Pesticides, Pollinators, and Pestilence: Protecting Public Health and Pollinators

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Tick and mosquito control provides important public health protection, but can also affect pollinator populations. The effects are often dependent on specific local conditions, such as how close the pesticide application is to places pollinators frequent, and when they frequent them.  

Efficient tick and mosquito management has advantages for both public health protection and for pollinator protection.  When available resources are used efficiently, fewer people get sick, and the need for broad-scale interventions, which can be expensive and are likely to affect pollinators, is minimized.

Mosquito, Aedes aegypti
Mosquito, Aedes aegypti(Public domain.)

There is an urgent need to control insects that carry disease-causing agents because of the harm these diseases cause. However, many of the common control methods have unwanted side effects: they can harm populations of pollinating insects that do not spread diseases and, instead, provide important services. USGS researchers are collaborating with other scientists to shed light on pest-control methods that are least harmful to pollinators while protecting human, wildlife, and livestock health. 

Mosquitoes, for example, cause millions of deaths worldwide by transmitting diseases such as dengue and malaria.  In North America, there are thousands of reported cases of West Nile virus disease as well as Eastern equine encephalitis, St. Louis encephalitis and Lacrosse encephalitis.  Mosquitoes have also spread dengue, chikungunya and Zika viruses in the southern regions of the United States. Ticks, of course, are known not only for transmitting Lyme disease, but also such diseases as Rocky Mountain spotted fever and Powassan encephalitis.  

Insects such as mosquitoes and sandflies, and arthropods such as ticks, are known as vectors, the middlemen, so to speak, that actively transmit a disease or virus from an infected host animal to another individual. For example, if a blacklegged tick feeds on a mouse that is infected with the bacteria that cause Lyme disease, and then feeds on a person later, the Lyme disease bacteria can be transmitted to that person.

While protecting humans, wildlife and livestock from vector-borne disease is a major focus of public health, at the same time, there has been a decline in pollinator populations.  This is likely due to factors such as habitat loss, pesticide use and invasive species.  

The threats from these vectors are many, and there are multiple control methods available for ticks and mosquitoes, such as biological control, pesticide application and landscape changes.  The diversity of available methods allows selection of control methods to effectively protect public health, while minimizing adverse effects on pollinators.

 

Nymphal blacklegged tick
Nnymphal blacklegged tick, Ixodes scapularis.  Most human cases of Lyme disease in the U.S. are transmitted by bites of nymphal blacklegged ticks like this one.  The nymphs wait in or on the leaf litter for vertebrates (like people) to pass by, then they climb on and eventually bite, sometimes transmitting the bacteria to the host.(Credit: Graham Hickling, The University of Tennessee. Public domain.)

Control Methods and Possible Effects on Pollinators

Landscape Manipulation -- Modification to floral resources; or pollinator nesting habitat; changes in pesticide  distribution patterns

Biological Control --Predation, parasitism or infection of pollinating species

Pesticide Applications --Direct mortality of pollinator species; effects on behavior, reproduction, overwinter survival, resistance to diseases

 

Biological controls have been developed or proposed for mosquitoes and ticks, including viruses, bacteria, nematodes, and fungi that harm the vectors. Although some pollinators could be adversely affected by the fungus Metarhizium anisopliae, which is used to control both mosquitoes and ticks, applications could be targeted to minimize harm to pollinators. Two bacteria species used to control larval mosquitoes are Bacillus thuringiensis israelensis, and Lysinibacillus sphaericus, but because they are applied to water they would not necessarily affect pollinators.

Biological control also includes host management, such as the management or eradication of white tail deer populations, which are the most common adult hosts for populations of some tick species. Some research has suggested that lowering deer populations could potentially benefit pollinator populations because deer eat vegetation and more deer can potentially lower floral abundance for pollinators, but these effects would likely depend on local conditions.

Landscape manipulation to control ticks can involve opening up canopy and shrub vegetation, thereby creating drier ground conditions, which could increase the abundance of flowers. Although this could improve pollinator habitat, it might also affect their exposure to pesticides drifting from agricultural areas, depending on pesticide use patterns and wind direction. Other ways to manage ticks include controlled burns and removing lower vegetation and ground cover. This would make the areas more favorable for some plants and less so for others, with the effects on pollinators being dependent on local conditions and the type of management actions taken. A different example would be wetlands, which can provide important floral resources for bees, but may also be managed for mosquito control.

Vector Control with Pesticides

Pesticides such as organophosphates and pyrethroids are used to control adult mosquitoes, ticks and fleas.  Both are neurotoxins, and direct exposure is highly toxic to honeybees, although toxicity varies for other bee species. 

Broad, area-wide applications of pesticides are likely to affect pollinators more than targeted applications. The characteristics of the chemicals and how they are applied determine how they would affect pollinators, so that applications can be targeted to avoid those potential adverse effects on pollinating species. Some chemicals break down over several days, or leach into the soil and last weeks to months. There are other compounds that breakdown quickly in sunlight, while others are taken up by plants into the leaves, stems and pollen.

It’s possible to use time-based targeting to maximize mosquito and tick mortality, with the pesticides breaking down before pollinators are active. Effective targeting includes using approaches such as spraying at night with short-lived chemicals, as well as targeting specific locations for application during the best time of the season for the greatest effect on mosquitoes and ticks with the least impact on pollinators.

Monarch butterfly on flower
Monarch butterfly on flower. Credit Brian F. Powell(Public domain.)

Research for Decision-Making

Research to aid decision-making for management of ticks and mosquitoes can be expanded to include models that optimize managing those disease-causing species, while avoiding pollinators. Innovative research on well-targeted control methods is underway, and can contribute to this effort.  Examples include innovative trapping technologies and targeted manipulation of vector reproduction. 

 

To learn more about management of mosquitoes and ticks and minimizing the adverse effects on pollinators, please read: Management or arthropod pathogen vectors in North America: Minimizing adverse effects on pollinators, available online in the Journal of Medical Entomology.