Can garlic be the savory secret to a bug-free garden?
Companion planting with garlic, using garlic sprays and powder on plants can repel and even kill pests because of its potent smell and sulfur-containing compounds, making it a powerful natural insecticide and fungicide. However, it can also repel beneficial insects.
In this article, we will explore how garlic defends not just itself but other plants, the science behind its bug-repelling power, and how you can harness its natural pesticide properties in your garden.
Garlic keeps bugs away when used as a companion plant, and also when the extract is used as a pesticide.
Garlic (Allium sativum) is famous for its pungent smell. This smell also works as a deterrent to keep bugs away from the garlic plant. Additionally, there is evidence that when used as a companion plant, garlic can also keep bugs away from other plants.
In a study of rapeseed plants intercropped with garlic and onions, garlic was able to significantly reduce the population of aphids affecting the rape plants, resulting in lesser leaf damage compared to the plants that were not intercropped with garlic (Pahla et al, 2014).
Garlic was found to be the most effective as shown in another study that tested garlic, chives, and fennel (Hata et al, 2016). This research showed that garlic could reduce the population of two-spotted spider mites on strawberry plants by as much as 52%. In comparison, fennel and chives achieved a reduction of 42% and 50% respectively.
In addition to intercropping, garlic can also keep bugs away when used as an insecticide sprayed over plants. The insecticide may be in the form of “garlic water” made from pureed garlic and water, or the essential oil extracted from the plant through steam distillation.
Garlic is a natural deterrent for many pests due to its strong smell and taste. Insects, much like many animals, are repelled by strong odors, especially those that they associate with danger or potential harm.
However, the repellant properties of garlic go beyond its smell. Garlic contains several organosulfur or sulfuric compounds which can interfere with the nervous systems of insects and can thus kill them. Here are brief descriptions of each compound:
– Allicin (or Diallyl Thiosulfinate) is produced when garlic is crushed or chopped, an enzymatic reaction converting the compound alliin into allicin. Allicin is largely responsible for the strong aroma of fresh garlic. Not only does it repel insects, it can also inhibit the growth of fungi and bacteria.
– Diallyl Sulfide is the simplest form of the oil-soluble organosulfur compounds found in garlic. It is produced during the degradation of allicin. It has been found to have insecticidal and antimicrobial properties.
– Diallyl Disulfide is another oil-soluble organosulfur compound. It is more stable than allicin and can be detected in the vapors emanating from freshly crushed garlic. Diallyl disulfide has been shown to have insecticidal properties.
– Diallyl Trisulfide is one of the main components of garlic oil. It has been shown to have strong insecticidal and repellent activities.
Garlic repels a wide range of organisms, including insects, rodents, and even soil pathogens.
Many studies have proven garlic oil as an effective insecticide against the following insects and pests (Plata-rueda et al, 2017; Kim et al, 2011; Baker and Grant, 2018):
- Some aphid species
- Cabbage loopers
- Cabbage flies (adults and eggs, not larvae)
- Cutworm eggs and larvae
- Mealworm beetles
- Fruit flies
- Common house flies (adults and eggs, not larvae)
- Silverleaf whiteflies
- Gall midges (adults and eggs, not larvae)
Garlic may also repel some smell-sensitive animals such as deer and mice. It is also toxic for rabbits.
However, garlic doesn’t keep out all insect pests. For instance, the allium leaf miner insects are immune, as they feed on the leaves of garlic and other alliums.
Besides its insecticidal function, garlic is also a fungicide and has been shown to be effective against a broad range of soil fungal organisms (Sealy et al, 2007). It has also been found to exhibit antibacterial activity on a wide range of bacteria (Ankri and Mirelman, 1999). Garlic is also effective in the control of nematodes and weeds (Baker and Grant, 2018).
How to use garlic as an insecticide and fungicide?
Garlic can be interplanted with other crops to repel bugs. It can also be made into an insecticide to be sprayed on the leaves and stems of plants.
1. As a companion plant
Interplanting garlic with other plants is the best way of using garlic to keep pests away because studies show that intercropping with garlic repels only the pests and not the beneficial organisms such as lady beetles but using garlic extracts repel both (Baker and Grant, 2018).
But make sure that the garlic is not greatly outnumbered by the companion plant. The garlic should also be evenly distributed around the field and not limited to one section, to ensure that the entire field is infused with the bug-repelling compounds.
2. Garlic water spray
Homemade garlic sprays work equally well as commercially available garlic sprays. The idea is to extract the sulfur-containing constituent diallyl disulfide which does not get lost or degraded when mixed with water. But it must be used soon maybe within a few days as it degrades when coming into contact with air.
Here’s how to make garlic spray:
- Separate one medium-sized head of garlic into cloves, and remove the papery skin.
- Put the pieces and one cup of water into a blender, and puree.
- Add 3 more cups of water and 2 tablespoons of dish soap. The soap will help the garlic stick to the plant, and it also has some insecticidal properties of its own.
- Pour out the mixture, and let it sit for at least 24 hours to allow the water to absorb the sulfur compounds properly.
- Use a strainer to separate the liquid from the solid particles.
- Transfer the garlic water to a spray bottle.
Spray garlic water on the top and bottom of the leaves, as well as the stems of affected plants. It is best to spray plants in the mornings and evenings when insects are more active and there’s little risk of the sun burning the leaves.
This should be done once in 3 days and after every rainfall.
Garlic water can also be used as a soil drench to tackle fungi like pythium and other soil-borne pathogens.
Garlic insecticide sprays do not target any specific insect pests and will eliminate beneficial organisms along with the pests. So it’s best to spray plants with garlic only when you notice an infestation.
3. Dehydrated garlic powder or flakes
Dehydrated garlic powder or flakes are equally effective as garlic spray but last much longer.
To make garlic flakes, peel and slice fresh garlic cloves into small pieces. Spread the fresh garlic pieces on a baking sheet and heat to 150°-160°C (300°-320°F) in the oven until the pieces feel dry, light, and crispy to the touch.
The drying time depends on how thinly the garlic is sliced. It could take anywhere from 30 min to 45 minutes. It’s important to check the garlic slices every 15 minutes to prevent burning.
To make garlic powder, you can chop or mince the dried slices by hand or grind them in a blender, food processor, or spice grinder until they form a fine powder.
Garlic will not keep stem borer bugs away. It will also not work for natural pests of garlic plants.
Although garlic is a broad-spectrum insect repellant, it does not work on burrowing insects like sugarcane borer or rice stalk borer. This is because garlic sprays are contact insecticides, they remain on the surface of the leaves and stem and are not absorbed into the plant tissue like systemic pesticides. As a result, garlic will not work on any pests that live within the plant tissue or right under the surface.
Natural pests of garlic like bulb mites, and allium leaf miners are also less susceptible to garlic sprays.
Final verdict: Does garlic really repel bugs?
Garlic indeed acts as a deterrent for many pests, making it a valuable tool in natural pest control.
The utilization of garlic as a companion plant and garlic extracts in the form of sprays or powders have been shown to reduce bug infestations on plants.
However, its effectiveness varies, and it shouldn’t be relied upon as the sole strategy for all types of bugs or pests. Combining it with other pest control strategies would yield the best results.
Hata, F. T., Ventura, M. U., Carvalho, M. N., Miguel, A. Q., Souza, M. M., Paula, M. D. G., & Zawadneak, M. a. C. (2016). Intercropping garlic plants reduces Tetranychus urticae in strawberry crop. Experimental and Applied Acarology, 69(3), 311–321. https://doi.org/10.1007/s10493-016-0044-3
Kim, S. J., Chae, S., Youn, H. Y., Yeon, S., & Ahn, Y. (2011). Contact and fumigant toxicity of plant essential oils and efficacy of spray formulations containing the oils against B- and Q-biotypes of Bemisia tabaci. Pest Management Science, 67(9), 1093–1099. https://doi.org/10.1002/ps.2152
Plata-Rueda, A., Martínez, L., Santos, M. H. D., Wilcken, C. F., Soares, M. A., Serrão, J. E., & Zanuncio, J. C. (2017). Insecticidal activity of garlic essential oil and their constituents against the mealworm beetle, Tenebrio molitor Linnaeus (Coleoptera: Tenebrionidae). Scientific Reports, 7(1). https://doi.org/10.1038/srep46406
Pahla, I., Tumbare, T., Chitamba, J., & Kapenzi, A. (2014). Evaluation of Allium sativum and Allium cepa intercrops on the control of Brevicoryne brassicae. . . ResearchGate. https://www.researchgate.net/publication/267640366_Evaluation_of_Allium_sativum_and_Allium_cepa_intercrops_on_the_control_of_Brevicoryne_brassicae_Homoptera_Aphididae_in_Brassica_napus
Boone, C., Bond, C., Cross, A., Jenkins, J. (2017) Sulfur General Fact Sheet; National Pesticide Information Center, Oregon State University Extension Services. npic.orst.edu/factsheets/sulfurgen.html
Sealy, R., Evans, M., & Rothrock, C. S. (2007). The Effect of a Garlic Extract and Root Substrate on Soilborne Fungal Pathogens. Horttechnology, 17(2), 169–173. https://doi.org/10.21273/horttech.17.2.169
Ankri, S., & Mirelman, D. (1999). Antimicrobial properties of allicin from garlic. Microbes and Infection, 1(2), 125–129. https://doi.org/10.1016/s1286-4579(99)80003-3
Baker, B., & Grant, J. (2018). Garlic & Garlic Oil Profile. https://ecommons.cornell.edu/handle/1813/56126
Photo credits: Foto de Karolina Grabowska de pexels