiensis bacteria are the most widely known and effectively used. These diseases are discussed under Chemical Control in the next section.

CHEMICAL CONTROL

This manual is designed to augment the North Carolina Agricultural Chemicals Manual, not duplicate it. Specific chemical recommendations do not appear in the text. Since registrations and recommendations are always subject to change, their presence in this volume would necessitate its constant revision. The N.C. Agricultural Chemicals Manual is revised annually and provides current recommendations; it makes a good companion volume to this text. In no way are the suggestions for safe use and calibration for proper application in the N.C. Agricultural Chemicals Manual to be belittled or ignored. The following section on control is merely an expansion of explanations begun in that manual.

Types of Pesticides - Insect feeding habits determine, to some extent, the type of control chemical used. Pesticides can be classified as stomach poisons, contact-residuals, fumigants and systemics. In vegetable production, all types are important; however, fumigants are used primarily in greenhouses and are discussed later under Greenhouse Vegetable Production.

Chewing insects such as caterpillars, beetles, and grasshoppers are targets of stomach poisons. The diverse array of botanical, organic, and inorganic chemicals which comprise this group must be ingested before killing pests. Since piercing-sucking pests (aphids, mites, leafhoppers, whiteflies, etc.) feed below the plant surface, they may avoid contact with stomach-poison pesticides. Contact-residual chemicals such as malathion are not so easily avoided and, therefore, kill a wider range of pests. Contact-residual chemicals poison insects and/or mites which crawl on treated surfaces, eat treated leaves, or are sprayed directly. However, summer or dormant oils, which are also contact-residuals, kill by smothering tiny insects or mites.

Bacillus popilliae (milky spore disease, Japedemic, Doom) and Bacillus thuringiensis (Dipel, Sok BT, Bactospeine, Thuricide) contain actual bacterial spores (B. popilliae or spores and a crystalline toxin (produced by B. thuringiensis Berliner. Safe to humans, animals, and beneficial insects, these bacterial preparations are selective pesticides. Only certain groups of troublesome caterpillars are affected by the B. thuringiensis toxin and spores, and "milky spore disease" controls only Japanese beetle grubs.

The persistence of a pesticide after it has been applied depends to some extent on the chemical group to which it belongs. Materials like the plant-derived pesticide pyrethrin last less than one hour, although similar synthetic pyrethroid pesticides which last considerably longer are now being made. In general, insecticides classified as chlorinated hydrocarbons are long lasting although some break down fairly rapidly. Less persistent than chlorinated hydrocarbons, most organophosphates and carbamates do not remain effective for more than 1 to 4 weeks. Although all classes of pesticides are used to control vegetable pests, most of the chemicals recommended for pest control on vegetables must be relatively short lived so the produce will be edible.

Passed in 1954 and enforced since 1956, Public Law 518 (the Miller Bill) limits amounts of insecticides and fungicides which can remain as residues in or on raw agricultural commodities. These residue limits are known as tolerances. By adhering to manufacturer's directions, growers are assured of harvesting produce which will meet tolerances established by the Food and Drug Administration. Any marketbound produce which exceeds established tolerances is subject to confiscation.

Spray Effectiveness - An ineffective pesticide application usually can be attributed to one or several reasons: 1) poor timing, 2) incorrect chemical, 3) poor spray coverage, 4) unfavorable weather conditions, 5) too low a rate, 6) equipment failure, and/or 7) operator negligence. The first of these, poor timing, is by far the most common reason for failure. Pest life history, rapidity of host plant growth, and weather conditions all affect timing. Attention to current recommendations should aid timing, preclude the use of incorrect pesticides, and prevent the use of incorrect rates. Sometimes, however, special additives are necessary. For example, sprays tend to run off plants with waxy leaves such as cabbage. Wetting agents may have to be added for effective control.

Calm periods of the day, especially early morning and evening, are optimum times to apply pesticides and minimize drift. Since many broad spectrum chemicals are used in vegetable insect control, pesticide drift poses a particular threat to bees. Also, spraying around noon is not recommended even on calm days because the combination of bright sun and spray droplets on the foliage may cause injury to the leaves.

Home Garden - Amateur gardeners must select from several pesticides labeled for home use. Wise selection of a pesticide, therefore, involves some informed decision making. As a rule, buying quantities small enough to be used in one growing season is a good idea since it eliminates long term storage problems. The home gardener can lessen chances of having to purchase additional pesticides by initially selecting a product which can be used on a wide range of crops and against many pests. Since most home grown vegetables go straight from the garden to the table, the choice of an insecticide with a short waiting period between application and harvest is advisable.

A variety of applicators is available to amateur gardeners. Dusters apply pesticides in a form which requires no mixing. While not as efficient as sprayers, dusters offer quick, convenient and visible application. Dust when there is little or no wind to prevent excessive drift.

Trombone and hose-end sprayers also work well. With the trombone sprayer, pressure is developed by moving a slide which sucks up premixed pesticide with one motion and forces it out the nozzle with the next motion. The trombone sprayer is portable, but the spray is somewhat intermittent. Consequently, getting uniform coverage may not be easy. Hose-end sprayers use water pressure to dilute and deliver pesticides to the target pest. Concentrated pesticide is placed in the hose-end sprayer and partially diluted. Then water pressure siphons, dilutes, and propels the pesticide to the desired area.

Obviously, length of the hose limits the range of the hose-end sprayer. Sprayers which use air pressure to force diluted pesticide from a nozzle allow a very fine to coarse spray pattern. Clean sprayers after each use to prevent excessive corrosion. Although some sprayers are fitted with strong plastic parts which do not corrode, rinse them after each use. For the large garden, electric and gasoline powered models are available.

Care of spray equipment is not difficult. As long as a duster is kept dry, no other maintenance is required, except for an occasional drop of oil on the plunger rod or other moving parts. After each use, sprayers with metal tanks should be washed out three times with clear water to prevent corrosion from ruining even stainless sprayer tanks. For metal sprayer tanks, a tablespoon or so of household ammonia, shaken thoroughly, will neutralize corrosive effects of any insecticide residue and prolong the life of the tank. The tank should then be allowed to dry completely. The plunger rod should be lightly oiled. Avoid using herbicides and insecticides in the same sprayer.

Commercial Vegetable Production - Consumers set high standards for the minimum acceptable quality of produce. As a result, only very low numbers of insects and mite pests can be tolerated. Fortunately, a broad range of chemicals are cleared for use on commercially produced vegetables.

Soil-applied granules and fumigants, foliar sprays, and systemics all have a place in vegetable production. With so many kinds and formulations of pesticides available, it is wise to keep a close watch on the effectiveness of an insecticide. Should increasing ineffectiveness be due to increasing pest resistance, other pesticide materials can be substituted. Frequent substitution substantially lowers the chances that pests will develop resistance. However, most ineffectiveness is due to poor application techniques.

Scouting for pests is on the increase in commercial vegetable production. A check of fields every 2 or 3 days for insects, damaged foliage, insect eggs, and frass keeps the grower aware of the status of pest populations. Regular scouting, along with light trap surveys, can give early signals of building infestations.

For some insects such as soil pests, it is not possible to forecast outbreaks. Such pests, which are likely to recur year after year, need to be controlled routinely. In this way, damaging infestations will not have a chance to develop.

There is no need here to describe all types of equipment used to deliver pesticides in the field. However, a brief survey of basic equipment maintenance may be in order. Before the growing season and frequently throughout it, equipment should be calibrated, nozzles cleaned and adjusted, and defective parts replaced. Spraying with improperly adjusted equipment wastes time, money, and energy; it can also result in poor control, crop injury or illegal insecticide residue at harvest.

Greenhouse Vegetable Production - Control of pests in the greenhouse is often difficult because of lush, sheltered growing conditions. In general, insects, mites and slugs reproduce more rapidly in warmer temperatures. Also, periodic use of pesticides in greenhouses often reduces parasites and predators of greenhouse pests. Pests tend to be more resistant to pesticides than their predators and parasites.

Application of pesticides is almost essential if one is to continue in production of greenhouse crops. Eliminating weeds inside and outside the greenhouse reduces alternate hosts for vegetable plant pests. Screening doors and vents make it harder for moths and beetles to fly in and lay eggs or feed. But careful as the grower may be, sooner or later an insect or mite will come in on the clothes of workers or on introduced plants, in pots, or in soil, and threaten a crop unless the pests are chemically controlled.

Because of this, many growers apply pesticides on a periodic, prophylactic basis as "insurance" against accidental infestations. However, proper monitoring and periodic inspection is a better control strategy. Despite these precautions, insects or other pests may become established. Some growers treat whenever they discover pests. Growers should survey the plants daily or every other day to guard against extensive damage by insects, mites or slugs.

Pesticides are applied in greenhouses in many ways. Growers use aerosols, mists, smokes, fogs, dusts, sprays, drenches, and granules -- everything but aerial application and backups. Aerosols, smokes, mists and fogs must be applied when the greenhouse is closed (at night or in the winter). Since most pesticides are sensitive to ultraviolet rays, the later in the evening a treatment is made, the more effective it will be. Afterward, the house must be ventilated before workers can safely enter it.

Fogs, smokes and aerosols are generally applied per cubic foot. Compute cubic feet by multiplying the area of the floor times the average height of the roof; that is, the length times the width times the average height (Fig. 1). For houses with flat, sloping roofs, add the height at the eave to the height at the highest part, divide by 2, and multiply by the area of the floor. For houses with rounded roofs, measure the highest point in the middle and at 1/4, 1/2, and 3/4 of the distance to the low point. Add these four measurements, divide by 4, multiply by the area of the floor and the result is a close estimate of the volume of the house. (Although this estimate is a little low, rounded roof houses are usually covered with polyethylene film and are consequently very tight. Thus, applications in these houses are not dissipated as rapidly as applications to fiberglass or glass houses.) Fogs, smokes and aerosols generally need to be diluted by the air in the greenhouse to avoid damage to the plants.

Place smoke fumigators so that the smoke does not vent directly onto the plant foliage. Growers generally wear self-contained breathing systems or gas masks when applying fogs, smoke or aerosols. Some growers who use smoke fumigators determine the number of fumigators needed, based on cubic feet, and light those furthest from the door first. This allows the grower to vacate the house before it becomes dangerously filled with fumes. Two people should always be present when applying toxic substances in the greenhouse. If one person gets accidentally poisoned, the other can drag him to safety and call for help. Dusts and sprays are applied with conventional dusting or spraying equipment. Necessary safety clothing must be worn.

Apply granular insecticides with a hand-held shaker or some other device which does not grind up the granules. Wear boots, long pants, a long-sleeved shirt, rubber gloves, and a respirator. The plants and potting mix should not be handled until the granular pesticide has been washed from the foliage and watered-in thoroughly.

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