Gardens are in constant flux and, though we may not see it, there is always a battle of the bugs happening. Though gardens that have a balance of beneficial insects have more allies in this war, sometimes it is still necessary to help out with insect spray. Organic insecticide, if you must use insecticide at all, is the safest way to help stave off the bad insects while keeping as much of a balance in the garden as possible.
There are three main types of organic insecticides: botanical, biological, and oils and soaps, which fit into a general third category. Botanical insecticides are derived from plant sources such as neem or garlic, and essential oils such as clove, wintergreen, cinnamon, rosemary and peppermint.
Biological insecticides are derived from living organisms such as Bt (Bacillus thuringiensis), a naturally occurring bacterium, diatomaceous earth (a biological insecticide made from the crushed exoskeletons of microscopic sea creatures), milky spore (another naturally occurring bacterium), and Spinosad, which has a low toxicity to beneficial insects.
The third category includes soap, horticultural oils, iron phosphate and kaolin clay.
Neem suppresses feeding, prevents insects from molting and also has fungicidal properties. Garlic oil should be sprayed only as a preventative before pest populations expand, and not on blooming or fruiting plants. It will repel the pollinators. Botanical essential oils can also work to control root pests such as wireworms, cutworms, aphids, potato beetles, loopers, mites and whiteflies, and they are very safe.
Bt organically controls various caterpillar species and foliar feeding worms. This won't harm beneficial insects, bees, soil or humans. Diatomaceous earth dehydrates and kills pests such as ants, slugs, roaches, ticks and fleas. Milky spore attacks the larvae of Japanese beetles. According to "Grow Organic," a single application lasts 10 to 15 years.
All soaps and oil sprays must come into direct contact to pests to work, but are quite safe.
Though organic insecticides are considered much safer than chemical insecticides, it is still smart to wear protection when applying many of these to your vegetable garden. Wearing gloves and a face mask while using diatomaceous earth is especially recommended as it should not be inhaled. The one to worry least about is soap. Raymond Nones, author of "Raised-bed Vegetable Gardening Made Simple," recommends a spray bottle filled with 1 qt. water and 1/2 tsp. dishwashing liquid soap. It is also important to pay attention to application depending on the method being used. Horticultural oils for example must be sprayed on all surfaces of a plant's foliage and must come into direct contact with pests to work. Others, such as diatomaceous earth, must be applied again if it rains.
When possible, avoid monoculture. Nature is complex and naturally diverse, and doesn't have just one species growing over a large area all by itself. Monoculture is a human invention and it causes problems such as disease. Raymond Nones says that growing a variety of vegetables in close proximity to each other will lessen the chances of disease or insect problems. Also, keeping the garden clean and free of plants that have rotted or are dying, as well as fallen debris like leaves and fruit, will help to attract less bugs.
It is important to keep in mind that eliminating every last aphid, cutworm, slug and Mexican bean beetle is not the goal. The goal is to bring the bad bugs down to a tolerable level. Pest insects are needed, after all, for those good insects to stick around and help maintain the balance naturally. As written in "Grow Organic," by Doug Oster and Jessica Walliser, "solving pest problems organically means finding that delicate balance between wanting our gardens to thrive and maintaining a bit of reverence for nature's process." Using sprays should be your last resort.