Plant extracts and essential oils as biopesticides and good alternatives to synthetic pesticides

 

During the last half century, the protection of crops is depended on synthetic pesticides which are highly toxic and have adverse residual effects on crops, groundwater and soil. Moreover, their excessive application, leading to increased resistance of pests.  Few years back, when farmers used pesticides such as DDT and malathion, there was little understanding of how dangerous and long-lasting these chemicals are. It was only later that the degree to which these pesticides remain in the environment was discovered. Organophosphates designed to affect the brain and nervous system of insects, sometimes damage those of humans and animals. These pesticides killed bald eagles, birds, fishes and even people. The World Health Organization (WHO) reported that around two hundred thousand people were killed every year due to chemical pesticide poisoning. World Resource Institute reported that more than 500 insects were resistant to insecticides.

Many researchers have suggested using plant extracts and essential oils as good alternatives to synthetic insecticides. The bacterium Bacillus thuringiensis was the first biopesticide used and has the most widespread use till the day. Bacillus thuringiensis was isolated from a diseased silkworm by Japanese biologist Shigetane Ishiwata. Germany rediscovered it ten years later in a diseased caterpillar of flour moth. The nicotine was the earliest plant-based biocontrol used to control plum beetles during the 17th century. Then in 1835, Agostine Bassi demonstrated that white-muscadine fungus (Beauveria bassiana) could be used to cause an infectious disease in silkworm. Furthermore, experiments with mineral oils as plant protectants were also reported in the 19th century. In the latter half of the 20th century, research and development continued at a low level because of the widespread adoption of cheaper but more toxic synthetic chemical insecticides. During the early 20th century, studies of soil microbiology and ecology had led to the identification of many different microorganisms that act as antagonists or hyperparasites of pathogens and insect pests. A number of these were shown to be useful in field-scale inoculations, but few were developed commercially because of the rapid adoption of chemical pesticides during that time period.

Use of plants such as Azadirachta indica, Annona squamosa, Calotropis procera, Cannabis sativa, Pongamia pinnata, Datura metal, Melia azedarach, Moringa oleifera, Vitex negundo etc for prevention of insect and pests without any harmful effect have been mentioned in the ancient Indian literatures. These pesticides are readily biodegradable; do not accumulate in air, soil and water or on plants and quite safe for non-target human and animal subjects. Natural, plant-based pesticides are not only effective and inexpensive for protecting crops, but also safer and more environment-friendly. A recent study has shown the ovicidal and repellent activities of essential oils from Cymbopogon citratus, Cinnamomum verum, Eucalyptus globulus, Illicium verum, and Zanthoxylum limonella against cockroaches (Periplaneta americana). It is also reported that a combination of E. globulus and Rosmarinus officinalis essential oils showed a stronger insecticidal effect against adult P. americana than that of each individual essential oil. Biochemicals can act through a variety of mechanisms. Some act by inhibiting the growth, feeding, development or reproduction of a pest or pathogen. Essential oils from Neem tree alter the hormones of bugs so that they cannot fly, breed or eat. Natural pesticides have many advantages over synthetic ones and may be more cost-effective as a whole, considering the environmental cost of chemical alternatives. Moreover, natural pesticides are biodegradable and are less likely to harm humans or animals. In addition, they are cheaper and more accessible in less developed countries.

Table 1.

Examples of some commercially available biopesticides

Category	Type	Active ingredient	Product name	Targets	Crop
Micro-organism
Bacteria	insecticide	Bacillus thuringiensis var kurstaki	Dipel DF	caterpillars	vegetables, soft fruit, ornamentals and amenity vegetation
	fungicide	Bacillus subtilis QST713	Serenade ASO	Botrytis spp.	vegetables, soft fruit, herbs and ornamentals
	nematicide	Pasteuria usgae	Pasteuria usgae BL1	sting nematode	turf
Fungi	insecticide	Beauveria bassiana	Naturalis L	whitefly	protected edible and ornamental plant production
	fungicide	Coniothyrium minitans	Contans WG	Sclerotinia spp.	outdoor edible and non-edible crops and protected crops
	herbicide	Chondrostereum purpureum	Chontrol	cut stumps of hardwood trees and shrubs	forestry
	nematicide	Paecilomyces lilacinus	MeloCon WG	plant parasitic nematodes in soil	vegetables, soft fruit, citrus, ornamentals, tobacco and turf
Viruses	insecticide	Cydia pomonella GV	Cyd-X	codling moth	apples and pears
	anti-viral	zucchini yellow mosaic virus, weak strain	Curbit	zucchini yellow mosaic virus	transplanted zucchini and cantaloupes, watermelons, squash
Oomycetes	herbicide	Phytophthora palmivora	DeVine	Morenia orderata	citrus crops
Biochemical	insecticide	azadirachtin	Azatin XL	aphids, scale, thrips, whitefly, leafhoppers, weevils	vegetables, fruits, herbs and ornamental crops
	fungicide	Reynoutria sachalinensis extract	Regalia	powdery mildew, downy mildew, Botrytis, late blight, citrus canker	protected ornamental and edible crops
	herbicide	citronella oil	Barrier H	ragwort	grassland
	nematicide	Quillaja saponaria	Nema-Q	plant parasitic nematodes	vineyards, orchards, field crops, ornamentals and turf
	attractant	citronellol	Biomite	tetranychid mites	apples, cucurbits, grapes, hops, nuts, pears, stone fruit, nursery and ornamental crops
Semiochemical	attractant	(E,E)-8,10-dodecadien-1-ol	Exosex CM	codling moth	apples and pears

Source: Chandler D, Bailey AS, Tatchell GM, Davidson G, Greaves J, Grant WP. The development, regulation and use of biopesticides for integrated pest management. Philos Trans R Soc Lond B Biol Sci. 2011 Jul 12;366(1573):1987-98. doi: 10.1098/rstb.2010.0390. PMID: 21624919; PMCID: PMC3130386.