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Insect Management: Organic and Least Toxic Insecticide Options in the Home Garden
Home gardeners put a lot of time, effort, love, and money into this favorite of projects. We begin soon after Christmas looking at seed catalogs, dreaming of that first beautiful juicy red tomato and those other fresh veggies we will pick in the morning and serve for supper. But danger lurks as the weather warms and the days get longer. Pests! They fly, they crawl, and they come out of the ground to endanger your plants. Getting those plants to survive from seedlings through their lifespan is an on-going project. In this review, we will look at various ways to manage insect pests while minimizing dangers to those many beneficial insects, especially the pollinators.
Prevention, as the old saying goes, is worth a pound of cure. Some prevention strategies include choosing healthy plants if purchasing plants to transplant. Providing appropriate nutrients, water, and light during the growing season is another. And when a problem arises, react quickly to address a small issue before it becomes an overwhelming one.
Prevention includes good management strategies such as rotation of crops in the garden, managing weeds, soil improvements prior to planting, and the application of mulch. Plants should not be crowded so as to allow plenty of air circulation and sun penetration. Make a daily trip to the garden to check on things. Handpick insects you identify as a pest. Become familiar with the life cycles of common pests you see and time planting dates to avoid them. An example is the cabbage looper on cabbage, broccoli, and cauliflower, which can be largely avoided by planting in spring or fall during cooler weather. Another is the squash vine borer on summer and winter squash, and pumpkins. These can be avoided by waiting until early July to plant seeds. The plants will have matured after the adult borers have finished laying eggs.
If or when a pesticide becomes necessary there are effective, minimally toxic options from which to choose. There are good advantages to these options. Their use poses less risk to humans and the environment because they break down quickly so they do not accumulate in humans or the environment. Some of these organic and low toxic insecticides are pest specific, so they do little or no damage to other organisms. A word of caution: organic pesticides are still pesticides. Always read labels and follow all instructions. If spraying is necessary, always spray late evening to reduce risk of impacting pollinators.
What follows is some of the most common options to choose from:
Insecticidal soaps kill soft-bodied garden pests such as aphids, mites, and mealybugs on contact by smothering them and causing them to dehydrate. Because these products work on contact, both upper and lower leaf surfaces and stems must be sprayed. They are most effective when they dry slowly and not effective once dry; spray late in the day. Multiple applications may have to be applied. Hard-bodied beneficial insects such as ladybug and other beetles are not harmed. A word of caution: insecticidal soaps can cause damage if applied when plants are water stressed or in temperatures above 90 degrees. Some plants are sensitive, especially when used in direct sunlight. For first time use, test a small area and wait 24 hours before spraying a large area. Evidence of injury from phototoxicity includes leaf scorching and browning, defoliation, reduced flowering and stunted growth.
Some commercially available examples include Bonide Insecticidal Soap, Natural Guard Insecticidal Soap Concentrate, and Espona Earth-tone Insecticidal Soap Concentrate. A soap spray can be made at home. You will need a liquid dish detergent without scent, color, or chemicals such as Nature’s Promise or Ivory). Add 2-4 tablespoons per gallon of water, starting with 2 tablespoons. Shake and observe for a fair amount of bubbles. Add additional soap, 1 tablespoon at a time, up to 4, to produce visible bubbling when shaken.
Insecticidal Oils (Horticultural Oils)
A few definitions first.
- Horticultural oil=an oil used to control a pest on a planting
- Mineral oil=a refined petroleum-derived oil
- Vegetable oil=derived from the seeds of some oilseed crops
The safety to people, other mammals, and beneficial insects is similar to insecticidal soaps and the same cautions apply. Horticultural oils mode of action is also by smothering on contact. Spray late in the day to allow slow drying to get better insect control. Always follow directions on labels and cover all plant surfaces. Use a proper 1-gallon sprayer. Adjust the pressure to avoid large droplet sizes.
Some commercially available examples include Ferti-loam Hoticultural Oil Spray Concentrate, Bonide All Seasons Spray oil Concentrate.
Some types of vegetable oils are employed for insect control. Cottonseed oil is considered the most insecticidal. Soybean oil, commonly available, can provide fair to good control of some insects and mites. To make a spray at home, add one tablespoon of oil to one gallon of water. To disperse the oil, add liquid dish soap (type mentioned above). Start with 1 tablespoon. Shake. Oil needs to stay dispersed about 30-45 seconds after shaking. Add additional soap, if needed. Add to sprayer, shaking frequently.
These are naturally occurring toxins extracted from plants. ¹ They have the advantage of breaking down quickly leaving little risk of residues on food crops or to beneficial insects. Their disadvantage is precise timing and more frequent applications than other chemical insecticides.
Neem Oil is a botonical insecticide made from the extracts of Neem tree seeds. ¹It can be purchased as a hydrophobic extract or cold pressed and usually as a concentrate. Neem Oil spray is used to control aphids, leafminers, whiteflies, spider mites, scale crawlers, and beetles. Neem oil kills by suffocation but does have some insecticidal properties as well. Many commercial products are available such as Ferti-loam Rose, Flower and Vegetable Spray Concentrate, Bonide Neem Oil Concentrate, and Garden Safe Neem Oil Extract Concentrate.
Pyrethrin/Pyrethrum is produced from finely ground powdered flowers of a species of daisy. ¹ It is a contact insecticide and must be applied directly to the insect. ¹The insect is paralyzed, but may not be killed so it is frequently combined with another insecticide to ensure the insect does not recover. Pyrethrin products have a low toxicity to mammals so can be applied to food crops.
Commercially available products include PyGanic Crop Protection EC, Perti-lome Triple Action Plus II with Neem Oil, and Espoma Earth-tone Disease Control Concentrate (includes Sulfur, which is probably one of the oldest know pesticides in current use).
Other interesting options include garlic and essential oils. Garlic can be used to repel insects as well as nuisance animals though there is limited research showing effectiveness. Essential oils, including citronella, clove, cedar, cinnamon, mints and rosemary, have recently been marketed. They generally work as insecticides by contact killing or disrupting an insect neurotransmitter that is not present in people, pets, or other vertibrates.
These products contain microorganisms (viruses, bacteria, fungi, protozoa or nematodes) or their by-products. ¹Most of these are very specific with a narrow range of insects affected thereby sparing most beneficials. Commonly used microbial insecticides include Bacillus thuringiensis (Bt) and Bacillus thuringiensis var kurstaki (Btk). Bt is effective against young larval stages, not adults. The spray must be on the parts of the plants on which the insects are feeding and must be eaten by the insects to be effective. The insect will not die immediately but will stop feeding immediately. Treatment is best done late in the evening or on a cloudy day as it breaks down in sunlight. Btk is sold as floating dunks or pellets for use in ornamental ponds and similar places to eliminate mosquito larvae
Diatomaceous Earth and Kaolin Clay are two examples of minerals. Diatomaceous Earth is a non-toxic powder of fossilized, one-celled organisms called diatoms. ¹It is the “natural grade” type that is used as an insecticide, not the type used to filter swimming pools. It can be used to control ants, slugs, sow bugs, and soft-bodied insects. Note: it kills honey bees so do not apply to crops in flower. Kaolin clay acts as a barrier that disguising insect host plants with a white film on apple and pear trees, for example. It is a preventative, not a control.
In conclusion, with a bit of on-going attention in the garden you can indeed be eating that juicy red tomato and fresh green beans, among other fruits and vegetables from your yard. All without poisons and toxins that can harm the environment, the pollinators, and other beneficial visitors.
By Joyce Lyons, Master Gardener℠ Volunteer in Currituck County
¹ “Less Toxic Insecticides” Clemson Cooperative Extension Home and Garden Information Center.
“Insect Control: Horticultural Oils” Fact Sheet No 5.569 Insect Series/Home and Garden. Colorado State University Extension.
“Using Oils as Pesticides” Carlos E Bogran, Scott Ludwig and Bradley Metz. Texas A&M AgriLife Extension Service.
Organic Production Series – Insect Management. NC State University publication