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The Garden Scoop

Organic Pest Control

In recent years, as interest in more sustainable agricultural practices has grown among home gardeners, organic pest control options have become widely available in many retail outlets and garden centers.  I use many of these products in my own garden and find their origins in nature and modes of action against pests quite interesting. 

The term ‘organic’ means different things to different people.  For the purpose of this discussion, I am referring to gardening practices that meet current USDA Organic Standards.  In general, these standards prohibit the use of most synthetic fertilizers and pesticides, relying heavily on physical, mechanical or biological based farming methods. 

It is a common misconception that pesticides are not permitted under the USDA Organic Standards.  However, there is a growing number of pesticides commercially available to home gardeners which meet USDA Organic Standards.  These products are derived from plants, microorganisms and other naturally occurring sources.  They typically breakdown in the environment quicker than synthetically produced pesticides and, when applied correctly, offer equally effective levels of control. 

Pyrethrum is an organic approved pesticide derived from dried flower heads of several species of chrysanthemums.  Harvested flowers are dried and turned into a powder or solvents are used to extract oils from the flowers.  The resulting insecticide has been used across many disciplines to control everything from human lice and mosquitoes to cockroaches, beetles and flies.  In horticulture, it is used as a non-selective insecticide for many common pests.  It works as an insecticide by quickly penetrating the nervous system to induce loss of motor skills or paralysis, often in minutes.  However, insects do have some natural defenses to this product.  The digestive tract of many insects contains enzymes that can detoxify pyrethrums before death.  Therefore, proper dosing is extremely important and reapplication may be necessary.  Since this pesticide is non-selective, it can have negative impacts to beneficial insects.

A more selective organic-approved pesticide is Bacillus thuringiensis (Bt), a naturally occurring soil bacterium.  Bt strains are very host-specific to a limited number of insect species including lepidopteran caterpillars and some beetles.  When Bt is consumed by a susceptible host, insecticidal proteins produced by the bacteria are activated in the insect’s gut.   These proteins bind to receptors in the insect’s intestine creating holes in the intestinal lining that shut down the digestive tract, resulting in death.  The insect carcass is then an ideal, nutrient-rich home for the bacteria to further propagate.   Although this pesticide is very host-specific, which is good for beneficial, non-target insects, it must be consumed by the pest and does nothing from general exposure.  Thus, applications should be carefully timed to ensure the pest actually consumes Bt.  In addition, Bt rapidly degrades in the environment, often requiring reapplication frequently to get effective control.

Spinosad is another organic-approved pesticide that originates in soil bacteria.  It is a mixture of two chemicals (spinosyn A and spinsoyn B) that are produced by the soil bacterium Saccharopolyspora spinosa and are collectively referred to as spinosad.  This insecticide targets the nervous system on contact or after ingested by causing muscles to flex uncontrollably, leading to paralysis and death in just a few days.  Interestingly, spinosad targets a wide range of pests, but spares many of our common beneficial insects, making it somewhat selective.  When wet it is toxic to bees, but is relatively safe upon drying.  So, applications can be timed during low bee foraging activity, such as nighttime, to limit impacts.  It degrades slower than Bt in the environment, lengthening the reapplication interval.  While this pesticide is not entirely selective, it does have the unique characteristic of targeting some of the worst pests while sparing many of the beneficial insects we want.

Although it technically isn’t a pesticide, Kaolin clay can be mixed with other ingredients and sprayed on foliage to limit or stop insect feeding.  It works by exclusion, creating a physical barrier that deters many leaf-feeding pests.  Humans have used Kaolin clay and other constituents of mineral soil for thousands of years to protect crops, so this method is not new.  Kaolin clay has a low shrink-swell capacity and a low cation-exchange capacity, meaning it is relatively inert, reacting with fewer things than other clay particles.  These unique qualities allow it to be combined with other substances (to keep it in suspension) and effectively sprayed on plants.  The fine leaf coating that is produced not only deters feeding of insects and other animals, but also helps protect against diseases and environmental stresses, such as heat stress.

Another common misconception about organic-approved pesticides relates to their toxicity.  Many believe that these products are less toxic than synthetically produced pesticides.  In some cases organic pesticides are less toxic or sometimes very benign substances (like Kaolin clay or Bt), but in other cases they may have similar toxicity to their synthetic cohorts.  As with any pesticide application, label instructions must be followed exactly to ensure safety and effective pest control. 

Organic pesticides provide an additional suite of options for gardeners interested in pest control with a bit lighter footprint on our environment.  With the growing number of products available these days, it is now easier than ever for home gardeners to move toward organic methods.