Vegetable Garden Pest Control


By National Gardening Association Editors

Photo by Anderwood

Even in the best-managed vegetable gardens--ones with soil rich in compost, and a diversity of plants to encourage natural predators--certain pests will occasionally get out of hand.

There is a nugget of truth in the old maxim that insects are most likely to gang up on plants that are already unhealthy because of a soil nutrient imbalance or drought. This is the case, for examply, with some aphid outbreaks. Unfortunately, the maxim doesn't come close to explaining all our pest problems.

For instance, vegetables are bred largely for yield and flavor, often at the expense of natural resistance to pests. Furthermore, all vegetables are tender and nutritious, and this fact is not lost on a wide array of insects. With cabbageworms, hornworms, bean beetles, and Colorado potato beetles, the better you've made the soil, the more they like your vegetables.

If these or similar insects are in your neighborhood and you are growing their favorite crops, you are almost certain to have a pest outbreak. Given these realities, what should you do? Most likely you'll consider using some kind of insecticide.

Here we summarize the latest experience and expert advice about the sprays and dusts used to control pests in vegetable gardens.

Integrated Pest Management

Vegetable garden pest control begins with basic good gardening common sense, such as choosing varieties that are resistant to pests in your region, preparing the soil well and providing regular irrigation.

It helps to have in your garden a diversity of plants and habitats. Water, even a very small pond, is attractive to many insects and other creatures. Likewise, an abundance of flowering, nectar-bearing plants will encourage and sustain parasitic and predatory insects.

The next step in a least-toxic pest control strategy is to employ barriers, such as row covers, to exclude pests altogether. Using a pesticide, any pesticide, is always the measure of last resort. You spray or dust late in the game, when the pest insect is clearly way out of control and an important crop is at risk.

When a pest first arrives, or when prior experience tells you it soon will, the best approach is to develop a strategy of control. Integrated pest management (IPM) is a pest problem-solving process that includes considerations such as pesticide resistance, natural biological controls and pollution, in addition to problems caused by the pest. IPM integrates many pest-control methods and minimizes insecticide use, particularly of the more toxic, broad-spectrum kinds.

When a problem does occur, it is essential to correctly identify the cause. The beetle you see near a hole in a leaf may be a predator. But if it is damaging your plants, simply pick it off. Also consider that doing nothing at all--letting nature take its course--is often the best approach. Always use simple, noninvasive remedies first.

Sensible Insecticides Used Responsibly

Sometimes pest problems are not adequately managed by natural, cultural or mechanical control methods. Insecticides are often the only control option that remains. The prime factors in determining pesticide safety are:

  • How specific it is to particular insects?
  • How toxic it is to humans and nontarget organisms?
  • How quickly does it degrade.

Remember, in a vegetable garden it is virtually impossible to spray just the one thing you want to spray. Other crops (perhaps ready to pick) are always nearby, so you want to stay away from insecticides that don't break down quickly.

Choose an insecticide that is as specific to the pest at hand as possible and then use as little as possible. If only one spray will do the job, use only one spray. For the long-term health of your garden, the less spray you use, the better.

Remember, too, that just because an insecticide has a botanical origin or is considered acceptable to organic gardeners, it still contains a toxin and is not automatically safe for humans.

Vegetable Pest Remedies

Gardeners today have at their disposal a handful of effective and safe pesticides. When you have to spray to save your crop, here are the insecticides to consider using, with their characteristics, positive and negative.

Bt. The bacterium Bacillus thuringiensis (Bt) was identified in 1911 by the biologist E. Berliner, who found it infecting pupae of the Mediterranean flour moth and other insect larvae living in grain warehouses in the German town of Thuringia. It wasn't until the 1960s, however, that entomologists learned how to make it into a powerful and very pest-specific insecticide.

Advantages of Bt include safety--it is essentially nontoxic to humans, other mammals and birds. The label specifies no waiting period between application and harvest. It is also highly selective so is easily incorporated with existing natural controls. A limitation of Bt is its slow action. After pests consume it, their feeding slows down. But their death won't occur for two to five days. Bt is also perishable. Most formulations are less effective after a few years of storage.

Bt exists naturally in most soils. Different strains of Bt occur that produce protein crystals toxic to certain insects. The strain for most caterpillars is B. t. kurstaki. Commercially prepared Bt spray or powder has no effect on adult butterflies or moths. Remember, however, that not all caterpillars are pests.

Strains of Bt have been developed for a few other pests. Some leaf-feeding beetles (including Colorado potato beetles) are susceptible to B. t. tenebrionis (also known as San Diego strain and M-1), for example. Because Bt is a near-perfect insecticide, there is danger of overuse. Any overused insecticide will gradually become less effective as insects evolve defenses to it. Some insect pests, such as the diamondback moth and Indian meal moth, were once susceptible and are now at least partially immune to Bt.

Bt is in the news because plant scientists have learned how to insert its genes into corn plants making the entire plant toxic to corn earworm. While Bt is favored and accepted by all organic gardeners, genetically engineering Bt toxins into other plants is not.

Diatomaceous Earth (DE) This is a powder-like dust made of the silicate skeletons of tiny marine creatures called diatoms. Millions of years ago as they died, their skeletons gradually accumulated into deep layers that are mined today from deposits where oceans or large lakes once covered the land. DE acts like ground glass, cutting into the waxy coat of insects and causing them to dry out and die. It is not toxic if eaten, but is irritating if inhaled.

DE is available in two different forms. One form is used primarily in swimming pool filters. It is not an effective insecticide and is dangerous to inhale (it can cause a lung disease called silicosis). In your garden, use only the natural grade of DE. Still, it is wise to wear goggles and a dust mask during application.

Dust DE onto leaves and stems to control pests such as aphids, Colorado potato beetle, immature forms of squash bug, Mexican bean beetle or whitefly. Or spread it as a barrier to slugs and snails. It works best in dry situations. It is not selective and kills spiders and beneficials as well as pests. Don't overuse it.

Particle Film Dusts. Dusts are among the oldest types of insecticides, but the newest kinds are very carefully designed for maximum efficiency. Only one type is currently available to home gardeners, Surround At Home. It's made of an engineered kaolin clay. Applied with a pressurized sprayer, it coats plant leaves making them less familiar and less comfortable to pests. Since these are not insecticides, there is no danger of a pest developing resistance. And the kaolin, a common ingredient in medicines, is non toxic.

Horticultural Oils These are most often highly refined extracts of crude oil. (Some vegetable oils, such as cottonseed and soybean oil, are also sometimes used.) They are increasingly recommended for vegetable garden pest control because they present few risks to either gardeners or desirable species and integrate well with natural biological controls. Also, oils dissipate quickly through evaporation, leaving little residue.

Oils kill insects by plugging the pores through which they breathe. Oils can damage plants if applied at excessive rates or on particularly hot (above 100°F) or cold (below 40°F) days.

Spray oils in vegetable gardens to kill aphids, leafhoppers, spider mites and whiteflies. A few drops of oil in the ear tips of corn controls corn earworm.

Insecticidal Soaps These are specific fatty acids that have been found by experiment to be toxic to pests, primarily soft-bodied insects such as aphids, mealybugs, spider mites and whiteflies. Surprisingly, adult Japanese beetles are also susceptible. Most nontarget insects are unaffected, and toxicity to animals is nonexistent. Soap insecticides act fast and leave no residue. You can use them on vegetables up to the moment of harvest.

Advantages of soaps to home gardeners include safety to both the applicator and nontarget insects. They a selective, so are easily incorporated with other, natural biological controls. Some plants, such as peas, are readily burned by soaps, and their effectiveness is greatly reduced if mixed with hard water.

Don't use liquid dishwashing detergents or hand soaps. Though many will kill insects, they might hurt the plants, too. Some fatty acids are toxic to plants, the reason a soap-based weed killer is now available. Dish soap manufacturers change the oils used in their formulations regularly, based on cost and availability, so the brand that worked fine for your neighbor last year might severely stunt your Brussels sprouts this year.

Neem This is an extract derived from the crushed seeds of the tropical neem tree (Azadirachta indica). Though intensely studied for many years now, it is still a new botanical insecticide. The primary active ingredient is the compound azadirachtin, although the oils and other ingredients also have some insecticidal effect.

According to EPA regulations, neem is exempt from food crop tolerances because it is considered nontoxic.

Neem works both as an insecticide and as an antifeedant. It kills insects in the juvenile stage by thwarting their development, and is most effective against aphids, thrips and whiteflies. There is no quick knockdown with neem, but a week or so after application, you'll notice a steady decline in the number of pests. It is not effective against adult insects (though it may interfere with egg production), and has little impact on beneficial insects.

As an antifeedant, neem is effective against Japanese beetles. Apply neem before the beetles appear and reapply after rainfall. Once beetle numbers build up on the plant, neem no longer discourages them.

Neem sprays degrade very quickly in water. Mix only the amount you need and apply all of it immediately. On the plant, neem retains its activity against juvenile insects pests for about one week.

Pyrethrins Derived from the painted daisy, Chrysanthemum cinerariifolium, pyrethrins are considered one of the most important natural insecticides. When you must use a broad-spectrum insecticide in the vegetable garden or lose the crop, they are one of the best choices. Of low toxicity to mammals, they kill insects quickly. In sunlight they break down and are nontoxic within a day or less. For best results, apply them in the late afternoon or evening. Use pyrethrins for the hard-to-kill pests such as beetles, squash bugs and tarnished plant bugs.

The terminology can be confusing. Pyrethrum, discovered around 1800 in the Transcaucasian region of Asia, is the ground-up flowers of the daisy. Pyrethrins (almost always plural) are the insecticidal components of the flowers. Pyrethroids, such as cypermethrin, permethrin and resmethrin, are synthetic compounds that resemble pyrethrins. They are more toxic and more persistent than pyrethrins, so are much more toxic to beneficials. Though increasingly popular in commercial agriculture, home gardeners should avoid them.

Often, pure pyrethrins only stun insects. This is why they are often combined with synergists like piperonyl butoxide, chemicals that enhance the effectiveness of the active ingredients, thus enabling formulations with less pyrethrin to kill insects.

Eight Vegetable Garden Insecticides

Here are the insecticides you are most likely to need in a vegetable garden. Note that many others are available and sometimes recommended, but these are the safest and most effective for most home gardeners.

Name Use against How it works; how long it lasts How to use it
Bt. Toxic primarily to caterpillars Cabbageworms and cutworms. Use B. t. tenebrionis for Colorado potato beetle. Bacterial toxin; causes caterpillar death usually within 24 hours. Dissipates in 2 days or less. Available as spray or dust. Apply late afternoon and reapply after rain. Mix with insecticidal soap for better coverage.
Diatomaceous earth (DE). Toxic primarily to soft-bodied insects, snails and slugs Discourages aphids, juivenile Colorado potato and bean beetles, leafhoppers, and thrips. Sharp-edged diatom skeletons scratch insect exteriors, causing them to dry out. More effective combined with pyrethrins. Use natural grade DE, not the kind used in swimming pool filters. Wear dust mask.
Particle film dust. Coats leaves and repels insects Japanese beetles, tarnished plant bugs, and Colorado potato beetles. Specially refined kaolin clay uses microscopic particulates to form barrier film on leaves. Apply with pressure sprayer. Mix with Bt or pyrethrins.
Horticultural oils. Toxic to many insects and mites Kills aphids, corn earworms, leafhoppers, spider mites, whiteflies. Kills pests by suffocating them. Use highly refined summer oils. Do not apply to drought-stressed plants, or on hot, cold, or very humid days.
Insecticidal soap. Toxic mostly to soft-bodied insects Kills aphids, earwigs, grasshoppers, Japanese beetle adults, leafhoppers, spider mites, whiteflies. One of the safest insecticides. Fatty acids destroy the cellular membrane of insects on contact. Mix with warm soft water and be sure to cover both sides of leaves. Can burn leaves during hot weather.
Neem. Toxic to juvenile forms of some pests; also a repellent Kills juvenile aphids and Colorado potato beetle. Repels whiteflies, Japanese and Colorado beetle adults. Affects growth hormones of some insects causing them to stop feeding. Effect lasts about 1 week. Apply liquid spray morning or evening with humidity is high.
Pyrethrins Toxic to a broad spectrum of pests Controls most vegetable pests included flea, potato, and bean beetles. A nerve toxin, often combined or DE. Degrades rapidly. Apply dust during cloudy weather or early evening.

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