To Neem or Not to Neem, That Is the Question

by Hamutahl Cohen

You’ve probably seen Neem oil recommended in blogs, gardening forums, and on the shelf at your local gardening store. Neem is derived from the seeds of the Azadirachta indica tree, and is one type of horticultural oil that is used by gardeners looking for alternatives to synthetic insecticides. But is it effective? Is it benign? This post explores the pros and cons behind neem and other horticultural oils.

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What Are Horticultural Oils?

Horticultural oils are either plant-based (like neem, canola, or clove oil) or mineral-based (refined petroleum products), and they work mostly by smothering soft-bodied pests like aphids, scale, and whiteflies.

Some plant-based oils do contain chemical compounds that can do more than smoother —neem oil contains azadirachtin, which disrupts insect development and feeding behavior.

Horticultural oils are typically considered low-toxicity for humans, and can be used on a very wide range of plants, including vegetables, fruits, ornamentals, and houseplants.

The PROS

1. Broad-Spectrum Neem oil is effective against a range of insect pests and some fungal diseases, yet remains relatively safe for humans, pets, and other animals. According to the Environmental Protection Agency neem oil “has been shown to have minimal impact on non-target organisms” (EPA, 2012) such as birds and mammals.
2. Reduced Resistance Potential
   Unlike synthetic insecticides that often target a specific physiological pathway (and thus promote resistance over time), neem oil affects multiple aspects of insect development, which makes resistance less likely to develop quickly.
3. Organic-compatible and easy-to-apply
   Neem and other horticultural oils are generally approved for organic gardening. interventions. They are easy for home gardeners to use since you can spray from a store-bought bottle and avoid any special equipment.
4. Low Residual Activity
   These oils break down quickly in sunlight and soil, reducing long-term environmental contamination and residue on edible crops.

The Cons

1. Phytotoxicity Risk
   If you use oils in high temperature or direct sunlight, it can lead to leaf burn and plant damage. Apply it early in the morning or late in the day to minimize this risk.
2. Non-Selective Action
   Neem and other oils can still harm beneficial insects if sprayed directly. Lady beetles, lacewings, and bees can be affected by fresh residues. You can time your application to avoid flowering periods, or spray during the evening when bees are less active to avoid non-target impacts.
3. Repeat applications sometimes required
   The effects of oils can take days to manifest and may require repeated applications (e.g. every 10 days) for best results. Gardeners expecting immediate eradication may be disappointed. Oils often work best in conjuction with other control strategies (e.g. pruning out infested areas, releasing beneficials, etc.)
4. Storage and Shelf Life
   Oils can degrade over time, especially when exposed to light and heat. They can go rancid or lose efficaciousness. Check the expiration date on your bottle and store in a cool, dark place.

Concluding Thoughts

I like horticultural oils. They can be effective tools and are safe for people to use. But they need to be used with some consideration, particularly timing to avoid non-target impacts to beneficial insects and leaf burn. Let me know your experience with oils.

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References

EPA. (2012). Neem Oil; Exemption from the Requirement of a Tolerance. Federal Register. https://www.federalregister.gov/documents/2012/05/31/2012-13143/neem-oil-exemption-from-the-requirement-of-a-tolerance