Plant Extracts as Natural Insecticides

Part of the Young Naturalist Awards Curriculum Collection.

by Kalia, Grade 8, New York, 2011 YNA Winner


Every year my family and I wage a constant war for homegrown fruits and vegetables. We weed-whack, fence, and cover our plants to keep them from rabbits and deer. However, the killers we cannot evade are insects. Which begs the question: Do we want organic produce or plentiful produce? This question inspired my science project theme: Can insecticides be created out of natural substances? Organic, plant-based pesticides that rely on plants' natural defenses against insects may not only be effective and inexpensive for protecting crops, but also safer and more environmentally friendly.


Agricultural crops are under constant assault by insect pests, making insecticides essential to reduce losses. Synthetic insecticides such as organophosphates are important, effective tools in modern crop management. However, they pose serious threats to the environment and to people. Humans come in contact with dangerous pesticides on food, in water and in the air near farms. This "pesticide drift" occurs when pesticide dust and spray travel by wind to places unexposed to pesticides. Almost 98 percent of sprayed pesticides do not reach their targets. They penetrate to groundwater, pollute streams and harm wildlife, including natural predators of the targeted pests. Older pesticides such as DDT killed bald eagles, birds, fish and even people (Carson).

Many farmers will not use synthetic pesticides, and some consumers will only buy organic produce. Mass production farms rely on synthetic pesticides, however, because they are cheaper than organic ones. 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 (Carson). Organophosphates designed to affect the brain and nervous system of insects, sometimes damage those of humans and animals.

Many plant species produce substances that protect them by killing or repelling the insects that feed on them. For example, the Douglas fir has a special sap that wards off beetles if it is attacked. Neem trees produce oil that alters the hormones of bugs so that they cannot fly, breed or eat (National Academy of Sciences 1992). It is possible to create effective, natural insecticides from these substances to protect crops that, unlike wild plants, may have lost their capability through cultivation to cope with pests. Natural pesticides have many advantages over synthetic ones and may be more cost-effective as a whole, considering the environmental cost of chemical alternatives. Natural pesticides are biodegradable, barely leave residues in the soil and are less likely to harm humans or animals. In addition, they are cheaper and more accessible in less developed countries.

Farms provide food for people and increase pest populations. Mass production of food relies on densely packed plants of the same type (monocultures), which are vulnerable to attack by insects. In natural habitats insects have to hunt for food, but a farm makes it easy for insects because the crop is isolated. Insects can cause economic devastation. In New Jersey alone, farmers lose $290 million a year to insects. In underdeveloped countries, insect outbreaks can lead to starvation.


Green chilis

I produced natural pesticides from Eastern hemlock (Tsuga canadensis), green chili (Capsicum annuum) and garlic (Allium sativum). Research has shown that garlic and chili peppers successfully repel insects. I chose to conduct new research with Eastern hemlock needles because it is a hardy coniferous tree that has natural defenses against insects, like its relative the cedar, whose wood repels moths. I hypothesized that green chili would be the most effective insecticide, followed by garlic and Eastern hemlock based on the intensity of their smell and taste. I surmised the Eastern hemlock might be least effective insecticide because it has not been tested before and has a mild aroma.


Eastern hemlock needles

I tested my insecticides, each at three concentrations, on populations of greater wax moth larvae (Galleria mellonella), or what I called the "worms." The medium and lowest concentrations were diluted 3:1 and 5:1 from the fully concentrated solution. I created ten populations of 20 moth larvae each. After allowing the larvae time to get used to their environment and eliminate natural deaths, I sprayed each population with the natural insecticide. Thereafter, I tracked their survival rates.


Processing the garlic

To prepare the Eastern hemlock, I cut off 200g of pine needles, chopped them up in a food processor and added one liter of distilled water. It was important that there was nothing in the water that might affect the pesticide. I concentrated it by boiling to extract the substances from the needles, and strained out the sediment. I did the same with 400g of peeled garlic cloves and 250g of green chilies. By weighing out a cup of each, I calculated the concentration of plant substances per liter of solution.

Preparing the Habitats

Moth larvae habitats

I divided food among the 10 habitats, and placed 20 moth larvae in each. After four days, I sprayed the moth larvae with the insecticides. I calculated how many milliliters per spray by counting the "pumps" to fill 50ml. The worms received the following amounts of each insecticide: 18ml garlic, 19ml Eastern hemlock and 11ml green chili. The moth larvae received less of the green chili pesticide, leading me to believe that it is potent in even lower concentrations than the other two substances.

Every day I tracked the numbers of dead and live moth larvae on a chart, as well as the escapees. I graphed the cumulative mortality of the population for each concentration of the insecticide and for the control group (Graphs 1 through 5). I analyzed (1) the mortality rate (how many died on each day), (2) how many days were needed to achieve lethality to 50% of the population, or LD 50, which is the standard used when testing commercial products, and (3) total mortality after 15 days. I sprayed the control group with water to rule out the possibility that spraying (or poor care) contributed to deaths.

Observations and Analysis

The control group did not have deaths until Day 11, six days after spraying, while all of the other groups had deaths starting in Days 6-8, two to four days after spraying. The final mortality in the control group was only 30%, while the other groups had a final mortality of 65% to 95%. I concluded it was mainly the insecticides that killed the worms.

Graph 1: Garlic Effectiveness

The highest concentrations of all of the insecticides caused mortality within one to three days of spraying (Graphs 4 and 5). With the chili and Eastern hemlock, mortality was observed after the first day. The green chili was the most lethal, killing 95% of the worms by Day 15, or 10 days after spraying. The garlic and Eastern hemlock killed 75% to 80% by Day 15. At full strength, all three pesticides achieved 50% mortality by Day 10, five days after spraying.

The Eastern hemlock at full strength and diluted 3:1 worked nearly the same, with five days to 50% mortality for both, and the same final mortality of 75% (Graph 3). When diluted 5:1, the Eastern hemlock was almost as effective, with a final mortality of 60%. This makes me think that I could have diluted it further.

A line graph titled “Chilies Effectiveness.”
Graph 2: Chilies Effectiveness

The green chili solution worked similarly (Graph 2). All three concentrations achieved 50% mortality after five days. The highest concentration and the 3:1 dilution achieved 95% mortality by Day 15, and the 5:1 dilution achieved 80% mortality.

The garlic solution had slightly different results (Graph 1). The highest concentration achieved 50% mortality after five days, the 3:1 dilution after six days and the 5:1 dilution only after 13 days. The full-strength solution achieved 80% mortality by Day 15, but the 3:1 and 5:1 dilutions only achieved 65% mortality.

Discussion and Conclusions

A line chart showing the effectiveness of hemlock.
Graph 3: Hemlock Effectiveness

There are four ways a natural insecticide can work: by ingestion, through contact, as a deterrent, and by disrupting developmental processes. Ingestion is when the moth larvae consume the pesticide and are poisoned. Contact poisoning is when the solution kills the moth larvae through their skin or other tissue. A deterrent is when the insecticide prevents the moth larvae from feeding, and they starve. Finally, certain pesticides, notably oil from the neem tree, disrupt the hormones that control molting and other processes. I researched the substances in green chili, garlic and Eastern hemlock to understand how my natural pesticides might have worked.

Green chilies and other hot peppers contain a natural substance called capsaicin that creates the hot, spicy effect. Capsaicin at 10 parts per million causes a persistent burning sensation. The intense flavor comes from the large hydrocarbon "tail" of the molecule. Capsaicin works by opening doors in the cell membranes that enable calcium ions to flood into the cell, making it trigger a pain signal that is transmitted to the next cell, and on and on. Extremely high concentrations of capsaicin are toxic. Capsaicin destroys cells by stopping the production of certain neurotransmitters that enable cellular communication.

Chart comparing the effectiveness of different pesticides at full strength
Graph 4: Comparison of Pesticides at Full Strength

By boiling the chilies, I isolated and concentrated the capsaicin and other chemicals. Because greater wax moth larvae are small, soft-bodied insects, sufficiently high concentrations might contain enough capsaicin to destroy cells and kill them. Another possibility is the acidity of hot peppers: the soft skin of a wax moth might be damaged by the pepper's chemicals.

Graph 5: Mortality Timing of Pesticides at Full Strength

Garlic produces allicin, which gives garlic its smell and healthful properties. Garlic does not contain allicin itself, but when the cloves are crushed, two chemicals inside react to form allicin. This is why garlic does not smell until you crush it. Allicin has been shown to have antifungal, antibiotic and antiviral properties, and researchers believe it may help to prevent cancer. Garlic oil has been used as an insect repellent, and may be toxic to certain insect eggs. It is possible that in high concentrations, the antibiotic effects of garlic become lethal to the moth larvae. My garlic-based insecticide was highly concentrated. Garlic was somewhat slower to cause 50% mortality but it had the second highest eventual lethality. It may have acted as a repellent to the worms, making them not eat their food, but it may also have had contact-based effects.

I decided to use Eastern hemlock needles, because, unlike the chili and garlic, I was not able to find any existing research on insecticides using them. Various evergreen-based oils, primarily from cedar, are used to repel insects, especially moths. This is why closets are often lined with cedar. My Eastern hemlock insecticide may have acted as a repellent to the worms. However, the pesticide started taking effect immediately, which suggests to me that it was either a contact or an ingestion pesticide. However, through my research, I was unable to identify any specific harmful chemicals in Eastern hemlock. The Eastern hemlock needles at full strength and diluted 3:1 worked almost the same, with the same time to 50% mortality (five days), and the same final mortality (75%). When diluted 5:1, the hemlock was almost as effective, with a final mortality of 60%.

All three of my natural pesticides were made from common plants that grow in many parts of the world and can be purchased quite cheaply. It was easy for me to produce them in my kitchen. My method is already used with other plants such as Indian neem tree leaves, and could be expanded to industrial capacities. Chilies, garlic and Eastern hemlock needles are safe and nontoxic for humans. (In reasonable quantities, they all seem to have healthful effects for humans.) I used no chemicals or other dangerous substances to make the insecticides.

My products could be expected to break down naturally outdoors and not cause any long-term toxic effects. I do not know if these natural insecticides might have any harmful effects on animals.

Potential sources for error in my experiment included uneven spraying, inappropriate temperature and lighting, and rough handling, especially during the daily counting, which could have harmed some of the moth larvae. My methodology included several things to prevent and/or correct for these sources of error. I used ten separate populations of moth larvae, which allowed for three concentrations of each insecticide, as well as a control group. I also started with 20 moth larvae in each population in case there were natural or accidental deaths.

Other potential experiments could include trying different concentrations and varying the spraying frequency of the insecticides, as well as spraying the larvae and their food separately to see if the insecticide was effective through ingestion or as a deterrent, as opposed to killing by contact. I could try different species of pests to see which insecticides were most broadly effective. I could also try different parts of the ingredient plants, or mix solutions together to create more potent insecticides. I could try to isolate allicin to see if it is the cause of garlic's lethality, and I could test other natural antibiotics to see if they have similar effects. I could also try testing relatives of my insecticides such as different types of peppers or evergreens. I think this experiment could lead to many more pesticides that could improve the farming processes we use today.


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