I recently had a post about 
Organic Pest Control  which summed up the results of a survey conducted by Mother Earth  Magazine asking gardeners across the country which methods of organic  pest control they found the most effective.
Although I had 
heard of most of the methods of control  mentioned in the article, I have to admit that I didn’t fully  understand what some of them were. So today I am going to go into a  little more detail describing the various types of organic pest control  which were mentioned in the article, as well as a few others.
First, it is important to make sure that the insects you are trying  to kill are actually bad bugs, and not good ones. If you aren’t sure of  the identification of an insect, there are many sources 
listed in this post which can help you with your ID.
For more details about many of these methods, there is a great page on the Montgomery County Maryland website called 
Pesticide Alternatives.
Manual methods – things you can do:
1) Handpicking– This method of pest control means  just what it says. You walk through your garden and remove the insects  by hand. This method works well for large insects such as beetles and  hornworms. Of course, you have to do something with the bug once you  pluck it. If you don’t have chickens or ducks to feed them to (see  number 9, below) then you can carry a small container of water and soap  or isopropyl alcohol to drop the pests into.  The deceased insects can  then be returned to the earth to provide garden nutrients.
2) Hose ‘em off – Similar to handpicking is spraying  them off with a strong stream of water. This method should be used when  you are watering, anyway, so you don’t waste water, and works, to some  limited extent, for soft bodied insects such as aphids and spidermites.  Be sure and spray the bottoms of the leaves, too.
3) Right Plant, Right Place–  Plants will generally have more pest resistance, and be healthier in  general, if they are grown in the right spot. Get to know your site  conditions and choose plants accordingly.
4) Growing resistant varieties– Plant breeders are  creating cultivars that are more resistant to pests and diseases all the  time. Ask for these varieties when making your plant selections at  local nurseries or through mail order catalogs. Keep in mind that, in  general, 
native plants are often more resistant to native pest species.
5) Timed planting– You can avoid certain garden  pests by planting when they aren’t around. Some pests lay eggs only at a  certain times in the spring. Planting to avoid these times can help  avoid the pest.
6) Companion planting – Companion plants in your  flower and vegetable garden will attract beneficial and predatory  insects to your plants—while repelling unwanted garden thugs. For  example, marigolds repel nematodes; mints (potted to prevent  overgrowth)  repel cabbage pests and aphids; rue deters Japanese  beetles; sweet basil controls tomato hornworm, repels aphids,  mosquitoes, mites and acts as a natural fungicide and slows the growth  of milkweed bugs (and don't forget pesto!); tansy used as a green mulch  can repel cucumber beetles, Japanese beetles, ants, squash bugs.  Chrysanthemums, chives, onions and garlic repel many pests, so plant  them near your vegetable crops.
7) Crop rotation – From season to season, plant  different crop species in the same location. This will help you  discourage pests which were attracted to the first crop and keep pest  populations from building up.
Chemical controls – things you apply
8) Insecticidal soap– Insecticidal soaps work by  penetrating  the insect’s outer layer (cuticle) and dehydrating them and  is useful for soft-bodied insects such as aphids, mealybugs and  whiteflies. Although insecticidal soaps are more eco-friendly than  harsher chemicals (they are non-toxic to humans and test animals and  they biodegrade rapidly in the soil) there are some things to keep in  mind. First, all soaps have phytotoxic properties, so you should test  them on a few leaves before spraying an entire plant.  They will also  kill many beneficial insects so make sure you know which insects you are  spraying. Insecticidal soaps can be purchased at many garden supply  stores now or you can make your own.
9) Horticultural oil – Horticultural oils work by  smothering pests and their eggs, and may also have some repellent  properties. Superior oil products control a wide variety of insects  while going easy on beneficial insects. You can purchase horticultural  oil or make your own.
10) Bt (Bacillus thuringiens),is a  microbial biological control method which works on various larval insect  pests. Bt is applied to the plant, where it is ingested by the  caterpillar, which is  poisoned, paralyzed and eventually dies. Bt will  not harm the majority of beneficial insects in your garden. However, it  will kill butterfly larvae, which of course, are caterpillars. Bt can be  purchased under many brand names.
11) Neem– (also known as azadirachtin) is an  insecticide extracted from  seeds of the neem tree. Neem works as a  broad-spectrum repellent, growth regulator,and insect poison. It  discourages feeding  by making plants unpalatable to insects. If they  eat the treated plant anyway, it inhibits their ability to lay eggs.  Neem works on a wide range of insects, however, caution should be used.  It is described as being ALMOST non-toxic to mammals and beneficial  insects, but should not be used on food plants.
12) Garlic-oil spray-  Garlic oil kills insects, but  not selectively, so it will kill the good bugs as well as the bad bugs.  It works well on aphids, squash bugs, whiteflies and other insects.
13) Diatomaceous earth– Diatomaceous earth (DE) is a  nonselective, abrasive dust commonly used to kill pests in the home and  garden. It works by physical rather than chemical action. DE is the  fossilized silica shells of algae called diatoms, which are covered with  sharp needlelike projections that penetrate an insects cuticle, causing  the insect to die of dehydration. You can dust plants and soil with DE  to control crawling pests like slugs and snails, aphids, caterpillars  and other insects. DE is considered nontoxic to mammals, but can  irritate mucous membranes, so a dust mask should be worn while applying  it. .
14) Milky spore disease – This product kills grubs of Japanese beetles and other related beetles but is harmless to other organisms.
Call out the critters – Beneficial animals and insects
15) Beneficial insects –  Beneficial insects are good bugs that eat bad bugs, and include  ladybugs, green lacewings, parasitic wasps and others. You can garden to  attract beneficial insects  or you can purchase them from garden supply  stores. For more information, see:  
Lady Bugs
16) Beneficial birds – Many birds (including  hummingbirds) also eat garden pests. Gardening to attract birds will  bring you a beautiful natural form of pest control that will sing for  its supper. Read: 
10 Tips for a Wildlife Friendly Garden and 
Gardening for Hummingbirds
17) Poultry predation– Apparently, chickens and  ducks love things like grubs, hornworms and other meaty garden pests. If  you have the time and space to raise poultry, this sounds like a fun,   eco-friendly solution.
18) Beneficial nematodes, Beneficial nematodes are  underground pest hunters that control over 250 different species of  insects that spend some part of their lives underground. They are a very  efficient organic insect control method and kill most insects before  they become adults. This includes lots of common lawn and garden pests  such as grubs, fleas, mole crickets, Japanese beetles and weevils.
Using barriers
19) Floating row covers – Floating row covers are  lengths of fabric  which are lightly draped on plant foliage, creating  barriers from pests. They let in more than 80% of the sunlight, as well  as the rain and other irrigation. Covers must be draped loosely to allow  for plant growth, but secured at the bottom to prevent intruders.
20) Rigid collars – For the plants, not the insects.  Collars can be made of cardboard, plastic or metal cans, open at both  ends and placed around the plants. These work for some cutworms, but not  climbing cutworms.
Many of these products can be purchased online or at your favorite nursery or hardware store.
**The source for much of this material came from two  books from our home bookshelf:  
Rodale's Chemical-Free Yard & Garden and 
The Organic Gardener's Handbook of Natural Insect and Disease Control.
Here are some other great resources for the Metro DC Area:
Pesticide Alternatives – Montgomery County Maryland website
Try Pesticide Alternatives (pdf file) – Maryland Department of Agriculture
Minimum Chemical Gardening – Virginia Cooperative Extension