[Annual Rodent Control Issue] Making the Right Rodenticide Choice

When we talk about rodents, rats and mice inevitably steal the spotlight, in part because they are among the most destructive rodents and because they present unique control challenges. Rats and mice damage property, contaminate food, transmit disease and cause harm to our environment, so speed and efficacy are of primary importance as you handle infestations. Add to your challenges the fact that these vermin reproduce quickly, controlling populations before they become unmanageable is key.

Ted Bruesch, technical support manager at Liphatech, recently shared insights into the history, formulations and uses of rodenticides in a webinar. What follows are some highlights of his discussion.

Rats, Cats, the Pied Piper & You. The evolution of rodent control dates back to ancient Egyptian times, when cats were worshipped for, among other traits, their rat-catching capabilities. Later, dogs were bred to serve as rat killers, and they became trusted companions to medieval rat catchers, the first pest management professionals on record. Rat catchers employed a broad range of traps and other devices, as well as potions, in plying their trade (of course, the legendary rat catcher of Hamelin had his own unique bag of tricks).

As modern-day rodent control took hold in the 1920s and early ’30s, potions became rodenticides and, in the United States, the government began taking a role in their regulation. Today, the PMPs who tackle rodent management are highly skilled, trained individuals with a variety of rodenticides and devices to choose from as they protect public health and property.

Generation after generation has attempted to identify the ideal rodenticide. Active ingredients (AI) running the gamut from arsenic, thallium and compound 1080 (sodium fluoroacetate) to cyanide and red squill had their day, and many of those were used as late as the early 1970s.

PMPs often mixed their own baits back then, combining various active ingredients with grain or seeds for mice and with hamburger, fish parts or vegetables for rats. Bait materials were chosen for their real or perceived ability to mask the bitter taste of the active ingredient. Rodents must ingest enough of a rodenticide for it to be lethal, after all, and “bait aversion” (refusal to eat) occurs if rodents don’t like the smell, taste or newness of a food. Because their senses of smell and taste are highly developed, rodents can detect active ingredients at levels far below the parts per million (ppm) present in rodenticides.

For example, zinc phosphide is used at 20,000 ppm, and rodents can detect many chemicals at less than 1 ppm. Active ingredients are getting progressively more robust, however, so we’ve seen ppm’s reduced to about 750 for cholecalciferol, 250 for warfarin, 100 for bromethalin and 50 for first-generation anticoagulants chlorophacinone and diphacinone, and second-generation anticoagulants brodifacoum, bromadiolone and difenacoum. Most recently, second-generation anticoagulants difethialone and brodifacoum have been formulated at 25 ppm.

“Bait shyness” is another hurdle PMPs must overcome in managing rodent populations. When a rodent gets sick shortly after eating a new food (e.g., rodenticide), it makes the connection and refuses to eat that food again. That’s why slower-acting rodenticides tend to have a higher long-term success rate than fast-acting ones.

The Big 10. Today, 10 active ingredients are being used in rodenticides in the United States: acute toxicants bromethalin, cholecalciferol and zinc phosphide; first-generation anticoagulants warfarin, chlorophacinone and diphacinone; and second-generation anticoagulants brodifacoum, bromadiolone, difenacoum and difethialone.

Acute toxicants work quickly — three to four days for cholecalciferol, one or two days for bromethalin and only an hour or so for zinc phosphide — so they are prone to bait shyness. Their high AI concentrations also make them prone to bait aversion. The key to success is to incorporate acute toxicants short-term into a program that eliminates competing food. Once you’ve knocked part of the population down, you can move to a more palatable ingredient offering long-term efficacy. Another downside to using acute toxicants is that no antidotes exist for them.

First-generation anticoagulants were introduced with the launch of warfarin in the late ’40s and improved with chlorophacinone and diphacinone formulations. Because these ingredients are effective at much lower concentrations, they minimize bait aversion. And because they work slowly, they minimize bait shyness. They also have a readily available antidote: vitamin K1.

Second-generation anticoagulants offer even more, as they provide all of the advantages of first-generation anticoagulants while working with just a single feeding. First-generation products require a feeding period of a week or more, but brodifacoum, bromadiolone, difenacoum and difethialone each have the power to kill a rodent with less than one 3-gram feeding.

Improvements during the past four or five decades have included progressively lower risks to non-target animals like pets or wildlife, increased efficacy and improved forms.

Forms and Uses. Rodent infestations might be challenging, but they are definitely conquerable. With a variety of potent ingredients and a broad array of forms, there is an appropriate, effective material for every rodent control situation. The most popular forms follow.

Liquid bait. If Norway or roof rats are the problem, liquid bait can be effective. These rodents are most likely to ingest a lethal dose of this diphacinone bait where their water supply is limited. Mice, on the other hand, get most of their moisture from food, so they aren’t as likely to go for this type of bait. When you use liquid bait, be sure to anchor it securely, since spillage is often a concern.

Tracking powder. Three types of tracking powder are available: chlorophacinone, diphacinone and zinc phosphide (labeled for mouse control only). These are all restricted-use pesticides due to their high toxicity; their concentration is much higher than baits with the same active ingredients. They require meticulous handling due to a variety of risks: A rodent can run through the powder and then contaminate other surfaces with it; a draft can cause the powder to become airborne; and the long-lasting nature of the rodenticide means it will stay active for years, possibly harming a non-target animal or a child at some time. These materials are labeled for use in void, stations or tubes, runways, indoor burrows or burrows that likely serve as entry points to the building.

Meal bait. Meal bait is perfect for finicky pests. Its palatability is high, tasting much like the rodent’s regular diet. The downside is that it’s not easy to secure. Like liquid bait, it can scatter. It’s not moisture-resistant either, so it might remain palatable for only a few hours if placed in burrows in wet ground. It’s an excellent choice, however, for burrows in dry ground.

Bulk pellets. Resembling a seed, the pellet mimics rodents’ natural food source. It is an outstanding material for managing Norway rats, provided you are able to place the pellets deeply (at least six inches) into the burrow. Pellets aren’t always the best choice for mice, though, because mice will carry and deposit them in unwanted places such as toy boxes and shoes. If you are baiting burrows, be sure to retreat them weekly, and watch them even after you have wiped out the population, because rats can be attracted to residual pheromones around the burrows for years.

Place packs. Meal and pellet rodenticides are both available in place packs, which offer the advantages of being premeasured, minimizing operator contact with the active ingredient, protecting the bait from moisture and eliminating the melting you might experience with other forms. Place packs can be secured in bait stations with a paper clip or a skewer.

Molded, extruded and paraffin blocks. Block baits were created to address the issue of moisture. Initially, bait materials were mixed in a vat with wax, then heated and poured into molds, cooled, released from the molds and packaged. Eventually these blocks were extruded, or squeezed out of the vat, instead. Although rodents aren’t particularly fond of wax, blocks offer excellent water protection and consistent efficacy in high-moisture environments. Molded blocks tend to have a relatively high wax content, while the amount of wax in extruded blocks varies widely from brand to brand. Paraffin bars, with their high wax content, are ideal for sewer infestations, often secured by wire above the water flow.

Soft bait. Soft bait has become very popular in Europe, and it’s quickly gaining momentum in the United States — with good reason. This mixture of milled grain and vegetable oil is highly palatable because it has no wax. It also has no peanut or tree nut ingredients. It does, however, have a bittering agent to protect children from ingesting it. Skewered on either a vertical or horizontal rod, soft bait doesn’t melt off even after extended periods of summer heat.

The author is a frequent contributor to PCT magazine. She can be reached at ddefranco@giemedia.com.