Plant Pests and Their Control

A good gardener must learn to recognize plant symptoms associated with insects and disease and take whatever measures are necessary to control the insects and diseases involved. Injury to garden plants is caused by a variety of agents. Animals of various kinds feed on plants. Some inject toxic substances that result in gall-like formations (abnormal growths). Plant diseases can be caused by bacteria, fungi, and viruses. Some disease symptoms may result from physiological disorders. Symptoms of mineral deficiency, various types of winter injury, and injuries caused by air and soil pollution are nonparasitic. Weeds take their toll by lowering crop production.


Rodents feed on roots and stems of woody plants and occasionally on fleshy, underground storage organs like lily and tulip bulbs. Field mice often girdle the base of tree and shrub stems. Members of the rose family and winged euonymus are favorite sources of food for mice. When hungry enough, mice also girdle pines and many other woody species. When the bark is chewed all around the stem and through the cambium to the wood, the tree gradually dies from starvation of the roots, since food manufactured by the leaves moves downward to the roots through the inner bark. Mice are most likely to feed on bark during the winter months, when food is scarce. They work under the snow. Sometimes, when grass and weeds are allowed to grow up around the trees and mice can be protected from their enemies under this cover, girdling occurs in the fall of the year. The safest method of protecting trees and shrubs from mouse damage is to put a cylinder of lA inch mesh hardware cloth around the base of the tree. The top of the cylinder should be high enough so that mice will not enter if snow becomes deep. Tramping the snow around the base of the tree when snow is deep keeps the top of the cylinder above the snow line.

Rabbits, both cottontail and jack, also feed on the bark of trees and shrubs, especially during the winter months, when other food is scarce. They feed above the snow line and often cut off entire branches. It is more difficult to protect plants from rabbits than from mice. A cylinder of chicken wire can be used, but this must project well above the snow. Repellents, such as Goodrite Zip, sprayed on the trunk and lower branches in the fall help too. Dried blood in mesh bags hung from branches of the tree also repels rabbits.

Pocket gophers kill trees by feeding on roots. Fairly large trees up to 4 or 5 inches in diameter sometimes die rather suddenly. When this happens, the tree can easily be lifted out of the soil, with all its roots chewed off. The mounds of pocket gophers give warning of their presence.

Woodchucks, striped gophers, and chipmunks also cause problems. These rodents are fond of fresh foliage and often eat tender vegetables and young flowering plants.

Moles produce another type of injury. They feed on soil insects and push up ridges of soil, thus making the surface unsightly. Getting rid of the soil insects is the most effective method of ridding your yard of moles.

The best control for rodents of all kinds is to encourage and protect their natural predators: fox, owls, and hawks. Sometimes, when the natural predators have been killed by man, the gardener must resort to trapping and poison baits to protect garden crops from rodents. Your county Agricultural Extension Service can give you advice on how best to keep the rodents under control.


One hates to think of deer as being destructive but occasionally, when deer populations have become too high, they cause considerable damage. Not only do they feed on tree branches, but they also destroy trees by polishing their horns on the trunks of young trees. They seem to pick the choicest young trees to work on. There is no practical control against deer other than keeping their population down by controlled hunting. Fences are of little help since deer can easily jump most of them. Repellents may be effective for short periods, but these must be replenished at frequent intervals. Human hair, stuffed in a nylon stocking and hung on a tree, has been used to keep deer away from individual trees. Deer also respect an electric fence.


Birds are especially destructive on ripening fruits of all kinds. In the home garden they can strip the fruits from a cherry tree in a few hours. Robins are among the worst offenders. The only practical solution to this problem is to cover plants with bird netting as the fruits start to ripen. Devices to scare birds away may work for a while, but birds soon become accustomed to them. Commercial growers have sprayed cherry trees and grapes with Mesural to keep birds away from ripening fruits. Check with your County Agricultural Extension Agent to see if this chemical is approved for this purpose in your state.


Insects can cause damage to garden plants and trees. Canker worms can defoliate large elm trees in a few days. Cabbage worms perforate heads of cabbage and make them worthless. Striped cucumber beetles perforate the leaves of cucumbers and other vine crops in a few hours. Learning to recognize these troublesome insects should assist you in preventing serious damage to your plants.

Insects are classified according to the damage they inflict on plants. Chewing insects eat parts of both vegetative and reproductive organs. These insects include worms, caterpillars, beetles, and grasshoppers. Sucking insects pierce the plant organs and suck juices from the plant. A curling and distortion of the affected part of the plant results. Aphids, leaf hoppers, and plant bugs have sucking mouthparts. Rasping insects, the most common of which are thrips, scrape the surface of the leaves and other plant organs. And some insects cause gall formations.

Troublesome insects can be controlled in a variety of ways. Rotation of crops and sanitation greatly reduce insect problems. Natural enemies of insects also aid in insect control. Predatory insects like lady beetles and praying mantis help, and many birds also feed on insects. Spraying to control insects should be kept to a minimum. Sometimes, when nothing else works, a timely spray application saves a crop or plant from serious damage. Consult your county Agricultural Extension office fof recommended chemicals to use for each insect problem.

Red Spiders

Red spiders, or spider mites, can be very troublesome during hot, dry periods. Most plants are susceptible, but damage is usually greatest on evergreens and members of the rose family. In the house, red spiders affect a large variety of plants including ivies and roses. An off-color or rusty appearance of the foliage usually indicates the presence of red spiders. To confirm this, take a sheet of white paper and hold it under a plant branch. Tap the branch. The red spiders will fall on the paper and can readily be seen as tiny specks. If these specks move, your plant assuredly has red spider. Washing the foliage with a spray of water helps remove the red spider. This method does not kill the spiders and they will return to the plant. Spraying with a good miticide generally provides satisfactory control. Again, consult your county Agricultural Extension office for recommended controls.


In a wet season slugs may be a real problem. They resemble snails without shells, have soft bodies and are coated with a slimy substance. Slugs hide during daylight hours and move about at night. They have voracious appetites and feed on most garden plants. Good air circulation between plants arid exposure to sunlight reduce slug problems. Slugs are particularly troublesome to plants with large leaves and to plants with dense foliage close to the ground, such as strawberries. During the daytime slugs can be found under boards and large leaves, and in mulch material. Occasionally, boards are used to trap slugs. By turning the boards over, the slugs can be killed by dusting them with lime. Stale beer is also used to attract slugs. A shallow pan of beer is sunk in the ground to its rim. Slugs are attracted by the beer, fall in, and are drowned. Sometimes in a wet year when slugs are numerous, slug baits are used to attract and kill the slugs. Read the label to see if the bait is safe for your plants. Some baits may be used around flowers but not around fruits and vegetables.


Diseases in plants are caused by a parasitic organism or physiological disorder. Bacteria, fungi, and viruses produce diseases in plants. Symptoms vary, depending on the host and the causal agent. Leaf and fruit spots, blights, wilts, galls, soft rots, mildews, and rust are common symptoms. Learning to recognize the disease in early stages of development is important. Protective bacteriacides and fungicides can prevent infection from occurring but are generally ineffective once the disease organism has entered the plant. Sanitation, rotation, and planting disease-resistant varieties also aid in disease control. Consult your county Agricultural Extension office for the correct diagnosis of a disease and for recommended chemicals.

Reducing losses from virus diseases is aimed at controlling insects that transmit the disease. Removing diseased plants is also helpful in reducing infection. Protective sprays are of little or no value in controlling virus diseases.

Physiological diseases are treated by first determining the cause and then taking measures to correct the condition or conditions causing the problem. Symptoms of nutritional deficiency are not difficult to recognize; adding the proper fertilizer plus minor elements usually corrects the problem. Winter protection and the proper choice of plant species and cultivars can reduce winter injury. Air pollution poses a more difficult problem. Avoid planting varieties that are highly susceptible to air pollutants.


A weed is defined as a plant out of place. Not only are weeds unsightly but they also reduce yields by robbing crop plants of essential water and plant nutrients. Frequent cultivation is the best control for weeds in the home garden. Some of the preemergence weed killers can be safely used to kill annual weeds that come up after the crop plant is well established. Read instructions on the package to be sure that the weed killer is safe to use around the plants you are growing. Lawn weeds like dandelions can be safely killed by using selective weed killers.

In controlling weeds by cultivation, cultivate shallow and as often as necessary to kill the weeds. Kill the weeds while they are small and before they compete with your crop plants. Never let weeds go to seed in your garden. Failure to control weeds is the most common cause of not having a successful garden.

Chapter 6

Pest Diagram For Small Plan

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