While small-scale garden intrusions can be nipped in the bud with hand-plucking or a well-aimed water hose, swarms of destructive invaders may warrant calling in heavier artillery. Bacillus Thuringiensis, known as Bt, is a naturally occurring bacterial microbe that acts as a stomach poison, killing the targeted pest by starvation without harming humans, food crops or other wildlife. Chemical insecticides, nervous-system poisons, exterminate a wider range of pests but pose greater health and environmental hazards.
If you're looking to stamp out a tent caterpillar plague, gypsy moth infestation or bagworm attack, the best-known Bt strain, Bacillus thuringiensis kurstaki, is just what the gardener ordered. Btk is only effective on leaf- and needle-eating caterpillars. Trade names include Biobit, Dipel, MVP and Thuricide.
Because of their non-harmful, targeted properties, research and development of other Bt products is on the upswing. Bacillus Thuringiensus tenebrionis, Btt, kills some types of beetles. It's sold under a variety of product names including Trident, M-One, M-Trak, Foil and Novodor. Another strain (Bacillus thuringiensus israelensis), Bti, kills black fly and mosquito larvae.
Wait to apply until pests are feeding. You can use a dust, available by the bag or in a ready-to-use shaker canister, for flowers, vegetables, shrubs and other foliage. It also comes in a water-soluble formula for hose-end or handheld garden sprayers, the preferred method for treating trees or large-scale areas. The general rate is 1 to 2 teaspoons per gallon of water; refer to the brand you’re using for exact measurements.
Unlike chemical applications Bt loses potency within a few days and leaves no residue. It will degrade even faster in bright sunlight. You may not notice an immediate improvement; it takes time for the bacteria to enter the digestive tract.
Chemical insecticides are less selective and faster-acting than BT. Many products kill on contact. Products labeled “broad spectrum” kill hundreds of different types of insects; some also prevent or treat plant diseases.
Active ingredients are listed on package labels. Carbaryl, permethrin, malathion, pyrethrine and bifenthrin are among the most common. Some are more toxic than others. Some can only be used on ornamental plants. Even chemical insecticides for vegetable gardens may not be suitable for all vegetables.
Chemical insecticides come in ready-to-use sprays and dusts as well as water-soluble concentrates for garden sprayers. Dilution rates average 1 ½ to 2 teaspoons to 1 ½ to 2 tablespoons per gallon of water depending on the product and the type, size and growth stage of the plant. Dry granules that are sprinkled and then wet down with a hose to activate can also be used for spot treatment.
Application frequency is based on the type of plant. Package labeling lists how long to wait after application -- days to weeks -- before harvesting edible plants.
Both organic and inorganic insecticides create toxic chemical reactions. Limit exposure by wearing protective glasses or goggles, gloves, hat, long pants, a long-sleeved shirt and chemical-resistant footwear.
Choose a calm, overcast day with no rain in the 24- to 48-hour forecast. Pay close attention to even the slightest breeze to avoid blowback or overspray. Keep people and pets out of treated areas until dry.
Manufacturer websites including Bayer, Bonide, Ferti-lome, Dragon, Ortho and Spectracide offer easier-to-read labeling, instructions and product menus that help you narrow down the options. Your local greenhouse can also assist in choosing the best product for your situation.
- Colorado State University Cooperative Extension Service: Bacillus Thuringiensis
- Cornell University Cooperative Extension Service: Toxicology Network Toxicology Information Brief
- Cornell University Cooperative Extension Service: Carbaryl
- Cornell University Cooperative Extension Service: Malathion
- University of Kentucky College of Agriculture: General Use Insecticides for Home Gardening
- Gardensafe.com: Garden Safe Fruit and Vegetable Spray
- Fertilome.com: Dipel Dust Biological Insecticide
- Spectracide.com: Customer Help Labels and Material Safety Data Sheets
- Texas A&M University Agrilife Extension: Understanding Common House and Garden Insecticides
- U.S. Environmental Protection Agency: Types of Pesticides
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