If nontoxic controls won’t do the job, you may have no choice but to use a pesticide. “Pesticide” is a general term for any product that destroys or repels pests, or that prevents or mitigates their attack. The group includes insecticides, used against insects and related creatures of various types; fungicides, which control many plant diseases; and herbicides, which kill weeds.
The U.S. Environmental Protection Agency (E.P.A.) has strict rules for pesticide classification and labeling. Because all pesticides can be dangerous if used incorrectly, the regulations apply both to products we think of as quite benign (such as soap sprays) and to those so toxic they can be applied only in tightly controlled situations by persons certified to use them.
When you buy any pesticide, read the label carefully. Check the product’s common name (the name listed on the label under “active ingredients”), which often differs from the trade name; the insecticide carbaryl is marketed under the trade name Sevin, for example. In this book, we refer to pesticides by common name. Also look for the signal word on the label; it will give you an idea of a product’s immediate toxicity. The four signal words are “caution,” “warning,” “danger,” and “poison”; “caution” indicates the lowest toxicity, “poison” the highest.
As a general rule, start your control attempts with the less toxic products. Soaps, oils, sulfur, pyrethrin, and Bt all fall into this class of “milder” pesticides; when used carefully, according to label directions, they pose minimal danger to you and the environment.
They must, of course, be used with care: some individuals may be allergic to them, and they can cause eye or skin irritation. Soaps and oils are extremely dangerous to fish.
Reading a Pesticide Label
When you read a pesticide label, start by looking at the list of active ingredients. Here you’ll find the common name of each ingredient–information that’s useful when you’re comparing brands or following recommendations that refer to products by the common rather than trade name.
Also look for the signal word–“caution,” “warning,” “danger,” or “poison.” This word alerts you to the product’s immediate toxicity; “caution” indicates the lowest toxicity, “poison” the highest. As a general rule, it’s a smart idea to start your control attempts with the least toxic choice possible.
Labels specify which pests the product controls; give a list of plants on which it can be used, also noting whether it is safe for food crops; provide storage and disposal information; state any special precautions; and give first-aid instructions in the event of unsafe exposure.
Once you have purchased a pesticide, mix it exactly as the instructions direct. A solution that is too weak may be ineffective; one that’s too strong may kill the plant you’re treating and be harmful to you and the environment. Then apply the product precisely as directed. If the instructions tell you to wear goggles or a breathing mask, it is essential that you do so.
Soaps are especially effective in killing soft-bodied insects: when the spray hits them, it penetrates their bodies and causes cell membranes to burst. Unfortunately, soaps kill not only pests but beneficial insects as well–including bees, which are in decline throughout North America. You can, however, time the application of soap spray to minimize hazard to bees. Since bees typically return to the hive in late afternoon, spray in early evening; by morning, the soap will have dissipated. You’ll also save beneficials by resisting the urge to spray a wide area “just for good measure.” Spray only those plants exhibiting symptoms.
Some soaps are effective in killing weeds. Those marketed as weed killers have a higher proportion of soap to water than those sold to kill insects.
When you prepare soap sprays, keep in mind that they will be more effective when mixed with soft water than with hard water: soft water produces a sudsier spray that is better able to reach all surfaces of a plant.
TIP: A U.S.D.A. formula combining oil and soap is effective in killing soft-bodied insects. Mix 1 cup peanut, safflower, corn, soybean, or sunflower oil with 1 tablespoon liquid dishwashing detergent. To make the spray, use 11/2 teaspoons of the oil-detergent mixture for each cup of water.
Oils serve several purposes and come in two weights. The heavier sort, called horticultural oil or dormant oil, is applied in late winter (before foliage appears) to kill certain disease spores as well as eggs and dormant stages of some pests. It cannot be used after leaves emerge, since it will scald foliage. Some manufacturers combine horticultural oil with lime sulfur (see Sulfur,) and market it as “dormant spray”; read labels carefully if you want oil alone.
Summer oil is the lighter oil; it can be used on plants in leaf. Unlike horticultural oil, it generally doesn’t scald foliage. It can sometimes interfere with transpiration, however, so spray just one branch and watch it for signs of wilting before you treat the entire plant. Summer oil smothers both soft-bodied and hard-bodied insects when applied directly to them; it can also prevent rust and mildew spores from taking hold.
Sulfur–the finely ground mineral–is effective on a variety of flowers, fruits, and vegetables in preventing powdery mildew, rust, and black spot. It can also control mites and several insect pests. It is toxic to a few plants, including cucumber, raspberry, and apricot, so check the label carefully to make sure it is safe for your intended target. Unlike lime sulfur, elemental sulfur cannot be used in conjunction with oils: the combination will kill plants. Wait 1 month before applying sulfur to any plant that has been treated with an oil spray.
Buy sulfur as a wettable powder, then mix it with water and spray the solution to coat plant surfaces evenly. Keep in mind that sulfur is sometimes combined with other pesticides, many of which have higher toxicity levels. Read labels closely to see if you’re buying sulfur on its own or mixed with something else. The signal word will give you a clue; sulfur rates “caution,” but combination products of higher toxicity will be labeled “warning.”
Lime sulfur (calcium polysulfide) is effective against the diseases and pests noted above for elemental sulfur. It can be purchased separately or in combination with horticultural oil (as dormant spray). Because it is caustic, you are required by law (as stated on the product label) to wear protective clothing, goggles, and a breathing mask when you’re applying it.
In jungles, rain forests, and other wilderness areas around the world, researchers have found plants whose natural toxins will kill pests. One of the most effective of these is pyrethrin, derived from a daisy-flowered plant, Tanacetum cinerariifolium (formerly Chrysanthemum cinerariifolium). Pyrethrin kills many soft-bodied and some hard-bodied insects, has a low level of toxicity, and dissipates rapidly. It is not, however, harmless; mix, apply, and dispose of it as carefully as you would any other pesticide. It kills beneficial insects as well as pests, so use it judiciously. Some pyrethrin-based products are combined with other, more toxic pesticides; read labels closely to see what you’re getting.
Synthetic (laboratory-produced) pesticides are the solution of last resort, to be used only after you have exhausted all other, less toxic approaches to coping with a particular pest, disease, or weed. Despite their higher toxicity, such products sometimes offer the best–or only–way to save a precious plant from insect attack or halt a rampant disease or weed infestation.
Appropriate pesticides for specific problems are identified by name in the following sections on pests, diseases and weeds. Keep in mind, however, that pesticide regulations change rapidly. Some of the products mentioned in this book may later be removed from the list of pesticides approved in your state, or their application directions may change; and, of course, new products may be added. If you have questions concerning current regulations, consult a local nursery or your Cooperative Extension Office.
The insecticides below are listed alphabetically by common name (the name given on the label under “active ingredients”). If the trade name differs from the common name, it is given in parentheses.
Most of these products kill pests on contact or through direct ingestion. Acephate, however, is a systemic pesticide: applied to a plant, it is absorbed by the foliage or roots, and any pests that then ingest the plant’s juices or chew its leaves are killed. Systemic insecticides must never be applied to any edible crop. For products that can be used on food crops, check the label to see how many days before harvest the insecticide may be applied.
When using any insecticide, read the label instructions carefully; then follow them exactly.
Acephate (Orthene). Systemic. Use only on ornamentals–but since it is toxic to bees and other nectar-drinking beneficial insects, do not use on blooming plants.
Carbaryl (Sevin). Commonly used in vegetable gardens; usually effective against chewing insects (caterpillars, for example) but generally not effective against sucking types like aphids. It can even make infestations by sucking insects worse by destroying their natural enemies. Highly toxic to bees and earthworms.
Chlorpyrifos (Dursban). A control for lawn pests, many pests of ornamentals, and certain borers that attack shade trees. Do not use on food crops.
Diazinon. A broad-spectrum insecticide also widely used to control various lawn pests, diazinon is the only chemical control that can safely be used to control soil-dwelling pests in vegetable gardens. It is toxic to bees and birds.
Malathion. A broad-spectrum insecticide for use on both edible and ornamental crops. Toxic to bees.
Metaldehyde. The most common slug and snail control, metaldehyde is usually the active ingredient in various baits and some liquids. It may be used around vegetable and fruit crops, but it is toxic to pets.
Methiocarb (Mesurol). An effective control for slugs and snails; will also kill earthworms. Do not use on food crops.
Propoxur (Baygon). Common in earwig baits and wasp and hornet sprays. Do not use on food crops.
Bt (Bacillus thuringiensis) is a bacterial pathogen that paralyzes and destroys the stomach cells of insects that consume it: once they eat a Bt-coated leaf, they stop feeding, then die within several days. Because Bt must be ingested to be effective, it is more selective than sprays that simply kill on contact–and that means it’s less likely to upset the insect balance in your garden.
The type of Bt most commonly sold kills many leaf-eating caterpillars–including, unfortunately, those that will eventually become butterflies. If you want butterflies in your garden, try to use Bt only on those plants hosting pest caterpillars. You’ll also find special strains of Bt aimed at specific pests; Bt tenebrionis, for example, is effective against Colorado potato beetle larvae.