How Spatial Repellents Help Keep Insects Away

Stephanie L. Richards, PhD, Medical Entomologist

How can the public help to minimize mosquito exposure during outdoor activities?

  

We are all looking for ways to avoid exposure to mosquito bites due to the irritation and potential for pathogen transmission. Public health officials (e.g., Centers for Disease Control and Prevention, local public and environmental health organizations) advise the public to wear topical repellents that contain DEET or other active ingredients approved by the Environmental Protection Agency as well as long sleeves and pants to cover skin. Repellents provide a method where people can avoid contact with potential vectors and the sublethal effects of repellents may be considered an additional tool in disease prevention (in addition to vector control methods).

Brief History of Repellents

It is known that the ancient Greeks burned certain types of plants to repel biting flies (Norris and Coats 2017). Natural extracts such as citronella have been used in an attempt to prevent mosquito bites. DEET is an effective and commonly used synthetic repellent today.

What is the difference between topical and spatial repellents?

Topical repellents are applied directly to the skin or clothing and provide a surface barrier to blood feeding arthropods, such as mosquitoes and ticks. The idea is that, when a mosquito or tick comes into direct contact with a treated surface (e.g., skin, clothing) that contains repellent, the arthropod does not like the taste of the repellent, so it leaves that potential host. Topical repellents must be applied to all exposed skin in order to provide protection and, in some cases, must be reapplied periodically. Repellents, such as DEET, invoke a behavioral response by repelling mosquitoes and also may conceal the attractiveness of host odors (Norris and Coats 2017).

Spatial repellents are not applied directly to skin, but emit vapors from devices that are either worn by people (e.g., clip on devices) or used near people (e.g., mosquito coils/citronella candles). Most spatial repellents are volatile pyrethroids, although some other active ingredients are available (Bibbs and Kaufman 2017). Windy conditions and/or walking at a brisk pace may temporarily minimize the effectiveness of spatial repellents if the protective “cloud” around the individual is disrupted. In mosquitoes that have evolved insecticide resistance by developing a thicker outer covering (cuticle), spatial repellents may bypass this type of resistance as they are inhaled by the arthropod (does not require cuticle penetration). Scientists are studying the extent to which odorant receptors in mosquitoes may develop resistance to spatial repellents. There are other types of insecticide resistance that must be considered for both topical and spatial repellents, as well as other forms of insecticides. To date, most studies have evaluated spatial repellents on mosquitoes; however, some repellency has been noted in ticks and this should be investigated further. In addition to repellency of mosquitoes, some studies have shown mortality caused by spatial repellents; however, more studies must systematically be carried out on a variety of mosquito species under different environmental conditions to evaluate these effects (Bibbs and Kaufman 2017).

The behavior modification of probable vectors caused by spatial repellents has the potential to change the dynamics of pathogen transmission, hence is under study by scientists (Achee et al. 2012). If some people in a group are using repellents (topical or spatial), but others are not using repellents, those that are not protected may experience more bites (tick, mosquito) since those wearing repellents are “pushing” the arthropods away. The use of repellents causes arthropods to become disoriented and may interrupt host-seeking so, in other cases, there may be a protective effect for the group. More work is needed to evaluate these potentially protective effects.

In addition to surveillance-based targeted vector control programs, repellents can be an important tool for the public to help protect themselves against arthropod exposure. The public should be informed about the potential benefits and drawbacks of repellents.

 

References

Achee NL, Bangs MJ, Farlow R, Killeen GF, Lindsay S, Logan JG, Moore SJ, Rowland M, Sweeney K, Torr SJ, Zwiebel LJ, Grieco JP (2012) Spatial repellents: From discovery and development to evidence-based validation. Malaria Journal 11:164.

Bibbs C, Kaufman PE (2017) Volatile pyrethroids as a potential mosquito abatement tool: A review of pyrethroid-containing spatial repellents. Journal of Integrated Pest Management 8:21.

Norris EJ, Coats JR (2017) Current and future repellent technologies: The potential of spatial repellents and their place in mosquito-borne disease control. International Journal of Environmental Research and Public Health 14:124.

 

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