Pesticides for Vertebrate Pests

Pesticide Terminology

According to the U.S. Environmental Protection Agency (EPA), the federal agency responsible for regulating pesticides, a pesticide is any substance or mixture of substances intended for preventing, destroying, repelling, or mitigating any pest. The term “toxicant” is used in wildlife pest management to refer to a pesticide used to kill or impair a pest. Repellents deter animal activity while not causing permanent harm or injury and are classified as pesticides. Other pesticides include rodenticides (used for rodent control), avicides (used for bird control), and fumigants, a type of pesticide active as a gas.

Pesticides are directed at controlling target species, whereas non-target species include wildlife, livestock, pets, and people not intended to be affected by a pesticide application. Active ingredients are the specific chemical compounds that produce the desired outcome in the target species. Primary exposure refers to the effects on an animal that directly encounters the pesticide, specifically the target species. A mouse that dies from ingesting a rodenticide is an example of primary exposure. Secondary exposure refers to the effects on a predator that has eaten an animal with primary exposure. An owl that eats a poisoned mouse is an example of secondary exposure.

Regulations and the Pesticide Label

The EPA oversees all aspects of pesticide registration, production, testing, labeling, transportation, storage, and application. In addition, state departments of agriculture, health, environmental conservation, and consumer protection regulate vertebrate pesticides. Some counties and municipalities may have their own additional restrictions. Local and state regulations must be equal to or stricter than federal regulations. Check with your local agencies to ensure compliance with all applicable pesticide rules. In many states, anyone using pesticides for commercial purposes must be a certified pesticide applicator, which involves completing coursework, passing an examination, and obtaining state licensure.

The EPA categorizes certain pesticides as restricted-use pesticides (RUPs). RUPs are available only to certified or licensed applicators. In contrast, pesticides not classified as RUPs are known as general use pesticides (GUPs) and can be purchased over the counter. Many GUPs contain the same active ingredients found in RUPs, but they pose less risk to people and the environment. They typically have lower concentrations of active ingredients, are sold only in small quantities (1 pound or less), or are formulated as wax blocks or soft baits, posing less risk to non-target species because rodents cannot easily move the toxicant.

Before purchasing and using a pesticide, read the label for specific information on target species, site of use, application methods, hazards, and safety requirements. Pesticide labels provide important information that helps mitigate the risk of harm to people and the environment. The label is the law – using a pesticide in a manner inconsistent with the label is illegal and punishable under federal and state laws. Always read the entire label before application. A thorough understanding of use, risks, and storage is crucial for the effective and responsible application of pesticides. Consult someone if you don’t understand any information printed on the label.

Pesticides and Formulations

Repellents
Repellents are classified by their mode of action–pain, fear, touch, or aversive conditioning. Be aware that manufacturers only must prove a product is safe when used as labeled to sell it as a repellent. They do not have to show it works, and many repellents have not been adequately tested.

Active ingredients that repel animals by pain include capsaicin and methyl anthranilate, which are irritants for mammals and birds, respectively. For the repellent to be effective, an animal must come into contact with it, either by tasting or inhaling it.

Active ingredients that use fear to repel animals include putrescent whole egg solids and coyote urine. Fear-based repellents cause target animals to perceive a predator as nearby and avoid the area.

Polybutene-based caulks are a sticky, non-drying polymer used as a wildlife deterrent. When applied to surfaces, the sticky texture creates an unpleasant sensation for animals, such as birds and squirrels, discouraging them from landing or climbing on treated areas. Because polybutene-based caulks trap dust and may stain surfaces, they should be used only on properly treated areas for short-term relief. Such repellent caulks usually are applied indoors.

Finally, repellents using aversive conditioning differ in that the reaction of target animals is delayed. Animals first consume the product, and within a few hours, they exhibit symptoms of illness. They associate the illness with the food or area where they fed. This is the mode of action for the anthraquinone-based goose repellent. Geese that consume turf sprayed with it become nauseous, and the birds subsequently avoid eating treated grass.

The effectiveness of repellents varies considerably depending on the motivation of the problem animals, alternative food sources, previous experience, and the active ingredients of the repellent. Repellents often fail to deter animals, and it is rare for them to resolve wildlife conflicts consistently. Consequently, landowners must learn to tolerate some damage when using repellents. Read labels carefully before application, as some repellents are not suitable for use on plants intended for livestock or human consumption.

Toxicants
As with repellents, toxicants have varying modes of action. Some reduce the ability of blood to clot (anticoagulants, such as warfarin), while others affect the nervous system (bromethalin), metabolic processes (phosphine), and heart function (vitamin D3). The selection of the appropriate active ingredient depends on the target species, potential risks, previous methods employed, application methods, and cost.

Rodenticides
Rodenticides are classified by their modes of action. Most registered rodenticides are anticoagulants that, when ingested, inhibit blood clotting. Animals die from internal bleeding (caused by stress) or bleeding from external wounds. Animals that die from anticoagulant exposure often show signs of bleeding near their mouth or anus. While the effects of all anticoagulants are cumulative, some may be effective with a single dose. Active ingredients include first-generation anticoagulants (multiple-dose rodenticides with active ingredients:  warfarin, diphacinone, and chlorophacinone) and second-generation anticoagulants (single-dose rodenticides including the active ingredients: brodifacoum, bromadiolone, difethialone, and difenacoum). Due to the low amounts of active ingredients, anticoagulant rodenticides are seldom fast-acting. Vitamin K1 can be used as an antidote to counteract the effects of most anticoagulants.

A rodent that has eaten an anticoagulant (i.e., primary exposure) has concentrated levels of that compound (especially after successive feedings) in its body for several days. A predator or scavenger that consumes intoxicated rodents (i.e., secondary exposure) may receive a high dose of the toxicant, which in turn can lead to impaired blood clotting and death. Brodifacoum and diphacinone are particularly toxic to dogs.

Other rodenticides are acute (i.e., fast-acting) toxicants or non-anticoagulants, and they have alternative modes of action. Acute toxicants can cause a rapid decline in a rodent population, often within a day, whereas anticoagulants typically reduce a population within 1 to 3 weeks. Acute rodenticide active ingredients include zinc phosphide, strychnine, bromethalin, and cholecalciferol (vitamin D3). They are helpful when the disease hazard is high or a very large population must be reduced quickly. Only use an acute toxicant once or twice per year in the same location to avoid the development of bait shyness in rodents. No antidotes exist for acute toxicants; therefore, even greater care must be taken when applying them.

Baits and Bait Stations
A bait is a pesticide formulation with active ingredients and attractants (e.g., grains, fats, and flavor enhancers) that entice target animals to eat them. Baits may be formulated into blocks, pastes, place packs, and loose grains.

Bait blocks are the most common formulation for controlling rats and mice closely associated with people. They are easy to use, highly effective, and can be secured in bait boxes. Some are refined and concentrated forms of naturally occurring compounds, such as capsaicin, blood, urine, strychnine, and carbon dioxide. In contrast, others are synthesized for their desired effects, including phosphine, bromethalin, and brodifacoum.

Bait stations come in various shapes and sizes, all of which are used to protect the bait, increase rodent consumption of the bait, minimize access to the bait by non-target animals and children, and allow placement of baits in locations  where it otherwise would be difficult because of weather or potential hazards to non-targets.

The EPA requires that all rodenticide bait products marketed to residential consumers be sold and used in protective, tamper-resistant bait stations, also known as bait boxes.

Some bait stations are large enough to accommodate bait and water inside. Place bait stations where rodents are active, especially where signs of their presence are evident, such as fresh droppings and gnaw marks along walls, under pallets, and behind equipment. Secure bait stations with screws, anchor bolts, or other fasteners. Use locks, seals, and concealed latches to make bait boxes tamper-resistant. Check stations regularly, ideally daily during the first week, and at least once a week thereafter. Ensure an uninterrupted supply of fresh bait for at least 2 weeks, or until all feeding ceases. Gradually reduce to monthly inspections.

Avicides
Some toxicants are available for use on birds not protected by federal or state laws (e.g., European starlings and pigeons). However, these are registered for use only by certified applicators trained in bird control. Fewer toxicants are available for bird control compared with those for rodents. Starlicide Complete™ (p-n2-toluene) is available to certified pesticide applicators for controlling starlings at livestock feedlots.

AvitrolTM (4-amino-pyridine) is a restricted-use pesticide (RUP) classified as a chemical frightening agent (repellent). It is applied to disperse birds rather than kill them. The chemical is mixed in a ratio of 1 treated corn kernel to 99 untreated kernels. When the bait is ingested, affected birds have convulsions, exhibit erratic behavior, and die. In doing so, they frighten other birds from the area.

Fertility control agents are considered pesticides and must be applied by a certified applicator. Products used to reduce the potential for eggs to hatch (e.g., OvoControlTM, active ingredient nicarbazin) are pesticides registered by the EPA. In some states, even food-grade corn oil is considered a pesticide when applied to eggs to reduce hatchability. State pesticide regulations vary considerably. Anyone who makes a commercial application must be licensed.

Other Vertebrate Pesticides
While only a few toxicants are available for controlling rodents and birds, even fewer are available for controlling other vertebrate species. The active ingredients, bromethalin and warfarin, are registered for mole control in turf. They are formulated as pellets, plastic worms, or gels and can be applied only in underground tunnels frequented by moles.

Fumigants
Fumigants kill animals via toxic gas. They are typically used for mammal control in burrows or dens away from human structures. Charcoal-based gas cartridges are GUPs and may be used for woodchucks in burrows. Fumigants containing aluminum phosphide as the active ingredient are Restricted Use Pesticides (RUPs) and are available only to certified applicators. Aluminum phosphide is highly toxic, and its use has been more strictly regulated since the deaths of two children in Utah in 2010. Fumigants carry multiple primary hazards, including the risk of fire and corrosion. Seek training before using fumigants.