Are organic pesticides safer for my garden?

A Question of the Week
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Based on what we hear at the UNH Extension Infoline, many home gardeners are interested in having less impact on the environment and their personal health, but still want to use something to kill bugs and weeds. Many people are turning to organic and natural pesticides as a safer alternative to synthetic pesticides. Organic pesticides generally come from things in nature that can be used to control pests. This includes substances derived from plants, minerals, and microorganisms. Many organic pesticides are less toxic than their synthetic counterparts, but that doesn’t mean they are safe or won’t cause environmental harm. Just like synthetic pesticides, organic pesticides are formulated to kill. Even if the active ingredients come from a natural source, they are at much higher concentrations than they would ever be found in nature.

Before contemplating using any pesticide, you should consider its inherent risks. The risk of causing personal injury depends on the toxicity of the material. All registered pesticides have been tested to determine their LD50, which is essentially a measure of the acute toxicity of a product based on the lethal dose it takes to kill 50 percent of the test sample. A low LD50 means that it takes very little pesticide to cause harm, and the material is more toxic to humans. A high LD50 indicates the opposite. All pesticide labels, both synthetic and natural, include signal words that specify the toxicity of the product. CAUTION means low toxicity, WARNING means moderately toxic, and DANGER means highly toxic.

 The choice to use organic or natural pesticides also stems from interest in protecting the environment. With this objective in mind, you must take into account the dosage that is required to control the pest. In some instances, a small amount of synthetic pesticide may equal a large amount of organic to solve the same issue. Another key point is that organic doesn’t mean that only the target pest will be killed. Other non-target beneficial organisms can be impacted as well.

Care should also be used around homemade pesticides. You can find lots of homemade recipes online, but be very cautious using these substances because their efficacy, toxicity, and environmental effects have not been university tested. Despite being made of common household materials like dish soap and vinegar and having seemingly fewer risks, these concoctions may have unforeseen consequences and cannot be relied upon to have the same effectiveness as a registered pesticide.

Two of the most common organic pesticides used by gardeners are neem and spinosad. Neem oil comes from the seed of the neem tree, which is native to Southeast Asia. It is used as both an insecticide protectant. When applied directly to insect’s bodies, it can block their breathing openings and cause suffocation. Neem is also known to interfere with insect growth and behavior. Additionally, it can prevent the germination and penetration of certain fungal spores into leaves, most notably powdery mildew. Some benefits of neem include low mammalian toxicity and limited persistence in the environment. However, neem oil is not effective against every insect and disease, and often must be frequently reapplied to reach the desired outcome.

Spinosad is a selective broad-spectrum insecticide that is derived from a bacterial species. It is labelled to control a wide variety of insect pests that attack vegetables, including caterpillars, thrips, and leafminers. It is both a nerve and stomach poison, killing pests that consume it and come in contact with it. Unlike other broad-spectrum insecticides, it does not harm many of the beneficial insects once it has dried, such as lacewings, parasitic wasps, and lady bugs. However, spinosad is very highly toxic to bees when wet and should not be used where bees are present.

Ultimately, pesticides should always be viewed as a last resort. Focusing on maintaining the health of plants through proper culture is a more sustainable and long-term solution. Healthy, vigorous plants are much more resilient to pests and are less likely to need chemical intervention. If an issue does arise, make sure to correctly identify the pest. Many insect and disease issues can be solved in the home garden by hand-picking insects or clipping and disposing of diseased plant parts. When all else fails, choose the most effective, least toxic pesticide to do the job, which very well could be an organic option. And remember, always read and follow the instructions on the product label.


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Got questions? The Ask UNH Extension Infoline offers practical help finding answers for your home, yard, and garden questions. Call toll free at 1-877-398-4769, Monday to Friday, 9 a.m. to 2 p.m., or e-mail us at answers@unh.edu.