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5. Cultural and Physical Controls: The first line of defense against garden pests -- and the most environment-friendly
by Jack Ruttle
The techniques described here are the absolute first line of defense against garden pests and diseases. Many of the practices that fall under the "physical and cultural" rubric are old-fashioned, good-gardening common sense. They're things you do before you even think about mailing off an order for ladybugs or checking to see if the gaskets in the sprayer are up to another season of use. Of all pest-control methods, these are some of the most environment-friendly.
Cultural Controls
These are an assortment of good gardening practices that should always be used, whether pests are present or not. Failure to do these things sets the stage for some serious insect and disease problems. Make the following cultural controls a part of your gardening routine.
Choose The Right Plant For The Site
Make sure the plants you select are suited to the soil, moisture and other conditions in your garden and receive the proper amount of sun or shade. For example, in the wild azaleas typically are shaded by the canopy of surrounding trees, so planting most azaleas in full sun instead of the dappled shade to which they are adapted is an open invitation to the lacebug. Plants that match your site conditions will be healthier than those that don't, and therefore better able to withstand depredation by pests.
Look For Resistant Varieties
Plant breeders have produced a good number of plants that are resistant to diseases and, to a lesser extent, insects. To find these varieties, study catalog descriptions for references to the ability to ward off pests. For example, some vegetables have good resistance to nematodes, the microscopic worms that infest roots. Some species of birch, including paper birch and black birch, are less susceptible than others to the bronze birch borer.
Rotate Crops
It's a good idea to move most crops around the vegetable garden every year if you have the space. Many disease organisms live in the soil or on the soil surface. Likewise, several major insect pests restrict themselves to a relatively small patch of ground, traveling only short distances in their search for food. Some of these insects and diseases can overwinter in the soil. Planting the same crop in the same spot year after year encourages these pests.
Plants that are in the same families tend to be susceptible to similar insects and diseases. When planning rotations, treat all kinds of beans as essentially the same variety. Eggplant, potatoes, tomatoes and peppers are all in the nightshade family. Cabbage, broccoli, cauliflower, kale and Brussels sprouts are basically the same in the eyes of most pests. Spinach and beets are close cousins. Carrots, parsley and dill are closely related. Cucumbers, melons and squash are all prone to the same pests. Move all of these vegetables around the garden in groups.
Mix Up The Planting
This is another prudent practice for vegetable gardeners with plenty of room. Unfortunately, there seems to be little truth to the cherished old idea that certain plants -- like tansy or garlic or marigold -- have a mysterious power that protects neighboring plants that we like to eat from insects. But there are other reasons for mixing plants of one kind among others. Plant diseases spread like wildfire from one plant to another. So do some insects. They move readily right down a row or through a field. But if you break up the planting with flowers or unrelated vegetables, the pest population explosion can be slowed down. Mix plants of similar sizes and shapes and season of ripening to avoid shading and make the most efficient use of space.
Beneficial flies, wasps and lacewings seek out sources of pollen and nectar for themselves, then look for nearby pests in which to lay their parasitic eggs. So it is a good idea to mix food plants for them in with your vegetables. Dill, lovage, thyme and the mints are favored food sources for many beneficials.
Clean Up The Garden
Many insects and diseases overwinter on the debris of the plants they plague in summer. It's just good sense to gather up spent stalks of vegetables, flowers and weeds and cycle them through the compost heap. You should also compost the mulch you've applied under the plants in the annual flower and vegetable gardens. The ideal next step in these areas would be to plant a cover crop. Working the ground to make a seed-bed for the cover will kill some forms of wintering insects, and expose others to the elements as well as foraging birds and other predators. The cover crop itself will enrich the soil. If you can't plant a cover early enough for germination or growth, the next best thing is to apply a fresh mulch, which will prevent winter rains from compacting the soil and leave the soil ready for planting with minimal preparation when spring arrives.
Plant At The Right Time
By planting some vegetables early or late you can avoid the worst infestations of some pests. For example, potatoes do well in cool weather, and early-planted potatoes can get strong enough before beetle populations peak to withstand a moderate infestation and still yield a respectable crop. See the Encyclopedia of Pests for additional planting tips.
Solarize The Soil
You can take advantage of the sun's rays to heat the soil to a level that kills many diseases, insects and weed seeds. This is a relatively new practice that works best where the weather is sunny. The main problem for most people is that you need to be able to take a patch of ground out of production for at least a month during the prime growing season. If cloudy weather moves in, the solarizing period may be even longer. However, if you have a bad soil disease problem, or nematodes or serious infestations of weeds, you may want to give this method a try.
To solarize your soil, cover the patch of ground with a sheet of clear polyethylene. First you need to till the soil thoroughly. The plastic should be in as close contact with the soil as possible, so the surface tilth should be fine and smooth. The dark soil is your solar collector, so after you've prepared the ground, wet it; water darkens the surface and the heat travels deeper into the ground. Then put on the plastic, burying the edges in a shallow trench along the perimeter. It's good to monitor the temperature with a soil thermometer. The soil needs to get into the 150°-plus range.
When the process is complete and you begin to plant, disturb the soil as little as possible. Below the 3" to 5" depth, you will begin to encounter weed seeds that are still viable, but if they remain buried, they'll stay dormant.
Physical Controls
Let's be blunt: These are practices that crush, drown, burn, incarcerate or exclude pests from garden plants. The most basic and most potent physical control agent of all is you, with your sharp eyes, powerful fingers, quick mind and big (to a bug), heavy feet. To put a prettier face on all this, it's worth quoting an old saying that the best fertilizer of all is the footsteps of the gardener. Of course, the person making those footsteps is not there fertilizing but simply attending to the plants, looking for problems and nipping them in the bud.
The first step is to identify a specific pest or disease as the culprit. Do nothing until you have identified the problem creature. If possible go into the garden once a day. When you are really stumped by damage you see on flowers or foliage, take a magnifying glass into the garden for a close look. Or sit quietly nearby for five to 10 minutes to see what moves when you stop moving. Squirt the plant with a stream of water to roust insects hiding beneath the foliage. And occasionally go out at night with a flashlight to see who is "working" your garden on the night shift.
Large populations of sowbugs will eat seedlings, new roots, lower leaves and fruits lying on or near a damp soil surface. Traps baited with diatomaceous earth attract and kill sowbugs and their relative, pillbugs.
Handpicking
More times than not, you can solve the pest problem the minute you see it, by picking it up and pinching it between thumb and forefinger -- the yuckiest but still most popular insecticide of all.
Handpicking works best on the slowest insects, like those still inside their eggs (often in masses on the undersides of leaves) or caterpillars. Once you see cutworm damage, you can often find them by scratching around in the soil near the most recent destruction. Japanese beetles and Colorado potato beetles are also fairly sluggish. Catching cucumber beetles or flea beetles, on the other hand, can be difficult because they are extremely nimble.
Increase your chances in the handpicking game by going out in the coolest part of the day. Use a squirt-gun full of mildly soapy water to dowse the wings of flying insects to slow them down. Create places for pests to congregate, then visit daily to put an end to the party: cucumber beetles will gather under wilted squash vines, slugs will gather under a piece of board, and earwigs will gang together inside tubes of rolled-up newspaper or hollow sections of bamboo.
Gather insects in containers of soapy water, where they will drown. (When the containers are full, you can dump them on the compost pile.) Older gardening books often recommend cans of gas or kerosene. But these are petrochemicals, not-so-distant relatives of synthetic pesticides. They hold no advantage over soapy water in killing the insects, are difficult to dispose of and make a mess if spilled.
Barriers
Rowcovers
The thin, lightweight, spunbonded polyester fabrics developed in recent years for vegetable farmers and gardeners let in rainwater and plenty of light for growth but are impenetrable even by small insects. Rowcovers were first marketed as frost protectors, but are getting more and more use over raised beds to exclude flea beetles, cucumber beetles, maggot flies and the like. The material is draped loosely over a patch, raised bed or single plant. It is so light that the plants lift it easily as they grow; there's really no need for metal hoops or other structures to hold the fabric up. Remember, though, that the covers need to be removed from some crops long enough to permit pollination.
Cutworm Collars
Cutworms are caterpillars that hide by day and come out at night to encircle the stems of seedlings, tender plants and even the leaves of trees. In the case of transplants, they eat away a ring of tissue, and the plant topples. However, it's easy to protect your transplants by putting paper collars around their stems.
The collars don't have to be very big. They should be about 3" high and must encircle the plant. They can be made of newspaper, tarpaper, cardboard or even plastic, which can be reused year after year. About half of the collar should be underground.
Root-maggot Shields
To prevent cabbage maggot flies from laying eggs at the base of young transplants of cabbage, broccoli and other members of the mustard family, make 4" to 6" squares of tarpaper. Punch a little hole in the center, then make a slit from the hole to the edge of the square. After planting cabbages, broccoli or any other brassicas, slip the tarpaper shield over the ground at the base of the plant. The maggot can't get at the soil near the stem to lay its eggs.
Tree Wraps
A band of sticky material on the trunk of a tree will prevent some pests such as gypsy moth larvae from climbing the trunk. About a foot off the ground (that is, above the tips of surrounding foliage or flowers) tie on a heavy paper or plastic band, making it tight both top and bottom, then cover it with Tanglefoot, a thick petroleum goo, or comparable product. Don't spread the sticky stuff directly on the bark, especially on young trees.
Fruit Bags
One of the simplest ways to prevent insect and disease damage to apples is to fasten a paper bag around each one. Admittedly, the tree is no longer quite so attractive; on the other hand, the result is extra-fancy fruit. Buy small brown paper bags, or cut up the long thin kind that wine bottles come in. The method also works well on low-lying tomatoes to prevent slug damage. For grapes, use larger bags.
Traps
Sticky Yellow Boards
Aphids, thrips and whiteflies are attracted to bright yellow. Paint a square of plastic or wood yellow (the ideal color is Rustoleum Safety Yellow), cover it with Tanglefoot and place it near the plants under attack to permanently lure away some of the horde. Place the traps at foliage height and within a foot of the plant where the pests are most dense. Shake the plant and many pests will fly directly to the trap.
Japanese Beetle Traps
These traps are baited with both a food-scent lure and a sex pheromone lure. The beetles fly in, hit a metal baffle and fall into a bag, which can be emptied periodically. The beetles are strongly attracted to the trap, but are just as happy to land on a nearby food source and begin mating there. So place it far away from the plants you want to protect, a hundred feet or more. Empty the traps several times a week. Japanese beetle traps are sold in almost any garden center or hardware store in spring.
Pheromone Traps
Sex pheromone traps are available for gypsy moths and several kinds of fruit pests. However, these traps attract only male insects and do not provide good control. They are really intended to help monitor a growing insect problem so that people can predict the ideal time for taking other control measures like spraying.
Slug Traps
Slugs are strongly attracted to the smell of alcohol and other fermentation byproducts, and will gladly crawl into a dish of beer or sugar water and yeast. These traps should be emptied of drowned slugs daily. Set the traps in a depression so they are almost flush with ground level. (They should be raised ever so slightly so that soil doesn't get in.) Rig up a shingle or piece of board to keep the sun and rain from exhausting the brew.
Diatomaceous Earth And Sprays Of Water
You may think of these as sprays or dusts because that's how they're applied. But they are physical controls, not poisons. A strong spray from a hose will knock off aphids and spider mites, and they'll be unable to crawl back onto the plant. Diatomaceous earth (D.E.) is mined from deposits of the microscopic skeletons of diatoms. It is applied dry with a duster, and kills soft-bodied insects like whitefly and aphids by physically scratching them and causing dehydration. Use a mask to avoid damage to your lungs. And be aware that some D.E. products are mixed with pyrethrum, a botanical poison.
Jack Ruttle was editor of Organic Gardening for 12 years and currently is senior editor of National Gardening magazine.