Summer's Bad Guys
Since I last reviewed home garden pest control 10 years ago, the philosophy and options have changed dramatically. Today, gardeners realize that a balance of insects, good and bad, is most healthful for both the garden and gardener. For the most part, gone are the days of spraying indiscriminately to eradicate pests. Now, the focus is on good gardening practices, such as soil building and planting a diversity of crops, combined with barriers and traps. If sprays are used, less toxic alternatives are increasingly chosen because these products are easier to apply and are widely available.
We'll start by covering control measures, and recap with a list of the most common garden pests and the most effective controls for them.
Pest control begins with sound gardening practices. Building healthy soil, choosing resistant varieties, cleaning the garden in fall, rotating crops, and timing planting to miss the most harmful stage of the pest all reduce problems. There is also my favorite control for small gardens: handpicking. You'll be amazed how, by visiting the garden frequently to squish eggs, larvae, and adults, you can single-handedly (no pun intended) reduce a pest population. Controlling insects, such as cucumber beetles, leafhoppers, and aphids, that transmit viruses also helps reduce spread of these diseases.
The next level of control involves barriers such as row covers and traps.
If you need to spray, choose one that attacks specific pests. Many of the biological sprays work for these. If a biological spray for a particular pest isn't available, try soaps and oil. Finally, if no other alternative is available, spray neem or pyrethrins. A combination of controls, such as trapping and spraying with soap, is also effective.
Sound gardening practices will help next year's garden and help to reduce this year's pest problems.
Garden cleanup. Remove and destroy infected fruits and flowers during the growing season. This can also reduce levels of pests such as plum curculio this season, lessening the damage to the crop both this year and the next.
Resistant plants. Choose the right varieties to reduce certain pest problems. For example, grow Siberian iris, which resists iris borer, instead of bearded iris; or try butternut squash, which resists squash vine borer, instead of other varieties.
Early planting. Time vegetable plantings to avoid pest populations and reduce the need for controls. Planting early avoids the height of infestation of insects such as Mexican bean beetle, leafhopper, and squash vine borer.
Water spray. A jet of water from a garden hose will often dislodge aphids and spider mites. Once off the plant, they tend not to climb back on.
Whether you pick, trap, or put up a barrier, these are the simplest controls.
Handpicking, squishing. It's not pretty, but it works well on small plantings. Depending on the pest, you may be picking adults, crushing larvae, or squishing eggs, especially on the leaf undersides. To kill borers, poke into their holes with a piece of wire to pierce them.
Homemade barriers. Barriers are tried and true. Two of the simplest are wrapping newspaper around the stems of tender transplants to prevent cutworms from attacking, and placing a square of cardboard on the soil around cole crop plants to prevent the root maggot fly from laying its eggs.
Homemade traps. Like barriers, these are simple and effective. Place moistened, rolled newspaper beside garden rows to catch earwigs attracted to the moist, dark environment; throw out the paper each morning. Attract snails and slugs with beer in a shallow plastic container sunk to ground level; empty the tub daily.
Row covers. These nonwoven fabrics let in air, light, and water but prevent adult insects from laying eggs on plants. Place row covers over seedlings and remove if it gets too hot or if pollination is required.
Sticky traps. Different colors of traps attract adult insects to the sticky coating which they adhere to and then die. Use yellow traps for aphids, leafhoppers, and thrips; white traps for whiteflies; blue traps for thrips; and red, round traps for apple maggots.
A nontoxic scent (pheromone) is often used with traps to attract or confuse adult insects, and to disrupt breeding and egg laying. The most common is a Japanese beetle trap but, unless you're trapping beetles on a community-wide basis, it attracts more beetles than it catches. Lures and traps for specific insects such as codling moths and peach tree borers are only effective to indicate peak population levels and timing of sprays in large, orchard-sized plantings.
A healthy garden needs a balance of good and bad insects. Attract beneficials to your garden by planting a diversity of herbs, flowers, and vegetables, providing shrubs for hiding, and a nearby water source. Lures will also attract native beneficial insects.
Numerous predatory and parasitic insects that attack a range of garden insects are available. Some, such as praying mantis, will eat almost any pest and should be avoided, while others, such as the thrips predator, only attack specific ones. Improved instructions, handling, and shipping have enabled gardeners to be more successful using beneficial insects. However, the timing of the release is critical. Beneficial insects need time to get a pest population under control, and, of course, you should avoid spraying pesticides after you release the beneficials.
Biological sprays are probably the safest sprays to apply in the garden. Most gardeners are familiar with Bacillus thuringiensis (Bt), but in recent years, the discovery of different strains of bacteria and fungi has expanded the number of pests you can control. These pesticides target specific pests, are relatively nontoxic to beneficial insects as well as animals and humans, and can be very effective. Remember that these are living organisms, so to survive they must be applied under proper conditions.
One well known biological spray, milky spore disease (Bacillus popillae), used for more than 50 years to control Japanese beetle grubs, has come under question by scientists. Recent research at the University of Kentucky indicates that supplementing the natural population of this bacteria in soils with the product doesn't increase the number of beetle grubs killed.
New on the market are biologicals such as Botanigard (Beauveria bassiana), a widely occurring fungus that attacks whiteflies. It is being used effectively on whiteflies in greenhouses. Research is currently being conducted for its use on a broader range of garden insects including caterpillars.
Bacillus thuringiensis (Bt). This is probably the most widely used biological control. The bacteria attack specific insects: B.t. kurstaki. attacks most caterpillars (which may be the young of butterflies you are trying to attract to your garden), B.t. 'San Diego' attacks beetles such as Colorado potato beetle. These kill only at early larval stages, so you must spray at the first sign of infestation. They're most effective mixed with a sticking agent and sprayed in early morning or late afternoon. It may take a few days before you actually see the insects dying.
Beneficial nematodes. These microscopic wormlike creatures will parasitize a variety of soil-dwelling insects such as Japanese beetle grubs, borer larvae, and even beneficial ground beetles. For best results, spray in early morning or evening on large areas when the soil is moist and warm, and reapply annually.
Grasshopper protozoa (Nosema locustae). This parasite affects only grasshoppers and some crickets, is most effective on the young larvae, and takes two to three weeks to kill. It is best applied in spring at the first signs of grasshoppers and is most effective when treating a large area.
These poisons should be your last resort against an invading insect population. They tend to act broadly, killing some beneficials as well as target pests, and must be used according to instructions, since they can also harm plant leaves. As with any pesticide, read the label carefully to determine the best timing and application.
Garlic spray and hot pepper spray are two newer products that act primarily as repellents. They are most effective in greenhouses against soft-bodied insects such as aphids, spider mites, and whiteflies. They don't impart their flavor to vegetables. Researchers are testing their effectiveness on a broader range of pests in the field. Gardeners may want to experiment with these.
Synthesized chemical sprays such as phosmet (Imidan) are the most effective on tough pests such as curculios. Some botanical sprays such as rotenone are losing favor because of their toxicity, and manufacturers are considering not reregistering these pesticides for some uses. Currently certain formulations of rotenone are not sold in Colorado and California.
Diatomaceous earth (DE). These fossilized shells of water-dwelling diatoms have sharp edges that pierce and dessicate the bodies of many insects such as slugs, earwigs, and flea beetles upon contact. Sprinkle it around or on plants, and reapply after a rain.
Horticultural oil. Years ago, most gardeners were familiar with dormant oil spray which could only be applied on dormant deciduous plants to smother insects (especially at the larval stage) and eggs. New, refined petroleum-based formulations can be sprayed any time of year on most plants.
Vegetable oil-based products are also available, but these are best used as a dormant spray to kill overwintering eggs. Always check for any harmful effects by test-spraying a few leaves first. Don't spray on drought-stressed plants, when temperatures are above 90°F, or when humidity is high. Don't mix sulfur with the spray.
Phosmet (Imidan). This is a synthesized chemical that controls curculio and many other insects. It works best sprayed on fruit trees when flower petals fall and 14 days later. Use it with extreme caution: it is toxic to bees, fish, and wildlife.
Insecticidal soap. Soaps have become the mainstay pesticide spray for organic gardeners. Better formulations and added stickers improve their effectiveness. They kill or repel a wide variety of insects, including aphids, leafhoppers, and spider mites. Cover plants well with the spray, but don't spray if temperatures are above 90°F. Check the label for plants that should not be sprayed.
Neem. The latest in plant-based insecticides, this extract from the seeds of a tropical tree comes in two formulations: extract and oil. The extract's active ingredient is azadirachtin, which repels some insects, suppresses feeding, disrupts mating, and kills a wide range of insects including aphids, Japanese beetles, and leafhoppers. The oil form is mainly used to suffocate soft-bodied insects and beetle larvae. Both are most effective used as a preventive spray or when insect populations are low. Neem is probably the safest botanical for animals and humans, but some research has shown it can harm some beneficials such as ladybugs.
Pyrethrins. Derived from the crushed flowers of African daisies, pyrethrin is an effective spray that works quickly on many adult insects. Although it is harmful to bees and some beneficials, such as ladybugs, it's safe for animals and humans.
Beneficial Insects and the Pests They Control
Charlie Nardozzi is a senior horticulturist at National Gardening.
Photography by Suzanne DeJohn & National Gardening Association.