Biochemical Biopesticides: Plant Extracts

Plants are, in effect, natural laboratories in which a great number of chemicals are biosynthesized. Many plants have developed natural, biochemical mechanisms to defend themselves from weed competition and animal, insect and fungal attacks. Some of these chemicals discourage feeding by insects and other herbivores. Others provide protection or even immunity from diseases caused by some pathogens. Still others help plants compete for resources by discouraging competition among different plant species. By studying the diverse chemistries of many different plant species, scientists have discovered many useful compounds that can be used as biopesticides.   

Plant extracts have long been used to control insects.  Dating as far back as 400 B.C., children were deloused using a powder obtained from the dried flowers of the pyrethrum plant (Tanacetum cinerariifolium).  The first botanical insecticide dates back to the 17th century, when it was shown that nicotine from tobacco leaves killed plum beetles.  Today, there are a number of biopesticide plant extracts being marketed as insecticides. These products fall into several different classes:

Insect Growth Regulators
Plant extracts that prevent insects from reaching the reproductive stage. 
         
Feeding Deterrents
 A compound that, once ingested by the insect pest, causes it to stop feeding and eventually, to starve to death.  Crop damage is inhibited and the insect eventually starves to death. 
         
Repellents
Typically compounds which release odors that are unappealing or irritating to insects.  Examples include garlic or pepper-based insecticides.  

Confusants
Compounds that imitate food sources and are used as traps or decoys to draw damaging insects away from crops.  Confusants can also be formulated as concentrated sprays designed to overwhelm insects with so many sources of stimuli that they cannot locate the crop.  
Not only are plant extracts used directly as insecticides, but they are used also as a source for synthetic insecticides based on analogues developed in the laboratory.  Scientists have modified molecules found in plants to be more toxic or more persistent.  Common examples of this can be found in the pyrethroid and neonicitinoid families of insecticides, derived from molecules isolated from plants like pyrethrum (T. cinerariifolium) and tobacco. 

Plant extracts can also be used as bioherbicides, and are based on several different modes of action:

Plant Allelopathy 
A term used to describe the biochemical interaction that inhibits the growth of neighboring plants by another plant.   Black walnut trees( Juglans nigra), for example,  produce the allelochemical juglone, which is toxic to many other plants.  Many recently discovered allelochemicals have potential for development as natural product herbicides.

Plant Growth Regulation
Some plant oils can act as effective contact herbicides through a variety of mechanisms such as disrupting cell membranes in plant tissue, inhibiting amino acid synthesis, or precluding production of enzymes necessary for photosynthesis.

Mechanical Control
Some plant extracts are powerful natural agents which act directly on weeds.  D-limonene, for example, is an effective degreasing agent which strips the waxy cuticle from weed leaves, causing necrosis, dehydration and weed death.

Plant extracts are also used in commercial agriculture as fungicides and anti-microbial tools in food safety applications.  They rely on the following modes of action:

Fungicidal Control
Some plant extracts act as contact fungicides.  Some disrupt cell membrane integrity at different stages of fungal development, while others inactivate key enzymes and interfere with metabolic processes.   

Induced Resistance
Crops treated with some plant extracts produce and accumulate elevated levels of specialized proteins and other compounds which inhibit the development of fungal and bacterial diseases.  In effect, the crop’s immune system is triggered to defend against destructive diseases.  

Using plant extracts as biopesticides offers growers many unique benefits.  Generally, plant-based compounds degrade rapidly, reducing the risk of residues on food.  Many of these products have very short preharvest intervals.  Most products show wide windows of crop safety and resistance to these compounds is not developed as quickly as with synthetic pesticides due to multiple modes of action.  Many plant extracts used as insecticides are fast-acting, quickly inhibiting insect feeding and additional crop damage.  And, because they act on the insect’s gut and  rapidly decompose in the environment, many plant extract insecticides are more selective to insect targets and safer to beneficial insects.