Organic natural pesticides break down more rapidly than chemical pesticides, according to South Australia's Zero Waste Program, but they also need to be used with caution. Over-exposing vegetable plants to sprays of natural ingredients can sometimes burn them. Natural pesticides are often a part of an integrated pest management program that includes companion planting and beneficial insects such as ladybugs and praying mantis.
Baking soda is a natural pesticide for mold and mildew problems on squash, pumpkins, cucumbers and tomatoes. Use a mixture of 1 tsp. baking soda, a few drops of liquid dish soap and 2 qt. of water. The soap helps the spray stick to the leaves and is easily washed off at harvest time. Spray on the affected areas to prevent fungal spores from establishing themselves. Repeat twice weekly until mold or mildew disappears.
Milk is lethal to red spider mites and mildew. Commercial chemical pesticides that are commonly used on mites and mildew are harmful to breathe. Zucchini, lettuce, cucumber and tomatoes are often subject to mildew problems. Red spider mites can affect roses. Prepare the spray by mixing equal parts milk and water. Repeat the spray process every two days.
Harmful insects can be removed from broccoli, kale, kohlrabi and other cool-season vegetables by using a strong blast of water but sometimes more help is needed. Dish soap spray is a natural pesticide that kills aphids, caterpillars, thrips, scale insects and mites. They are paralyzed by the soap and starve. Mix 1 tsp. in a spray bottle of water. Coat the upper and underside of leaves and rinse it off the next day. Repeat every two to three days for two weeks. Wash vegetables after harvest to remove any soap residue.
Mix ¾ cup of finely chopped garlic in 2 tbsp. mineral oil and soak it overnight. In the morning add 2 tsp. of dish soap and 1 cup of water. Use this mixture in 1:50 diluted form in a spray bottle. Diluted garlic is a general-purpose natural pesticide that is especially useful for control of aphids, caterpillars and snails on vegetable plants. It is not harmful to pets or humans when used in a diluted form. Rinse vegetables thoroughly after harvest to remove any residual garlic odor.
Brown rot, mildew and aphids can be a problem on tomatoes and squash. Chamomile tea is the mildest form of natural pesticide to use for these common diseases. Make a pot of tea by steeping chamomile leaves, let it cool, and then spray directly on leaves. Continue to spray every few days until the problems abate.