Enhancement of essential oil yield in Ocimum americanum L. across ontogenetic phases by Bacillus subtilis MTCC 8141 and Trichoderma harzianum MTCC 936 and its termiticidal efficacy against Odontotermes parvidens

For centuries, farmers have relied on synthetic chemicals to stop insects from destroying their crops. While these man-made poisons work quickly, they leave behind a trail of environmental damage, harm creatures that are not the target, and eventually lead to insects that become immune to the poison. In response, scientists have turned their attention to nature's own chemical arsenal: essential oils. These are the fragrant, volatile liquids found inside many plants, which often contain complex mixtures of compounds that can kill or repel insects in various ways. Unlike a single synthetic chemical that might target just one part of an insect's body, these natural oils often attack on multiple fronts, making them a promising, biodegradable alternative for pest control. However, the amount of oil a plant produces and the specific chemicals inside it can change drastically depending on how old the plant is and what lives in the soil around its roots.

Researchers in India set out to see if they could use helpful soil microbes to boost the production of these natural insecticides in a specific plant called Ocimum americanum, a type of wild basil. They focused on two microscopic helpers: a bacterium called Bacillus subtilis and a fungus called Trichoderma harzianum. These organisms are known to help plants grow by making nutrients more available and stimulating the plant's own defenses. The team wanted to know if adding these microbes to the soil would not only make the plants bigger but also increase the amount of essential oil they produced and change the oil's chemical makeup to make it more effective against a destructive pest: the termite. They also needed to find the perfect time to harvest the plants, as the oil content fluctuates as the plant moves from its young, leafy stage to its flowering, reproductive stage.

The scientists grew the basil plants in a field, dividing them into four groups. One group received no treatment and served as a baseline. The other three groups were treated with the bacterium alone, the fungus alone, or a mixture of both. They monitored the plants through three distinct life stages: before they flowered, while they were flowering, and after they had finished flowering. At each stage, they harvested the leaves to measure how much oil could be extracted. The results showed that the plants treated with the microbes grew significantly larger than the untreated ones. The most dramatic growth occurred when the plants were flowering; the plants treated with the mixture of both microbes and the bacterium alone produced the heaviest leaves. More importantly, the amount of essential oil extracted from the leaves peaked during this flowering stage. The plants treated with the bacterium alone yielded the highest oil concentration at 1.28 percent, closely followed by the mixed treatment at 1.27 percent, compared to just 0.90 percent in the untreated plants.

To understand what made these oils effective, the researchers analyzed their chemical composition. They found that the oil was dominated by a compound called camphor, which is known for its strong scent and insecticidal properties. In the plants treated with the bacterium, camphor made up more than half of the oil, a higher proportion than in the untreated plants. This suggests that the microbes did not just increase the volume of oil but also influenced the plant to produce a higher quality, more potent version of it. The researchers then tested this oil against a specific species of termite, Odontotermes parvidens, using three different methods. In the first test, they exposed the termites to the oil directly. In the second, they let the oil's fumes fill the space around the termites without direct contact. In the third, they tested whether the oil could drive the termites away.

The results were striking. The essential oils from the microbe-treated plants were far more effective than those from the untreated plants. When exposed to the oil from the bacterium-treated plants or the mixed treatment, the termites died at a much faster rate. At the highest concentration tested, these oils killed 100 percent of the termites within one hour of direct contact and within one hour of exposure to the fumes. The untreated oil also killed termites, but it took longer and required higher concentrations to achieve the same result. Furthermore, the oils acted as powerful repellents. When given a choice between a treated area and a clean area, the termites overwhelmingly avoided the treated side, with the microbe-treated oils driving them away completely at the highest concentration. The study confirmed that the combination of harvesting the plants at the right time—when they are flowering—and inoculating the soil with these specific microbes creates a powerful, natural tool for managing termites.

This research demonstrates that the relationship between a plant and the microscopic life in its soil can be harnessed to produce a more effective natural pesticide. By simply adding these beneficial bacteria and fungi to the soil and waiting for the plants to reach their flowering stage, farmers could potentially harvest a crop that is not only larger but also richer in the specific chemicals needed to fight pests. The study suggests that this approach offers a viable path toward sustainable pest management, reducing the need for harsh synthetic chemicals while leveraging the plant's own biological machinery to protect itself and the crops around it.