Plant Pests People depend on plants for many reasons. They provide food, wood, fibers for clothing, even oxygen for us to breathe. We grow plants for these and many more useful products as well as for the beauty of a world filled with trees, flowers and other greenery. Living things that eat or damage the plants we grow are called pests. Throughout human history, crop failures due to pests have caused tremendous suffering. In 1845, the potato crop in Ireland was suddenly destroyed by a fungus disease. One day the potatoes looked fine; a few days later, the leaves had turned black and the potatoes were rotten! The disease destroyed most of the potato crop in Ireland for the next several years, causing the Potato Famine of 1847. It is estimated that in only a few years, as many as 2 million people died of starvation in Ireland because of this one plant disease. Several million more people fled to the United States to escape the famine. The appearance of vast destructive swarms of locusts, a type of grasshopper, are described in the Bible and still happen today. The locusts gather together in huge groups and fly hundreds of miles seeking food. A single locust swarm may contain millions of insects and darken the sky as it flies over! When the locusts land, they rapidly eat everything, stripping the area of all crops, tree leaves, and grass. Dramatic pest disasters such as the Irish potato famine and locust plagues are unusual. Most of the time pests exist in smaller numbers, causing damage to crops and plants, but not total destruction. People also create pest problems, such as when they grow large fields of the same crop, providing pests with huge food supplies, or when they transport crops with hidden pests from one area to another. Plants may also be damaged by air pollution and toxic wastes produced by people. The Enemies of Plants There are many kinds of plant pests. Insects, mites, birds, and animals sometimes eat crops before people can harvest them. Plant diseases can cause crops to suddenly die, or make the crop rot before it can reach market. Imported Pests In its native land, a pest has many enemies, such as parasites (a living thing that feeds and grows on another living thing), predators, and diseases, that keep its numbers small. However, in places where none of the pest's usual enemies exist, a pest problem can develop quickly. The Japanese beetle is a shiny green and gold insect. It lays its eggs in grassy areas, and the young, called grubs, eat grass roots for a year until they emerge as adults in midsummer. The adult Japanese beetle eats a wide variety of plants, including many cultivated plants. In Japan, this insect is not a pest because there are many other insects that eat the beetle. There are also many diseases that kill most of the grubs. In the United States, however, the Japanese beetle became a serious pest. The grubs damaged lawns, and the adult beetles ate just about everything in the garden! The number of beetles in many areas was astounding. Now this beetle is controlled with a combination of chemical sprays and a disease called milky spore. The milky spore disease, which is grown in laboratories, is made into a powder and sprinkled on lawns, killing most of the grubs without harming the grass. The gypsy moth was brought into the United States by a scientist who wanted to use it to produce a hardy 1 of 6 7/2/09 1:31 PM

silkworm. In Europe, its native land, the gypsy moth is not a serious pest. Unfortunately, it escaped from the laboratory and established itself in the forests of Massachusetts. The gypsy moth caterpillars eat the leaves of most trees. In the absence of the European parasites, diseases, and predators that kill the gypsy moth, its population exploded in the United States. Without control measures, the caterpillars are capable of eating the leaves off of every tree in the forest--making it look like winter in June! The gypsy moth is spreading slowly southward along the East Coast of the United States. Over time, parasites will appear and diseases will develop that affect the gypsy moth in the United States. But left to nature, this process could take hundreds of years. Scientists are therefore developing diseases, parasites, and other enemies to control the gypsy moth. Many other exotic pests have caused large amounts of damage. The chestnut blight, a fungus disease, has killed the American chestnut tree throughout its entire natural range. Another fungus disease, Dutch elm disease, has killed many elm trees. The Mediterranean fruit fly, or Med fly, became a major pest after its arrival in California. The bacterial disease citrus canker caused similar problems in Florida when it was found in some nurseries. Weeds Farmers are always working to protect their crops from all kinds of enemies. They probably spend the largest share of their time and energy controlling weeds--any plant that is growing where it is not wanted. Weeds compete with the plant crop for water, sunlight, and soil nutrients. In addition, some weeds actually harm the crop. The witchweed is a parasitic plant that grows next to the crop plant. The witchweed attaches its roots to the crop roots, taking the food that the crop plant is making for itself. In fields with witchweed, the crop ends up stunted (smaller than normal). Dodder is another common parasitic plant. It looks like yellow or orange spaghetti. It twines around the stems of the crop, and sends plant shoots, or suckers, into the stem to take water and food from the crop. Insects and Mites Insect pests come in many shapes and sizes. Caterpillars, beetles, beetle grubs, grasshoppers, and locusts are all chewing pests. They damage the plant as they bite off and eat pieces of leaves, fruits, or roots. You can tell that a chewing insect is at work when you see the holes made in leaves and fruit. Some chewing insects are hidden from sight. They bore into the stems and twigs of the plant. Once inside, the insects feed on the plant. The plant, or part of the plant, wilts as the insect tunnels through it. Other chewing insects dig into the soil to get at the plant roots. The adult corn rootworm is a beetle that eats corn leaves and lays its eggs in the soil. When the grubs hatch from the eggs, they eat the corn roots. Because of the damage to the roots, the plants wilt and are smaller than normal; sometimes they even fall over when the wind blows. Aphids, thrips, mealybugs, leafhoppers, scale insects, spider mites and many others are called sucking pests. Their mouths are specially formed for piercing plants and sucking the juices. When sucking pests feed on plants, they often stunt the plant's growth. The leaves of the plant curl and twist and develop blotches of yellow or brown. Some pests live in close co-operation within the plant. The pest causes the plant to grow an elaborate swelling called a gall. Gall pests do this by making chemicals similar to the plant's own growth hormones. 2 of 6 7/2/09 1:31 PM

These chemicals cause the plant to form the gall. Then the gall-producing insect or mite lives and feeds inside the gall. Tiny wasps cause lots of different galls on the leaves and twigs of oak trees. Diseases Plants get diseases just as people do. Diseases can stop plants from growing or make them grow in unnatural ways. Often diseases will kill plants. Plant diseases are caused by organisms so small they can only be seen with a microscope. Viruses, viroids, fungi, bacteria, and microscopic worms called nematodes can all cause plant diseases. But organisms that cause plant diseases do not ever cause human or animal diseases. They are specialized to attack only plants and usually only certain plants. When a wheat plant has a disease called wheat stem rust, you can see rusty-colored spots and streaks on the stems. If you rub the spots, a very fine red-brown powder comes off. The powder is made up of the microscopic spores of the wheat stem rust fungus. These tiny spores are easily carried long distances on wind currents to unaffected areas. Entire fields of wheat can be infected with wheat stem rust; if the disease is severe enough, no grain will be produced. When plants are infected with powdery mildew fungus, white moldy spots suddenly appear on the leaves and stems of the plants. The white spots are made up of fungal strands and tiny chains of spores. Plant leaves curl and eventually die when attacked by powdery mildew. If the leaves die, they can't make the food needed to produce the crop. Powdery mildews are serious pests of grain crops, such as wheat, and of many vegetables and fruits. They also can be found on trees and flowers and even on grass. Like the rust spores, these mildew spores are easily carried on wind currents to infect new plants. Some bacteria, fungi, and nematodes grow inside the plant, clogging it up so water and nutrients can't flow from the roots to the leaves. When this happens the plant wilts, the leaves may turn yellow, and the entire plant may die. Trees, flowers, vegetables and other crops can all have wilt diseases. In other cases, a plant may wilt because its roots become diseased. Many nematodes and fungi also damage plant roots. Plants invaded by the root-knot nematode have lumpy knotted-looking roots. The tiny nematodes live inside the roots. Like the gall insects and mites, the root-knot nematode is able to make the plant produce these root-knot homes. When a plant has a lot of nematodes living in the roots, it may not have enough food left to grow normally after feeding all of the hungry nematodes. Sometimes insect pests will carry a fungus, virus, bacterium, or nematode from a sick plant and put it into a healthy plant. The Dutch elm disease is a wilt disease of elm trees caused by a fungus. Tiny insects called elm bark beetles carry the fungus from sick trees to healthy trees. When this beetle emerges as an adult from under the bark, it carries many of the fungus spores on its body. The newly emerged adult beetles fly to healthy elms to feed on the sap. When the beetle chews a small hole in the elm twigs in order to feed, it puts the spores of the Dutch elm disease fungus into the tree. Within a few weeks the fungus will grow in the tree's feeding network. Parts of the tree will wilt as the system becomes clogged and is no longer able to spread nourishment throughout the tree. Sucking pests such as aphids can transfer plant diseases. If an aphid feeds on a plant with a virus disease and then flies to a healthy plant, it can carry the virus to the healthy plant. Vegetable plants are often infected with virus diseases originally found in weeds. The aphids feed on the virus-infected weeds, then fly into the crop and transfer the disease to the crop. Cucumber and squash plants produce very odd looking 3 of 6 7/2/09 1:31 PM

lumpy fruit if they are infected with a virus. Controlling Plant Pests Over the long history of agriculture many methods and strategies have been developed to protect crops from pests. It is necessary to understand the pests so that methods can be developed to keep them from increasing in number--to the point where they damage the crop. When pests are few in number, they are usually not a problem. In fact, a few pests are needed so the population of a pest's enemies, which help control the pest, can be maintained. The basic rules of pest control are: Grow a plant that is not susceptible to pests or diseases; grow the plant when or where the pests are not present; and if the pest appears, destroy it. Many of the basic practices used in agriculture play a role in controlling pests and diseases. Farmers plan to use seeds from the best plants for future crops. Often the best plants are those with pest resistance. Deep plowing buries weeds and any pests or diseases remaining from the previous crop. Burning the field before planting has long been used to destroy weeds and pests. Simply cutting the weeds on a regular basis is also a form of pest control. The farmer can grow a mixture of many varieties of the crop, so even if a pest or disease destroys some varieties, some will survive. Growing a different crop each year, a process called crop rotation, helps control pest populations. The pests and diseases do not build up in the field as they might if the same crop were grown year after year. Using the natural enemies of pests is called biological control. The milky spore disease, which was used to fight the Japanese beetle, was discovered by scientists of the United States Department of Agriculture. It has been used successfully for more than forty years to control Japanese beetles. While the Japanese beetle was considered a pest, another beetle, called a lady beetle, or ladybug, was used as a biological control of a pest. More than one hundred years ago, ladybugs were imported into California to control a serious new pest of citrus plants, the cottony cushion scale insect. Within a few years, the ladybugs had spread throughout the entire state and had nearly eliminated the cottony cushion scale insect as a pest. Even today we think of the red and black ladybug as our friend. Insects and plant diseases can also be used as control agents for weeds. One of the first examples of using an insect to control a weed was the case of the prickly pear cactus in Australia. In the late 1800's, the cactus was introduced to Australia, where it thrived and grew into dense, impenetrable thickets covering millions of acres of land. A variety of insects and mites that feed only on the cactus were introduced to Australia to control the cactus. Scientists are very careful to test a potential weed-control insect or mite to make sure it does not like to eat any crop plants. Chemical controls, or pesticides, are relatively new in the long history of agriculture. The use of pesticides allows us to produce more food than ever before on less land. You can tell which insect or disease a pesticide is used to control by the pesticide's name. The first part refers to the pest, and the last part, "cide," is from the Latin word for "killer." Insecticides kill insects; miticides kill mites; herbicides kill plants (the word "herb" is from the Latin word herba for "plant"); a fungicide kills fungi. Like any rule, however, this one also has an exception: Chemicals used to kill bacteria are usually called antibiotics, not bacteriacides. Pesticides work in a variety of ways. Some pesticides poison the pest and stop basic life processes, such as the ability to breathe. Other pesticides stop the pest from growing normally. Insecticides that are similar to an insect's natural hormones cause the insect to grow abnormally. The insect may never grow into an 4 of 6 7/2/09 1:31 PM

adult, so it would never reproduce. There are also herbicides similar to plant hormones that cause the weed to grow in a twisted, abnormal way that prevents the plant from producing seeds. Some pesticides must be applied before the pest attacks the plant. Other pesticides, called systemics, are absorbed into the plant where they remain active inside the plant's system. Systemic insecticides are especially useful for the control of sucking insects. Some systemic fungicides can be applied to the leaves of the plant, and move into the roots where they control root rots. Other systemic fungicides are able to stop the growth of the fungus after it has invaded the plant. A wide variety of herbicides exist to control weeds. Often several kinds of herbicides are used throughout the growing season of a crop. Some kill any plant they contact; other herbicides only kill certain kinds of plants. Many herbicides are applied to the soil before the weeds even begin to grow. As soon as the weeds start to grow, the poison takes effect and kills them. Pesticides have been a tremendous benefit to people. Pesticides are used to perform some of the work that in the past was performed by the crop growers; so with the use of pesticides, fewer people are needed to cultivate the same amount of land. Chemical fungicides allow the farmer to grow fruit free of any spots and rot. Fruit and vegetables can be shipped long distances to cities without spoiling. The skillful use of pesticides can prevent the terrible disasters and crop failures that in the past have caused such great human suffering. After any pesticide has been used in the same place over a long period of time, the pest may become resistant to the pesticide--that is, the insect is no longer harmed by the pesticide. Resistance develops because the pest population is composed of many individuals with slightly different survival abilities. Within every pest population there are a few individuals that can tolerate a particular pesticide. When most of the pests that cannot tolerate the pesticide are killed, only the few tolerant individuals are left to reproduce, thus creating a population of resistant pests. Insects and mites were the first pests noticed to develop pesticide resistance. Now weeds and fungi are also known to have resistance to specific chemical pesticides. Great care must be taken to use pesticides wisely to avoid creating pesticide-resistant pests. Different kinds of pesticides should be used in rotation rather than always using the same pesticide many years in a row. Most pesticides will also harm people if large enough amounts are eaten, inhaled, or gotten on the skin. Pesticides can also kill the enemies of the pests, allowing the pest population to continue growing and creating a greater pest problem. This is why there is so much interest in combining a variety of pest control strategies for pest control. Future Technology The control of plant pests remains a difficult task requiring clever solutions to old and new pest problems. The search for better plants continues. Equally important is the preservation of populations of insects that may include important potential pest- or weed-control agents. It is now possible to add specific traits to plants using biotechnology. In agricultural biotechnology, a new product is created by transferring genetic material. For example, certain bacteria have a gene that directs the production of a toxin called Bt. The Bt toxin is poisonous to the corn borer caterpillar. Scientists have inserted the bacterial Bt gene into corn plants to create new strains that are resistant to insect pests. When the corn borer eats a corn plant that produces Bt, it gets sick and dies from the poison. The toxin is not dangerous to people, however. 5 of 6 7/2/09 1:31 PM

Experts estimate that weeds, pests, and plant diseases may destroy or damage as much as one third of the world's food. The number of people in the world increases every year, but the amount of land stays the same. It is important to understand pests and how to control them in order to allow our crops to make the most food possible. Many other ideas for new forms of pest control are being developed. New ideas and a better understanding of how pests harm plants and how plants defend themselves can all be used in the future to improve agriculture for all of the people in the world. Ethel M. Dutky Director, Plant Diagnostic Laboratory University of Maryland How to cite this article: MLA (Modern Language Association) style: Dutky, Ethel M. "Plant Pests." The New Book of Knowledge. 2009. Grolier Online. 2 July 2009 <http://nbk.grolier.com/cgi-bin/article?assetid=a2023350-h>. Chicago Manual of Style: Dutky, Ethel M. "Plant Pests." The New Book of Knowledge. Grolier Online http://nbk.grolier.com/cgi-bin /article?assetid=a2023350-h (accessed July 2, 2009). APA (American Psychological Association) style: Dutky, E. M. (2009). Plant Pests. The New Book of Knowledge. Retrieved July 2, 2009, from Grolier Online http://nbk.grolier.com/cgi-bin/article?assetid=a2023350-h CLOSE & 2009 Scholastic Inc. All Rights Reserved. 6 of 6 7/2/09 1:31 PM