Chapter 9: insects and other plant pests.
contact pesticide degree day gradual honeydew host plant instar metamorphosis molt nonselective pesticide pheromone trap selective pesticide systemic pesticide vector
The interaction between insects and plants can be positive or negative. Insects such as butterflies, moths, bees, and even ants pollinate plants and obtain nectar and pollen as food rewards. But some insects use the plants as food, usually to the detriment of the plants. In the same way that pollinators evolved to serve plants, some insects have evolved to feed off them. In other words, certain plants, sometimes referred to as host plants, attract specific pests. A good example of this is the butterfly milkweed plant and the monarch butterfly. The monarch butterfly is attracted to the butterfly milkweed plant, whose leaves contain a certain chemical compound that is toxic to birds but not to monarchs. Monarch larvae (caterpillars) accumulate this toxin while feeding off the plant, which then makes them toxic to would-be predator birds. Birds have come to recognize monarchs and, finding them distasteful, leave them alone. The milkweed plant is not so lucky, however, as a few monarch larvae can defoliate the plant in a short period of time. Table 9-1 shows major insect pests and the damage they cause.
In part because of the huge number of cultivated species, there are many pests of horticultural plants. Although some are specific to certain species or related species of plants, others are common to a wide variety of plants. Of the latter, aphid, scale, mite, beetles, and larvae of butterflies and moths (caterpillars) are common on a wide range of plant species. Common pests of greenhouse and indoor plants include aphids, whiteflies, mites, fungus gnats, mealy bugs, and shore flies (Fig. 9-1). Individual plants have pests that affect them specifically, and these should be researched and understood before undertaking cultivation of a specific plant or crop.
Insect Families and Related Species
Insects and their relatives are arthropods. This means they have segmented bodies and an exoskeleton and respire by gills or tubes called trachae. Arthropods include insects, spiders, mites and ticks, centipedes, and millipedes. Insects are distinguished from other arthropods in that they have three body segments, usually six legs, and a pair of antennae (Fig. 9-2). There are other characteristics that distinguish different types of insects including their mouthparts, wing structure, and the type of metamorphosis they undergo in their life cycle. Insects generally have one or two pairs of wings, although these may be absent. Mites belong to the Arachnida, having two body segments and eight legs (Fig. 9-3). They lack antennae. The major arthropod orders in which plant-damaging pests are found include the following: Arachnida (spiders and mites), Coleoptera (beetles), Diptera (flies), Hemiptera/ Homoptera (true bugs), Lepidoptera (butterflies and moths), Orthoptera (grasshoppers and locusts), and Thysanoptera (thrips).
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Insects may be identified by direct observation, by their frass, or droppings, and by the crop they are found on and the damage they do there. Some insects, such as aphids and whiteflies, feed on plant juices and excrete a sticky, sugary substance called honeydew, which attracts ants. Honeydew is also a good substrate for molds and fungi, and often black sooty mold is observed when such pests have invaded a plant.
It may be possible to identify a pest problem, even when the pest is not directly observed. Feeding damage often indicates the mouthparts of the offending insect. Some have chewing mouthparts, others have piercing-sucking mouthparts, and yet others have rasping, saw-like mouthparts. The resultant damage may appear as ragged holes in leaves, small, pin-prick-sized necrotic tissue, or ragged edges of a leaf (Figs. 9-4 through 9-15, see pages 154-157). Insects with chewing mouthparts include beetles, grasshoppers, grubs, and caterpillars. Those with piercing-sucking mouthparts include aphids, scales, whiteflies, and leafhoppers. Mites also pierce and suck. Thrips have rasping-sucking mouthparts.
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Insects can harm plants in a number of ways, with feeding being the most obvious. In addition, they also act as vectors for viruses or disease pathogens, they bore holes into plants to lay eggs inside them, and they exude honeydew to support fungal growth (Fig. 9-16). Nematodes are microscopic worms that may live in the soil or inside plants, depending on the species. The population of those that reside in soil can build up if susceptible plants are grown repeatedly in the same location. Crop rotation can help reduce nematode populations in the soil. Some species of nematodes are beneficial, as they eat plant pests and sometimes pathogens (Fig. 9-17).
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INSECT LIFE CYCLES
If you want to control pests on plants, it is good to understand the life cycle of the pest with which you are dealing. It is especially helpful to learn when they appear during the season, if they lay eggs, where they lay their eggs, and what stage of their life cycle causes the damage to plants. Furthermore, many insects experience more rapid development as the temperature warms up in the spring. Their emergence is determined by degree days (see chapter 7 for a discussion on degree days), not calendar days. Thus, the warmer the spring is, the sooner insect pests will emerge. The same is true in the greenhouse environment, in that the warmer it is, the faster insects will go through a life cycle, and, if a chemical control strategy is used, the more often chemical controls will be required.
Many species of insects undergo metamorphosis from egg to larvae to adult. In addition to these phases, some insects experience a pupal stage in which they undergo drastic changes in morphology, often from a crawling to a winged body (Fig. 9-18). Insect pests of plants may undergo either complete or gradual metamorphosis. There are two other types of insect metamorphosis, none and incomplete, but most insects in these groups are not plant pests.
COMPLETE METAMORPHOSIS. Complete metamorphosis has four stages: egg, larva, pupa, and adult. Butterflies and moths undergo complete metamorphosis. The larval form is called a caterpillar. This is the most damaging stage of the butterfly and moth life cycle. Larvae consume vast amounts of vegetation before pupation. Adults generally feed on nectar and are usually short-lived. The cycle is complete when the adults lay eggs. Other insects with complete metamorphosis, such as the Colorado potato beetle have larvae and adults that both damage plants.
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GRADUAL METAMORPHOSIS. Gradual metamorphosis has three stages: egg, nymph, and adult. Some species have several nymphal stages in which the body form is similar but becomes larger in each stage until finally the adult form emerges. Grasshoppers undergo gradual metamorphosis. Nymphs do not have wings, as adults do, but both stages eat vegetation and are therefore the most damaging stages of the life cycle. Egg-laying again completes the life cycle.
As nymphs and larvae grow, they may have to molt, or shed their skin. Molting involves the loss of the old exoskeleton followed by a period of vulnerability in which the new skin is soft and pliable. The stage between molts is called an instar. The number of molts and instars varies by species.
PEST CONTROL METHODS
When infestations are noticed early, it may be possible to eradicate them manually. This involves removing the offending pest by hand. This only works if there is lots of low-cost labor or very small acreage, as in the home garden. Favorable environmental conditions allow rapid life cycles and result in an exponential increase in insect pests. Often a pest such as tomato hornworm is not present in large quantities, so simply removing them as they appear is enough to maintain total control. Manual methods may be used in conjunction with other controls, depending on how many plants are infected and the convenience and efficiency of manual removal.
Cultural control methods rely on strict adherence to sanitary garden management and keen observation. Insects use scents called pheromones for communication and mate attraction. These may be synthesized and used to trap insects in special pheromone traps.
Many insects are attracted to the color yellow, so this attraction is exploited in the use of yellow sticky cards in greenhouses. Because thrips are more attracted to the color blue, blue sticky cards are used to attract them. Pheromone traps and sticky cards can be used to trap pests or to monitor their presence.
TRAP CROP. Trap crops are planted near the desired plant to attract pests to them, thus saving the desired food plant from invasion. It is even acceptable in organic gardening practices to then spray a pesticide on the trap crop to control the insect species, while maintaining a pesticide-free food crop.
MONITORING. Monitoring insect pests for threshold levels is an important component of integrated pest management (IPM). This tool can help in the decision as to when to apply pesticides and which pesticide to use. Threshold levels are based on aesthetic and economic considerations. For the cut flower and potted flowering plant market segments, little to no damage can be tolerated, whereas for other crops, some low level of damage may not harm the overall value of the crop. Economic considerations include the cost of applying pesticide balanced against the increased price that may be obtained by having a cleaner crop. As long as this is a negative value (i.e., the cost to apply is greater than the costs recuperated in sales), it is not feasible to apply pesticide. Costs of applying pesticides include the actual cost of the pesticide plus the time required for applying, use of equipment, and other incidental costs.
BARRIERS. Some greenhouses install physical barriers to pests and diseases. For example, ventilators may be covered with screening fabric to keep out small-sized insects. Double-door entryways may reduce the introduction of pests through doorways, and foot baths containing a disinfectant solution and placed at the entryway can reduce spread of disease pathogens carried on shoes.
Another type of barrier that can be readily applied in the garden is the use of row cover fabric to prevent pests from getting to the plants. Often this is necessary only for a window of time when the adult pests emerge and are looking for a place to feed. If they cannot access your plants, they will move on and not return. Understanding degree days can aid in monitoring for adult pest emergence and allow for effective preventive measures. Other barriers include collars placed around the base of individual plants or trees to prevent crawling pests from reaching the top of the plant. Tree wraps can protect fruit trees from rabbit and vole damage.
COMPANION PLANTS. Gardeners can benefit from the use of companion plants. This topic includes a lot of folklore, and scientists are just beginning to explore some of the claims. It is now known that some cultivars of marigolds exude a substance called thiopene through their roots that repels nematodes. Tomatoes are well-known for attracting root-knot nematodes. The nematode population builds up if tomatoes are grown in the same location in successive years. Marigolds can be used in rotation with tomatoes to effectively reduce root-knot nematode populations in the soil. There are many examples of companion plantings, but not all of them have been scientifically studied. There are probably sound reasons for the practice of companion planting, as this is a practice that has been used for a very long time.
CROP ROTATION. It is important to understand pest life cycles and whether they overwinter in the soil. In the case of those that do, crop rotation can provide a method of avoiding infestation in successive years. Because it also works for disease pests, crop rotation is always recommended. As a general rule of thumb, crops should be rotated with those from other families (i.e., do not follow a solanaceous crop such as eggplants or tomatoes with another solanaceous crop such as potatoes). Also, root, leaf, and fruiting crops should be rotated with each other.
Traditional breeding techniques have produced some crops with insect resistance, although these are not available on the same scale as those bred with resistance to various diseases. Plant breeders look for plants that produce chemicals toxic to insects or those that have traits that make them undesirable to insects, such as thick cuticles or dense trichomes. Other plants withstand or outgrow the damage caused by insects and may have qualities that can be bred into improved cultivars. Molecular genetic procedures are used to develop insect-resistant plants while bypassing the lengthy process of traditional breeding. One common gene that is introduced using this technique is Bt from the bacterium Bacillus thuringiensis, which allows the plant to produce its own insecticide. These crops will be resistant to certain lepidopterous insects.
When purchasing seeds or plants, seek out resistant and tolerant cultivars and avoid susceptible ones. As each crop species has a particular pest complex, familiarize yourself with these problems before selecting what you will grow. Also, certain pests are more of a problem in certain geographical areas, so consult your local extension service personnel for guidance and information.
Pesticides are available in a large variety of chemical and nonchemical formulations. In general, they may be either selective or nonselective in their activity on insects. Selective pesticides are very specific in which pests they control. Nonselective pesticides have a wider range and may kill all insects or a large group of related species. If you want to maintain beneficial insects, such as lady beetles and green lacewings, use selective pesticides.
Some pesticides may work only when they contact the insect directly; otherwise the pest will not be affected. These are called contact pesticides. Systemic pesticides are applied to the plant and translocated throughout it. When insect pests feed on the plant, they ingest the toxin and are killed.
Chemical controls fall into several major categories, based on their mode of action or their chemical class. Organophosphates, carbamates, insect growth regulators (juvenile growth hormones), and ovicides are some examples of insecticides. Conventional synthetic insecticides include chemicals such as diazinon, carbaryl, malathion, and methoxychlor.
Juvenile growth hormones target a specific pest at a specific point in its life cycle, causing its development to cease and not allowing it to metamorphose into its adult stage. Ovicides are effective on eggs. This is important, because many insecticides do not penetrate or affect the eggs and thus do not offer complete control of a pest. Other insect controls (biological, beneficial insects, natural derivatives, and horticultural soaps and oils) are discussed in the sections that follow.
The use of Bacillus thurengensis, or Bt, as it is commonly called, has made a major contribution to pest control in the United States in the past 20 or so years. It is a species of bacteria that kills lepidopterous insect larvae (caterpillars) when they ingest it. Bt is available in powders or can be mixed in a solution and sprayed on. It is not toxic to humans or animals. In fact, it is considered so safe that it is one of the controls approved for organic farming practices.
Predator insects work in several different ways. They may simply feed on the insect pest, or they may parasitize it. Parasitizing wasps and tachinid flies lay their eggs in a host insect, such as a caterpillar (Figs. 9-19 and 9-20). When the eggs hatch, the larvae feed on the host insect and kill it, after which the larvae emerge and complete their development. Green lacewing larvae (Fig. 9-21), lady beetles (Fig. 9-22), praying mantises (nymphs and adults), ambush bugs, and spiders are carnivorous and will feed on insect pests (see Fig. 9-17). Predator insects exist in the environment, and their existence should be encouraged by limiting the uses of nonselective pesticides. In addition to their natural occurrence, predator insects can be purchased, and their use will be more successful if certain guidelines are followed.
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There are naturally occurring chemicals that have pesticidal properties. These chemicals are often considered to be less toxic to the environment, although they may be more toxic than some synthetic chemicals. Nicotine is commonly used as a greenhouse fumigant. Pyrethrum is a toxic chemical produced by the flowering perennial, Chrysanthemum coccineum (Fig. 9-23). Synthetic compounds that resemble pyrethrum are called pyrethrins.
Azadirachtin is derived from the neem tree and has been found to have pesticidal activity. It is sold under a variety of trade names such as Azatrol, Align, Azatin, and Turplex.
Horticultural soaps and oils are effective against some pests and are a method of low to no toxicity or danger. Most insecticidal soaps contain salts of potassium or sodium, which dry the cuticle of soft insects such as whiteflies and aphids. Horticultural oils may be used to control aphids, lacebugs, spider mites, thrips, and whiteflies. The oil suffocates both adults and eggs and thus effectively reduces insect populations. Horticultural oil should not be applied when temperatures exceed 90[degrees]F or when plants are stressed or in bloom.
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Alien Pest Species
Some insects have become serious pests because they were introduced into an area where they lack natural predators. Some examples include gypsy moth, potato beetle, and emerald ash borer. Host plants also have not had time to evolve natural defense mechanisms against such exotic pests. Despite efforts by the Animal and Plant Health Inspection Service (see box) to prevent entry of potentially damaging pests into this country, they are still able to enter on clothing, airplane wheel wells, furniture, and so on.
IPM continues to develop as its use becomes widespread throughout the practice of agriculture and horticulture. Components of IPM in pest control include some of the areas discussed in other sections of this chapter: using genetic controls, using beneficial insects, using chemicals with lower toxicity, monitoring pest emergence and population size, and targeting susceptible stages of the pest life cycle. The USDA Agricultural Research Service (ARS) conducts extensive research on IPM programs for economically important crops.
ANIMAL AND BIRD PESTS
Animals and birds often cause greater problems than diseases or insects (Fig. 9-24). Deer eat many different fruits, vegetables, and landscape plants. In some areas they are not easily frightened by the presence of humans. As suburban areas grow into traditionally rural areas, this problem is expected to increase. For some people, the only solution is to abandon gardening altogether or to construct wire cages and fencing around everything. Some deer-resistant flowers and woody perennials are listed in chapters 16 and 17.
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Plant pests can be specific to certain plants, or they may generalize and feed on a large number of plant species. In addition to the damage they cause by their feeding, they may also transmit plant pathogens. Insects usually have three pairs of legs whereas spiders and mites have four pairs. Their droppings are known as frass, which can sometimes aid in identifying them. Honeydew is another type of insect excrement. Insects have complete or gradual life cycles. Their development from egg to larva to adult is strongly affected by temperature. Degree days have been calculated to measure this response in some insect pests. Insect pests may be controlled in numerous ways, including multiple techniques referred to as integrated pest management.
* Study one crop and a major pest of that crop. Research the pest's life cycle and conditions for its successful survival. Identify IPM techniques used to monitor and control this pest. What other plants does this insect feed on? Where does this pest lay its eggs?
* Interview a farmer or nursery or greenhouse grower in your area. Ask him or her about major pest problems and strategies used to control them. Write a report about your findings.
* Interview a long-time local grower about pest control strategies they use today compared with what they would have done 20 or 30 years ago. How has the challenge of pest control changed, based on their experience? What kind of changes do they see for the future of pest management?
1. What three body features distinguish insects from other arthropods?
2. Insect droppings are called--.
3. How many general stages are there in complete metamorphosis? Name them.
4. How many general stages are there in gradual metamorphosis? Name them.
5. What is molting?
6. What is an instar?
7. Compare and contrast selective and nonselective pesticides.
8. How does a systemic pesticide work?
9. What are pheromones? How does a pheromone trap work?
10. Discuss how parasitic wasps can control insect pests.
Lindquist, R. K. (1998). Identification of insects and related pests of horticultural plants. Columbus, OH: O.F.A. Services.
Nixon, P. L., Anderson C. D., Pataky N. R., Wolf, R. E., Ferree, R. J., & Bode, L. E. (1995). Illinois pesticide applicator training manual. Champaign, IL: University of Illinois at Urbana-Champaign.
Orton, D., & Green, T. Coincide. Carol Stream, IL: Labor of Love Conservancy.
APHIS: The Animal and Plant Health Inspection Service
APHIS "is responsible for protecting and promoting U.S. agricultural health, administering the Animal Welfare Act, and carrying out wildlife damage management activities."
Their mission was developed by the U.S. Department of Agriculture (USDA) in its efforts to ensure safe and affordable food. If such protection was not available, threats to America's food supply would be enormous. Some examples of APHIS protection involve the Mediterranean fruit fly and the Asian longhorned beetle. Both of these are major agricultural pests. If their entry into the country was not monitored, several billions of dollars in production and marketing losses would occur annually.
APHIS does its work by inspecting produce and livestock entering the country at our borders. They may quarantine products known to be vulnerable to a pest to determine whether the pest is indeed present.
Dr. Marietta Loehrlein currently teaches horticulture classes at Western Illinois University in Macomb, Illinois. She earned both her bachelor's degree in Agronomy and her master's degree in Plant Genetics at The University of Arizona. Her master's research project was concerned with germination problems associated with triploid seeds, from which seedless watermelons grow. Following that she worked for 5 years in a breeding and research program for Sunworld, International near Bakersfield, California. She worked with peaches, nectarines, plums, apricots, and cherries. Then she returned to school to earn her Ph.D. in Horticultural Genetics at The Pennsylvania State University. Her Ph.D. research focused on flowering processes in regal pelargonium (also called Martha Washington geraniums). While at The Pennsylvania State University, she bred a new cultivar of regal pelargonium, "Camelot." At Western Illinois University, Dr. Loehrlein teaches nine courses: Greenhouse and Nursery Management, Introductory Horticulture, Landscape Design, Landscape Management, Home Horticulture, Plant Propagation, Turf Management, and two courses in Plant Identification.
TABLE 9-1 Major Pests of Plants DAMAGING DESCRIPTION AND PEST STAGE(S) DAMAGE Arachnida (spiders and mites) Mites Adult Small, spider-like, having eight legs. Pierce and suck plant juices, creating pinprick holes. May construct webbing on plant. Some cause gall formation. Coleoptera (beetles, larvae worm-like or grub) Bean leaf Larvae Adult is yellow to red with beetle four black spots on back. Colorado Adults and Yellow- and black-striped potato larvae beetle. Larvae are red beetle with black legs and head and have two rows of black spots. Cucumber Adults and Larvae is Southern corn beetle larvae rootworm. Adult is (spotted yellow beetle with and black spots or yellow striped) with black stripes. Japanese Larvae, adults Adult is metallic green; beetle larva is a white grub. Various beetle Larvae White or brownish white grubs C-shaped larvae. Adults vary by species. Diptera (fly) Apple maggot Larvae Adult is a small, dark fly (maggots) brown fly. Larvae are white, tapered maggots Fungus gnat Larvae Small black winged adult. Larvae are translucent white with shiny black heads. Leafminers Larvae feeds, Adult is a winged fly adult whereas larvae punctures by resemble caterpillars. laying eggs Larvae create serpentine tunnels in inner layer of leaf cells where they pupate. Hemiptera/Homoptera (true bugs) Aphids Adult Pear-shaped, variously colored. Give live birth. Pierces and sucks plant juices, excretes honeydew. Chinch bug Larvae (nymphs) Black body with white and adults wings. Wing length varies by species. Larvae may be red or black, depending on stage. Eggs are white with a red dot. Leaf hoppers Nymphs Colors and sizes vary by species. Nymphs suck phloem and excrete honeydew. Mealybug Nymphs suck Nymphs suck phloem and phloem, excrete honeydew. excrete honeydew Scales (soft Adults Females wingless, covered and with waxy or hard armored) covering. Round or oval shaped. Tarnished Adults and older Brown bug with yellow, plant bug nymphs black, and white markings. Nymphs resemble adults but lack wings; they have wing buds instead. Whiteflies Adult pierces Pinprick holes in leaves, and sucks honeydew excretion. leaf juices Lepidoptera (butterflies and moths, larvae are caterpillars) Cabbage Larvae Green larva with white looper stripes running lengthwise, looper-type movement, adult is a brown and gray moth. Cabbage moth Larvae Green larva, adult is cream colored moth. Big holes in leaves to complete defoliation. Corn earworm Larvae Tan to green striped caterpillars European Larvae Adult moth is cream to corn borer beige with brown markings. Tomato/ Larvae Larva has green body with tobacco conspicuous "horn" on hornworm the rear end. Green frass, holes in leaves, leaves eaten down to the midrib, complete defoliation. Adult is a sphinx or hawk moth. Orthoptera (crickets and grasshoppers) Grasshoppers Adult Hind pair of legs enlarged for jumping. Large fan- shaped back wings provide ability to appear to fly. Mole cricket Larvae and Brown insect having two adult? pairs of wings. Front pair of legs shaped for digging, just like a mole's. Nematoda (various orders) Nematodes Adult Microscopic worm found in roots, stems, leaves. Thysanoptera (thrips) Thrips Adult and larvae Adult has slender pale to blackish minute body; color varies by species. Rasping mouthparts. Dermaptera (earwigs) Earwig Adult Dark brown to black, narrow long body with conspicuous pincers on rear. Stylommatophora (gastropods) Snails and Adult Slimy body with shell slugs (snail) or lacking shell (slug) and having antennae. AFFECTED PLANTS/ PEST ORGANS Mites Stippled leaves on many ornamental crops and greenhouse crops. Two-spotted and red spider mites favor dry or droughty conditions. Bean leaf Beans of various species beetle and corn. Larvae feed on roots and other underground plant parts. Feeding damage may girdle young plants. Colorado Potato, tomato, eggplant, potato pepper beetle Cucumber Chew holes and tunnel beetle into roots, stems, (spotted shoots, and leaves of and cucurbits, beans, corn, striped) peanuts, potatoes and other vegetables. Transmit bacterial wilt. Japanese Adults feed on ornamental beetle plant foliage and skeletonize leaves. Larvae feed on roots of living plants. Various beetle Feed on roots. Adults may grubs emerge and feed, as with Japanese beetle. Are easily found by digging in soil. May be Japanese beetle, May or June beetles. Apple maggot Maggots tunnel through fly flesh of apples, pears, and cherries. Egg punctures in fruit may be covered with white wax. Fungus gnat Lay eggs in soil, larvae hatch and feed on roots, spread pathogens, and may enter stems to feed. Leafminers Foliage on a wide range of host plants. Aphids Pinprick feeding holes; honeydew encourages sooty mold; can vector viruses. Plant becomes covered with aphids as they give live birth while feeding. Chinch bug Turfgrass species. Irregular patches of turf turn yellow and then die. Adults and nymphs feed on turf foliage in mainly sunny areas. Leaf hoppers Plant juices of many different plants and crops, including grasses, sedges, broad-leafed woody and herbaceous plants, and conifers. Transmit pathogens. Mealybug Many plants, major ones are hibiscus, grapes, foliage plants, flowers, and vegetables. Found on leaf nodes, leaves, crotches, roots. Honeydew results in sooty mold. Scales (soft Orchard trees, ornamental and shrubs; stems, leaves. armored) Soft scales produce honeydew and can move during their life cycle; armored scales cannot. Tarnished Terminal flower shoots plant bug punctured; flowers wilt and die. Leaf bud spotting, a general bronzing on leaves. Catfacing on fruit, fruit drop on tree fruits and vegetables. Whiteflies Many plants. Leaves. Eggs laid on undersides of leaves. Cabbage Plants in the Cruciferae looper (mustard) family. All parts of plant. Cabbage moth Plants in the Brassica family. All parts of plant. Corn earworm Corn, especially the ears; tomato, gladiolus, others. European Corn. Larvae bore into corn borer stems, stalks of corn, may cause lodging (toppling over) due to stem damage. Tomato/ Tomato, tobacco, other tobacco solanaceous plants. hornworm They feed on foliage and tomato fruits. Grasshoppers Irregular holes in leaves, chewing at leaf margin. Leaves may be eaten down to the petiole or midrib. Mole cricket Tunnels through roots of turf grasses. Nematodes Hundreds of species of ornamental and food crops are affected. Thrips Hide in the flowers or growing point of leaves, feed on developing leaves. Vector viruses. Cause ring-shaped blemishes on citrus. Earwig Flowers, vegetables, fruits; small, irregular holes in leaves. Eats decaying matter. Snails and Foliage of ornamental slugs plants, hostas.
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|Author:||Loehrlein, Marietta M.|
|Publication:||Home Horticulture: Principles and Practices|
|Date:||Jan 1, 2008|
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