Crop Pests Chapter 9 85 Crop pest prevention and control, as well as actual yield loss, account for a large proportion of the annual monetary cost experienced by producers. In addition, pests cost the consumer in terms of research dollars, potential health effects, and wages. Pests affect the crop in various ways, including decreasing photosynthetic capacity; competing for water, nutrients, and light; causing a loss of plant tissue; or initiating premature death. Although the possible causes of decreased crop health and growth are many, the majority of the problems are common and easily recognizable. These problems should always be considered and eliminated first ( if you hear hoof beats, look for horses, not zebras ). Knowledge of some primary pests of field crops, as well as some common characteristics of all pests, can assist a crop scout or producer in narrowing down the cause of a problem and in finding a solution as quickly as possible. Objectives Describe basic principles and terms used in identification of crop pests (insects, diseases, and weeds). Recognize the names and general methods of damage caused by important pests of corn, soybean, small grains, and forages. Discussion Crop Pests The degree of crop pest (insects, diseases, including nematodes, and weeds) infestation during each growing season depends upon several factors. The environment must be favorable, and optimal conditions vary for each pest species hot and dry, cool and wet, hot and humid. For many diseases, especially fungal diseases, a moist environment is necessary for infection. Any environmental condition that favors growth of the crop also favors weed growth, although optimal circumstances for crop development may allow the crop to outcompete other plants. In general, stressed plants are more susceptible to pest infestation, so management practices that produce healthy crops (such as providing proper nutrition) not only increase yield but also decrease the likelihood of pest outbreaks. Control Tactics Figure 9.1. Genetic tolerance to Northern corn leaf blight results in smaller leaf lesions. Producers have several common control tactics available to assist in prevention of pest infestation. Resistant varieties have been genetically modified, either through traditional breeding or genetic engineering, either to resist or to tolerate insect or disease infection (Figure 9.1). Recent genetic developments, such as Bt corn, have produced crops that actually kill the pests as they attempt to feed on the plant. For weeds, crops have been bred to be resistant to herbicides,
86 Figure 9.2. Gray leaf spot of corn is more prevalent in reduced tillage situations. allowing chemical application that kills weeds but not the crop. Use of resistant varieties has resulted in widespread decreases in pesticide use. Crop rotation can disrupt the life cycle of a soil-borne insect or disease pest that has a specific plant host feeding requirement. Weed control can be assisted through crop rotation by allowing use of differing herbicide modes of action. If a pest needs a specific food source, either crop debris or a weedy host, to complete its life cycle, tillage and cultivation are helpful. For example, incidence of gray leaf spot in corn can be reduced by tillage (Figure 9.2). A common alternative to chemical weed control in row crops is cultivation. Clean equipment and seed involve preventing the spread of pests by using sanitation in production practices. Adjusting the time of planting and harvest is also helpful. For example, wheat is commonly planted past the fly-free date to control Hessian fly. Corn with a high occurrence of stalk and/or ear rot can be harvested early to prevent stalk breakage and ear drop. Finally, chemical application, including insecticides, fungicides, and herbicides, is a useful tool for pest control. A careful balance of all control methods is important to preserving technology and the environment. Integrated pest management (IPM) is a term used to describe the utilization of many types of pest control methods. Insects Insect biology Figure 9.3. Grasshoppers can become serious pests of many crops. True insects are defined as invertebrate members of the phylum Arthropoda, and of the class Insecta characterized by bilateral symmetry, a chitinous exoskeleton, and bodies segmented into three separate regions (head, abdomen, and thorax). Insects have three pairs of thoracic legs, two pairs of wings, and one pair of antennae. Pests of field crops include various insects, such as worms, maggots, beetles, aphids, and moths. Several insect-type pests, including mites, are members of the class Arachnida. Insects can have one of two types of life cycles. Insects with a complete life cycle develop from egg to larva to pupa to adult. Often, larval insects vary greatly from adults of the same species in their habitat and feeding preferences. Insects in the pupal stage are often difficult to control. Insects with an incomplete or gradual life cycle hatch from eggs to nymphs, which resemble small versions of the adult insect. Nymphs and adults often have identical feeding sites. All insects go through several transformations, called molts, as they become adults. At each molt, the insect, termed an instar, has a specific form and size. Insect pests of field crops Figure 9.4. Adult Japanese beetles feed on foliage and flowers of crop plants. General insect pests Several insects are pests of more than one crop species. Grasshoppers eat any crop and any plant part, including leaves, stems, and fruit (Figure 9.3). Damage is from both the nymph and adult stages, as the nymphs begin feeding on plants in ditches and fencerows and then move into fields as they mature. Fall armyworm and armyworm usually feed on grass Chapter 9 Crop Sciences Laboratory Manual
plants but may also be found in legumes. The larval stage is destructive, feeding on plant foliage, mostly at night. The adult is a moth. The larvae of the seedcorn maggot feed on germinating seeds of corn and soybean, damaging the seedling and also allowing entry of seed rot diseases. The adult of this insect is a small fly. The larvae of the wireworm are small, yellow to red brown grubs that feed on the roots of a wide range of plants. The adult form of the wireworm is a click beetle. A more recent addition to the list of harmful insects is the Japanese beetle (Figure 9.4). The larva is a small grub that causes minor damage to plant roots, but the adult is a voracious iridescent beetle that feeds on the vegetation or flowers of a wide range of plants. Insect pests of corn 87 Figure 9.5. Corn rootworm beetle larvae can cause serious damage to corn roots. One of the most detrimental insect pests of corn is the corn rootworm beetle, with western, northern, and southern types found in the Midwest. The larvae feed on corn roots, causing decreased absorption and increased plant lodging (Figure 9.5). The adult beetle can cause major damage by feeding on pollen and silks (called clipping). Because this insect previously had laid eggs only in corn, with larvae feeding on corn roots the following year, crop rotation had been an important nonchemical control measure. Recent corn rootworm beetle biotypes have adapted to laying eggs in soybean, allowing a resurgence of this pest. The European corn borer (ECB) is another major corn insect pest. The first- and second-generation larvae feed on plant foliage, leaving a buckshot appearance as they feed through the leaves in the whorl. Third-generation larvae tunnel into the stalk, disrupting the flow of water, nutrients, and photosynthate, and encouraging stalk lodging and ear drop. The adult is a moth that does not cause damage. Figure 9.6. Corn earworm larvae feed on developing kernels. One of the lesser pests of corn is the black cutworm. The larvae of this insect feed on young corn plants at or just below the soil level, completely severing the plant. Stalk (corn) borer larvae chew through leaves and bore into the stalk. The larvae of the corn earworm feed on developing kernels, destroying grain and allowing the entry of ear disease (Figure 9.6). The adult form of each of these insects is a moth, which causes no damage. Another potential insect pest of corn is the corn aphid, whose nymph and adult forms can cover the tassel to such an extent that production of pollen is prevented. Western bean cutworm is a new pest of corn moving into Illinois from the western United States. Larvae of this insect feed first on the developing tassel and then move to the ear where they cause serious damage to kernels. The adult is a moth. Figure 9.7. Bean leaf beetles can be distinguished by a black triangle at the base of the wing covers. Advances in genetic engineering have led to the development of Bt corn, which has been modified to include the genetic code for the protein Bacillus thuringiensis (Bt). When the insect consumes plant material containing this gene, the protein enters the digestive system of the larvae and creates a crystal
88 substance that ruptures the insect s gut. Production of this crystal is specific only to certain insects of the Lepidoptera family, such as European corn borer, and is considered safe to all other life forms. Additional forms of Bt have recently been created for control of other corn insects such as armyworm and corn rootworm beetle. Bt corn is attractive for control of insects such as ECB that feed within the plant, making them difficult to control chemically. The use of Bt corn is increasing annually, and the use of insecticides is decreasing. Insect pests of soybean Figure 9.8. Soybean aphids are small insects found on lower leaf surfaces. Insect pests of soybean include the bean leaf beetle, which as an adult can be yellow, green, tan, or red, but which always displays a black triangle at the base of the wing covers (Figure 9.7). Damage occurs from both the larval and adult forms. The larvae feed on roots and root nodules but are typically not of economic importance. The adult beetle eats cotyledons, stems, leaves, and pods. The two spotted spider mite uses piercing sucking mouthparts to puncture leaf cells and remove juices. Infestations are often associated with hot, dry weather. The soybean aphid appeared in the Midwest from Asia in 2000 and is increasing in distribution rapidly (Figure 9.8). The aphids are small and yellow green, with distinct black appendages called cornicles or tailpipes appearing on the abdomen. This insect siphons fluid from the plant and, in addition to decreasing photosynthetic capacity, also causes a decline in plant pod number. Soybean aphids are also vectors to viral diseases, such as soybean mosaic virus (Figure 9.9). Insect pests of alfalfa Figure 9.9. Soybean aphids are vectors of soybean mosaic virus, which causes leaf puckering and necrosis. The alfalfa weevil larvae eat the growing meristem of alfalfa leaves, first leaving pinholes and then completely skeltonizing leaves. Adults also feed on leaves, but damage is usually of little economic importance. Adult weevils have a characteristic snout. The nymph and adult stages of the potato leafhopper suck fluids from leaves, initially resulting in a V-shaped yellow area at the tip of the leaflet, termed hopper burn. This pest is a green, wedge-shaped insect that is generally more damaging during dry conditions. The blister beetle larvae hatch and feed on grasshopper eggs, but the adult beetles feed on the flowers and sometimes the leaves of the plants. Blister beetles are most damaging as a chemical in hay, as their bodies contain a contaminant that result in large blisters on the skin or internal organs of people or animals that may become infected if not treated properly. Insect pests of small grains Figure 9.10. Phytophthora seedling blight of soybean results in stunted plants. Several insect pests can be found particular to cereal crops. The sluglike cereal leaf beetle larvae feed on the leaf surface, giving the plant a silvery appearance. The adult beetle has a metallic blue black head and wing covers with red orange legs and front. Adult cereal leaf beetles also feed on cereal leaves but cause little economic damage. The Hessian fly pupae, called flaxseeds, are found in the crown of plants. When Chapter 9 Crop Sciences Laboratory Manual
hatched, the larvae begin to feed on plant stems, causing stunting and death. Hessian fly adults, which live for only a few days, must lay eggs on live cereal plants. Waiting to plant until after the fly-free date can help control this insect by preventing the presence of a suitable egg host. Resistant varieties are also an important control method for this pest. Diseases Disease biology 89 Figure 9.11. Frogeyespot of corn produces distinctive leaf lesions. A plant disease can be described as abnormal definition of host plant that interferes with normal appearance, growth structure, or function. Plant diseases can be noninfectious, caused by nonliving factors (such as air pollution or herbicide damage), or infectious, caused by living (biotic) agents. The agent causing infectious disease (such as bacterium, fungus, virus, or nematode), is called the pathogen. The portion of the pathogen that causes the disease is called the inoculum. Diseases of field crops General diseases Figure 9.12. Soybean leaves infected with bacterial blight display chlorotic and necrotic lesions. Several general categories of diseases are common to many field crops. Seedling blight is a widespread term for diseases that affect young seedlings. Depending on the disease present, seedling blights can cause lesions and cankers on stems or soft rots of stems and roots. Pythium, phytophthora, rhizoctonia, and fusarium are all types of seedling blights (Figure 9.10). Leaf blight is also a general term for many types of leaf diseases, such as northern corn leaf blight or frogeyespot of corn, bacterial blight or bacterial pustule of soybean, bacterial wilt of alfalfa, and septoria of small grains (Figures 9.11 to 9.14). Figure 9.14. Pepperspot stemphyllium of alfalfa is a common leaf disease. Figure 9.13. Bacterial pustule causes necrotic leaf lesions.