Maryland Grows

Mosquito season is here, and many people turn to essential oils for mosquito control as a way of avoiding synthetic insecticides, but this is often met with a mixture of success.

The Environmental Protection Agency (EPA) considered essential oils to be minimum risk pesticides, so there is no testing being done on their effectiveness before they go to market, which allows for a wide range of products to be sold that may not work. While there is a growing interest in the use of essential oils as possible methods for controlling mosquitoes, most of these studies are focusing on how essential oils can be used when applied to the skin or fabric as a repellent rather than as a yard barrier spray. Garlic oil, Lemongrass oil, and Citronella oil are commonly used essential oils in barrier sprays, but there is little to no research on them showing their effectiveness on mosquitoes in the United States.

Many essential oils are used as organic herbicides and may be phytotoxic to sensitive plants, such as fruit trees and vegetables. If you are planning to use essential oils for mosquito fogging, it is recommended that you apply them first thing in the morning or at night to reduce the chance of burning your plants. To avoid harming pollinators and natural enemies, DO NOT apply to plants in full bloom.

Several essential oils, such as peppermint oil, geranium oil, thyme oil, and cedarwood oil, have been found to work as larvicides against mosquitoes. Most studies look at mortality rates over 24-72 hours so no recommendations for long-term control can be made at this time. There have been no studies done on the long-term effects that essential oils in water might have on non-target aquatic organisms such as dragonfly larvae, diving beetles, and water striders. An environmental friendly choice for a mosquito larvicide that has been readily tested would be Bacillus thuringiensis var. israelensis (Bti). Bti is a strain of soil bacterium, which targets only true flies (Order: Diptera). It is safe for vertebrates such as humans, dogs, birds, and fish, and it is safe for most non-target insect species such as dragonflies and bees.

Repellents work by releasing chemical vapors into the air, which blocks the mosquitoes’ ability to “smell” or detect their host. Many essential oils have been found to work as repellents in scientific studies. However, they vary in the length of their efficacy, the concentration needed, and their effectiveness against different mosquito species. For example, peppermint oil was found to work against C. quinquefasciatus, Anopheles culicifacies, and Anopheles annularis for over 6 hours but not at all against Ae. aegypti. Many essential oils are also highly volatile, so they offer great protection for a short period of time but soon are completely vaporized and leave the wearer without protection. An exception to this is an oil of lemon eucalyptus, which has a low volatility rate and provides very high protection over several hours (on average +4 hours).

The Center of Disease Control and the Environmental Protection Agency both have approved the use of oil of lemon eucalyptus as an organic insecticide, making it an excellent option for people who don’t want to use DEET or Picaridin repellents. No matter what repellent you choose to use, always apply sunscreen first. Never apply repellent under clothing, since the clothing will prevent the repellent vapor from dissipating into the air. Clip-on battery powered diffusers have recently become a popular way to use a mixture of essential oils to repel mosquitoes since they can continuously release the essential oil vapors into the air around the wearer.

Resources

• Active Ingredients for Minimum Risk Pesticide Use
• Controlling Mosquitoes

By Emily Zobel, Agriculture Extension Assistant, University of Maryland Extension, Dorchester County