The best pest control methods for the organic garden is to keep your garden healthy, pests don't become a problem until there is a weakness in the health of the garden.
The best method of controlling insects that damage crops is to keep your garden a secret. Don't invite them to the party. Think of insects like germs. There are always plenty of them around, but they don't become a problem until there is a weakness. What pest control methods for the organic garden work? Who cares if the sphinx moths fly around your garden, as long as they don't lay eggs on your tomato plants that will hatch into tomato hornworms? If we assume they are only going to lay eggs on weak plants, we can be assured they will leave alone our happy and healthy plants growing in humus rich soil. It is clearly better to avoid an insect problem than to have to deal with it. A nutrient balanced soil is the best way to get insects to avoid your garden.
Garden Pest Barriers
Another way to keep insects from getting to your plants is to create a barrier they cannot surmount. You need to know a little bit about the insect to erect the most effective barrier. Someone might have the image of a garden enclosed in mosquito netting. That might work but it is not very cost-effective. I say might, because there are a lot of different insects and if their motivation is strong enough, they are likely to find a way in. Netting can be a cost-effective barrier in some circumstances. Root maggots are the larval form of a fly that lays eggs on the stems of young plants. The flies may lay eggs on more mature plants, but if they do the plants are growing fast enough to survive. Young plants can be protected by a netting barrier laid right over the seedling or even over the seed bed before the seedlings emerge. A square foot of netting will protect a hill of cucumbers. Clearly there is not much work or material involved in this and the hill will produce a lot of cucumbers if helped through the tender stage.
Unhappy radishes will also attract root maggots. Since you would have to cover the whole row with netting to protect them, and considering that radishes reach maturity in a little over three weeks, a barrier hardly seems worth the effort. Radishes also attract flea beetles—usually radishes planted too early in the spring in my garden. While the radishes are shivering in the cold, growing slowly while they wait for the air and ground to warm up a bit more, the flea beetles are feasting, sometimes setting the plants back so severely I lose the crop. No problem: I just throw in some more seeds. Flea beetles will do the same number on broccoli directly seeded in the garden. Since I am going to transplant this crop later, the seedlings can all be grown in a foot-long row which can be covered easily. When the plants are four or five inches tall and ready for transplanting, there is enough when they cut down the leaf surface of transplants. After all, we prune larger plants when we transplant them so the damage to the roots is balanced with a decrease in the above ground portion of the plant.
While on transplants, we must not forget the insect most often combated with barriers: the cutworm. Cutworms also get around on wing but generally the grub is already in the ground when we have to deal with it. A collar of tar paper, cardboard, or some similar material that can be formed into a circle about three inches in diameter and a couple of inches tall and pushed into the soil an inch to make a wall around each transplant.
Diatomaceous earth can also be used as a barrier. This is a powder made up of calcified one-celled plants. To soft-bodied insects it is a sea of razor-sharp edges. What insects crawl on the surface of the ground to reach plants? None, actually, but slugs do. Slugs are mollusks, not insects, but gardeners can find little reason to make this distinction. A band of diatomaceous earth around the cabbage and/or lettuce if these are the crops being plagued, or around the whole garden, will keep slugs at bay if they are coming "from away" as we say in Maine. The diatomaceous earth cuts the bodies of slugs and soft-bodied insects that come in contact with it.
Garden Pest Traps
Slugs suit my purpose as an example of how traps can be used. They do most of their work at night when it is damp and cool. On a hot day, they can be found in cool, moist places. If you put some boards down in the garden, you may find lots of slugs under them in the heat of day. They also are attracted to malt. Dishes of beer will attract them ...and they'll drown happily.
Electric insect zappers will attract night-flying moths which lay eggs that turn into hungry worms. A light placed in juxtaposition to a sticky surface is a good combination for attracting and killing night-flying insects. It is preferable to the zapper because you can see the insects you are trapping and identify them. You will likely find that the majority are, at worst, benign.
Yellow cards covered with sticky stuff attract many insects. Sticky red balls hung in apple trees get covered with insects landing on them, also. These can be used as traps to decrease populations but they are more often used to monitor the number of insects present at any given time.
Integrated Pest Management (IPM)
IPM uses the sticky traps just described to time the spraying of pesticides. IPM is a giant step forward for commercial growers who used to spray large quantities of contact poisons as a matter of routine. If you were selling stuff for farmers to spray on their crops, wouldn't you want them to spray lots and spray often? Most, if not all, good farmers use IPM methods now. It has cut the use of pesticides in this country dramatically. If you are a gardener and you are spraying a contact poison like Sevin at the first sign of an insect, you are the last of a dying breed. Think about it.
Pests and the Timing of Garden Planting
Some people have reported success in avoiding insects by timing plantings so insects are avoided. For example, corn earworms generally do not survive winter much north of New Jersey. If you can figure out when they hatch and how long it is likely to take them to get out of your garden, you can time your planting so the ears form after the first flight and before the second. This won't always work because of variations in winter weather. However, if you have several plantings of corn and some have big problems and other don't, make a note of it. While you may not want to cut out any of the succession crops, you may decide one planting will be better for the bulk of the storage crop than another.
Timing can also be helpful in disrupting an insect's life cycle. Tilling the garden in the fall, for example, puts the cutworm larva up on the surface where birds can find them. I wouldn't recommend this unless I had a major insect problem that couldn't be reasonably dealt with in any other way. The disruption in the problem insect's life cycle is likely to be a disruption to a lot more friendly and benign insects as well.
Garden Pest Sprays
There are only two sprays that I will use: Bacillus thuringiensis (Bt) and Bacillus thuringiensis var. san diego—the former for cabbage loopers, cabbage worms, and tomato hornworms; and the latter for the Colorado potato beetle. I use them because they are non-toxic and because they only kill the target insects, and even then only if they eat a leaf that has BT on it.
The two most famous of these are ladybugs and praying mantis. Several kinds of wasps and lacewings are also helpful. You can buy some of these beneficial insects for release in your garden. I won't go into the gruesome details of how many aphids a hungry ladybug can eat in an hour. Consider what that hungry bug is going to do when the food runs out. "Fly away, fly away" to someplace where the food supply is better. I have got some of all of these insects in my garden or close by, I think, though I don't see them often. I'm glad not to, since that means there isn't much for them to eat.
If I come across a tomato hornworm carrying a load of white oval packages on its back, I leave it alone. The packages are eggs of a parasitic wasp. I want them to hatch so there will be more wasps around to seek out hornworms. The wasps are much better at finding them than I am.
I have been planting rows of beans next to rows of potatoes for over 20 years, because I read somewhere that the beans would discourage Colorado beetles and potatoes would discourage Mexican bean beetles. I am seldom bothered with Mexican bean beetles, but if the beans have had an effect on the potato beetles it has been very inconsistent. It says in Organic Plant Protection, by the editors of Organic Gardening and Farming magazine (Rodale Press, Inc., Emmaus, PA, 18049) that nasturtiums will repel aphids. In my experience I would be more likely to believe that nasturtiums are a catch plant for aphids.
It seems to me that companion planting is a nice idea but that the information available may be more anecdotal than scientific. Tansy is suppose to repel lots of insects, but who wants bales of tansy in the fall even if it would be great on the compost pile?
Low-Maintenance Pest Control in the Garden