Using IPM to Control Rodents – Part 1

You may wonder how to best get rid of rodents on your property, or heaven forbid, in your house. Unfortunately, that’s our situation – we’ve recently hired a company to seal our roof to prevent rats from entering our attic. (One month later, I think they’re finally sealed out.) You may also have heard some concern about rat poisons such as D-Con, which the EPA has determined pose a risk to children, pets and wildlife.

The following interview is with Gerry Miller, who worked 28 years with the California Department of Food and Agriculture and the Tulare County Agricultural Commissioner’s Office. Before retiring as a program manager, he worked as an Primary State Biologist and a Senior Environmental Planner coordinating the state and counties’ vertebrate pest control. Miller answered questions below about rodent control in urban settings and emphasized the need for an integrated pest management (IPM) approach that uses other methods besides poisons. Because of the length I’m splitting this one in two. A second article will deal with research about the secondary effects of rodenticides on our pets, wildlife and in humans.

First, some background:

• The EPA is most concerned about people using loose pellets of the most common rat poisons today. These are second-generation anticoagulants(D-Con and Hot Shot are common brand names for these rat poisons). which were developed because some rodents have developed a tolerance to the first generation products.
• These newer ones kill the rodents more quickly. The rodents essentially bleed to death and the toxins left in the carcasses can cause the same problems in anything that eats them, most often pets, birds of prey and other animals.
• These second generation anticoagulants include brodifacoum, bromadiolone, difenacoum, and difethialone.

Q: Why is it important to consider other treatment options besides rodenticides? Some things I’ve read about are sealing cracks and screening openings, removing food and water sources and keeping food in airtight containers. What about removing bird feeders?

A: Why use a rodenticide when you don’t have too. Additionally, many times killing the critter is like treating the symptom but not the cause of the infestation inside a dwelling.

I agree 100% to all but the bird feeders. In the business, we call sealing entry points ‘exclusion’. Many people underestimate the size of opening that a critter can get through. You need to know your target and screen appropriately For example, mice can get through an opening just over ¼ inch while rats can gain entry through an opening larger than ½ inch. And yes, remove food such as cat or dog food, water and shelter and you’re well on your way to solving your infestation. Bird feeders are fine if you can keep the critters from the food inside of the feeder or below where the food is distributed by the feeding birds.

The very best option is to first evaluate your infestation so that you know its species, population size and extent throughout your property. Close all sources of entry into the structure. If it continues after doing that and eliminating all sources of food, water and shelter, re-evaluate your property to determine the size and extent of your infestation. Determine your best option for steps forward, i.e. trapping, use of rodenticides, or consider a barn owl box to attract natural predators. Or do nothing if your infestation is at an acceptable level.

If you are not comfortable with the next steps forward, interview and hire a licensed pest control operator and listen to the options they bring forward and decide on what you want to do. If you want to carry out a rodenticide program, take the knowledge you’ve gleaned and head off to a retail store and read rodenticide label to find one that fits your situation. Talk to a knowledgeable salesperson for further knowledge. If you’re not comfortable, don’t buy the rodenticide, go home and do more research on the product manufacturer’s website to answer your questions. Make sure that you read and follow all label directions and safely control the application through all stages, including the proper disposal of any carcasses found. A good method is putting your hand in a plastic bag and picking up the carcass and disposing it in the garbage ( ok?).

Q: Is it true that all poisons carry some risks? In our case, we were concerned about the possible effect of anti-coagulants on the raptors on our property so we used a nerve toxin (bromethelin) because we thought it was safer. However, now we find out it’s highly toxic to birds. Do all poisons have secondary risks?

A: Everything we do involves some amount of risks. And yes, poisons do have secondary risks. That’s why it is so important to follow all label directions. The key to the safe and effective use of any poison whether it be a rodenticide or a chemotherapy compound, is to understand the compound’s risks and rewards, know how it is to be applied, and determine if the rodenticide is appropriate for your unique application. Do the risks out weigh the reward?

One of the problems is people use pesticides in a manner that was not approved. Rodenticide labels instruct the user to survey for and dispose of carcasses (in order to reduce risk to non-targets.) I’m not confident that homeowners do this unless a carcass starts stinking up in summer. They place the poison out and forget about it. They don’t want to touch a dead critter and dispose of properly. They just want the critters gone. Even if you use a commercial pest control operator to apply the rodenticide, it is the home owner’s or property manager’s responsibility to dispose of carcasses properly.

An example of not using pesticides correctly is the warfarin resistance that developed in some rodents in the U.K. The government agents weren’t able to administer the warfarin at the designated intervals due to their constraints. Instead, they administered it at longer intervals. The rodents were able to overcome the effects of the poison over this greater time period and thus developed this resistance. If it had been applied correctly, this would not have happened.

Q: I understand the EPA is trying to eliminate loose pellets and is concerned about their overuse. What about using loose pellets in a bait station?

A: Loose pellets inside an approved and properly used bait station is much safer and better in keeping the poison from non-targets. Many are extruded wax like “pellets” that may resemble dog food, candy, and were designed to be used in bait stations that are tamper proof and spill proof to contain the bait inside. But remember that most critters are leery of “new” things placed in their environment, so they need a reason to go inside of a bait station to eat. And good luck if readily available food and trash is all around and much easier to obtain.

I would caution folks to stay away from any claimed rodenticide sold in California that doesn’t have both US and Cal EPA registration numbers printed on the label. If you are unsure, contact your local County Agricultural Commissioner’s office and ask them if the product is legal to use. Make sure you read the label to see if you can apply the product as stated on the label. Any pesticide whether it be a rodenticide, herbicide, insecticide, etc. they must be used as instructed by the label. If not, don’t buy it, and hire a state licensed/ certified professional operator.

See Part 2 (published 7/26/13) for more info on other tips, some IPM examples, and more resources.