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August 12, 2014
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DESERT BLOOM: Organic Pesticides

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Angela

Organic PesticideOccasionally, horticulturists everywhere must confront the problems posed by unwanted guests. I’m not talking about people who overstay their welcome or steal vegetables; no, these unwanted guests can be insects, weeds, diseases, even cute little furry creatures.

Some insect pests, like houseflies, are simply annoying, but there are others who can wipe out a beautiful garden in no time. Anyone who’s seen a lush green tomato plant reduced to a skeleton will no doubt have strong feelings about tobacco hornworm, the very hungry caterpillar of a sphinx moth. A potentially delicious crop of melons can be destroyed by squash bugs, voracious villains that seem to appear overnight in droves.

Insects aren’t the only pests.

When weeds become too plentiful, they interfere with garden plants by pulling nutrients and water from them, by shading them, or by serving as a refuge for other pests. A few weeds even secrete chemicals that interfere with the growth of their neighbors.

One of the many good features of living in the desert is the lack of most plant diseases. We have a few major problems: powdery mildew (a particular problem for roses, although virtually every domesticated plant can suffer from it), crown gall, where woody plants develop tumors on the trunk, and fire blight, which kills many fruit trees, as well as roses and other ornamentals.

Usually, a healthy plant’s better able to resist attack by insect pests or disease, but there are times when an infestation or an infection occurs despite our best efforts. When any of these troubles afflict our ever so tenderly cared for gardens, it can be tempting to grab a container of poison and spray like mad. Seductive as conventional pesticides are, they can create further problems. While they promise to make the garden perfect, they might damage other plants, pets, the environment, even people.

If they want to be truly organic growers, then gardeners must resist that lure. Still, they need to protect their plants from invaders. Fortunately, there’s a fair number of control measures that are labeled for use in organic horticulture.

Insect pests have quite a few natural enemies, thank goodness. Parasitic nematodes, lacewings, ladybugs, trichogamma wasps, they’re all voracious, and don’t damage plants. Some of these beneficial insects are available locally, and all of them are sold over the internet. These good agents are still insects, however, susceptible to insecticides, so don’t release them soon after using one of the poisons, if you have used one.

Organic insecticides are poisons derived from bacteria, fungi or plants. There are also physical barriers, like floating row covers that loosely drape over the plants, preventing pests from attacking leaves. Horticultural oil smothers eggs and larvae, but make sure that it’s the right one for the season. You should apply dormant oil only when the plant is dormant. Those are used on trees and shrubs, mainly, although Colorado Extension advises applying mineral oil with an eye dropper to control corn ear worm. Summer oils are much lighter, but these can damage plants when the temperature’s higher than about 90°.

Boric acid poisons an insect’s stomach and nerves; it can hurt people in high doses. Diatomaceous earth damages the insect’s chitin shell, killing it without bothering us.

In addition to pulling or steaming them, one can control weeds with herbicidal soaps, horticultural vinegar, citrus oil, clove oil, and corn gluten meal. Except for the gluten meal, these’re all “burn down herbicides”, meaning they only kill the above-ground plant parts, not the roots, so the weeds might come back. Corn gluten meal goes on the ground before weed seeds emerge; it’s a “pre-emergent” or “weed preventer.”  

No matter what you’re applying, always read the label to use the product safely. If a product’s organic, it’ll have the OMRI (Organic Materials Review Institute) logo on the label. OMRI’s a national nonprofit organization that supervises all agricultural products to make sure they’re in line with the USDA national organic standards.

That’s a good thing, because too many products make claims to be “natural”, which has no meaning at all. Anyone can say the product’s natural, so what?

Just because a compound’s organic, doesn’t mean it is harmless. Pyrethrum and rotenone are both derived from plants and they’re both organic, but either of them can be highly toxic to fish if it’s applied too close to a body of water. 

For more information on labels and pesticides, whether organic or not, contact Cooperative Extension. We’re here to help.

For KNPR’s Desert Bloom, this is Dr. Angela O’Callaghan of the University of Nevada Cooperative Extension. 

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