Vegetable Garden Insecticides


Which one should you use when pests threaten your garden.

Even in the best managed vegetable gardens -- with soil rich in compost, and a diversity of plant species that encourage natural predators -- certain pests will 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 example, 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 will 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. Also described are two promising new insecticides.

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 many kinds of habitats. Water, even a very small pond, is very 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 least a 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 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; and how quickly it degrades. 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.

Eleven 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.

Adios (Sevin plus bait).

This environmentally smart new insecticide is aimed at adult cucumber beetles. The active ingredient, Sevin (see below), is inside a microcapsule of bait that cucumber beetles find irresistible. The bait is cucurbitacin, a natural compound in leaves of cucumbers, squash and melons. Once cucumber beetles encounter the Adios pellets on leaves they seek out more, gradually consuming enough Sevin to cause their demise. Very little Sevin is applied, between 2% and 5% the usual amount. Other insects, such as bees and wasps, don't eat the pellets so aren't affected. And the Sevin does not easily leach out of the pellets into the spray or irrigation water. (Adios is available from Naturally Scientific in Atlanta, Georgia; (800) 248-9970.)

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 1960's, 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 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 exits 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. var. 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 are developed for a few other pests. Some leaf-feeding beetles (including Colorado potato beetle) are susceptible to Bt. tenebrionis , 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.

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.
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. 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.

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*) or cold (below 40*) 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 non-existent. 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 non-target insects. They are selective, so 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.

Malathion

This is a garden insecticide of last resort. Though some consider it relatively safe, many home gardeners avoid using this insecticide simply because it is oily and smells bad. It is, however, widely available and often recommended for control of vegetable garden pests. It is toxic to many kinds of insects, but is less toxic to mammals than either rotenone or Sevin. Though destructive to many beneficial insects, it is less damaging than Sevin.

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. A concentrated formulation called Azatin (3% azadirachtin), is available from AgriDyone Technologies in Salt Lake City, UT; (800) 657-3090. Last year, the Environmental Protection Agency stipulated that Azatin was exempt from food crop tolerances because it is considered non-toxic.
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 "knock-down" 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, this is one of the best choices. Of low toxicity to mammals, they kill insects quickly. In sunlight they break down and are non-toxic within a day or less. For best results apply it 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 (most 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 much more toxic to beneficials. Though increasingly popular in commercially, home gardeners should avoid using them.
Often, pure pyrethrins only stuns insects. This is why they are often combined with a synergist, piperonyl butoxide, chemicals that enhance the effectiveness of the active ingredients, thus enabling formulations with less pyrethrin to kill insects.

Rotenone

This botanical insecticide is extracted from the root of many related tropical legumes, primariely Derris and Lonchocarpus. Though regularly used by organic gardeners, its toxicity to people is greater than common insecticides such as malathion and Sevin, and its residual toxicity lasts up to one week. However, like pyrethrins it does quickly break down in sunlight.
Apply rotenone in the early evening when bees are not active. Like Bt, it is more effective in a pH-balanced solution. Use a buffer solution if your water is strongly alkaline.
Rotenone is a broad-spectrum insecticide and should be reserved as a last resort against the hardest to control pests such as cabbageworms, Colorado potato beetles, flea beetles, fruit worms, Japanese beetles, loopers, Mexican bean beetles and weevils.

Sabadilla

This botanical insecticide is made by grinding the seeds of the sabadilla lily (Schoenocaulon officinale) into a fine powder. It's effective against a range of true bugs (Hemiptera) as both a contact and stomach poison. Highly toxic to bees, it is generally considered gentler than rotenone. Sabadilla is highly irritating to mucous membranes of mammals.
Like most other botanical insecticides, sabadilla breaks down very quickly in sunlight, but remains potent for many years if stored in a dry, dark place. And like other toxic sprays, use it infrequently, as a last step in your vegetable garden IPM program.

Sevin (carbaryl)

One of the most widely used insecticides in home vegetable gardens, it is controversial because it kills most kinds of insects. (The Adios product is one way it is used selectively.) Sevin is particularly deadly to honeybees and wasps, several of which are parasites of other garden pests. Because it kills beneficial insects so effectively, Sevin often causes outbreaks of so-called "secondary pests," such as spider mites. It is also toxic to earthworms. Sevin is a sledgehammer of a remedy. Use it sparingly, and only after pyrethrum or perhaps rotenone have failed to be effective.
Aside from its effectiveness, Sevin's virtue is low toxicity to mammals and birds. It is less toxic than the botanical insecticides rotenone, and it is less toxic to birds than to mammals. In a garden, however, Sevin requires two weeks or more to degrade, so it poses a significant hazard to a wide range of beneficial insects. Consequently, use it as a last resort, if at all, in your garden IPM program.

Insecticide users checklist * Use all kinds with care. * Study the product label before use. * Check sprayer nozzle and gaskets before using. * Never apply on a windy day and only when bees are less active, such as morning or evening. * Mix the least amount of material and dispose of excess according to label directions. * Use spreader-stickers or buffering solutions for improved effectiveness. * Wear long sleeves and pants, a mask and rubber gloves.
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