[Vertebrate Pests] City park rat management programs

Most cities on a global scale take pride in their city parks. It is well known how important parks are to the human condition for replenishment of our minds and souls. Parks are where most of us can temporarily escape from the daily stresses of work or life and find a bit of solace and solitude among the peacefulness of the trees, lakes, streams and wooded paths. Too, children and adults alike relish seeing wildlife or hearing songbirds, or getting a lucky glimpse of a hawk or some other majestic bird soaring above.

So, tracing back to our oldest cities, city planners have been mindful of the need to integrate parks among busy metropolises. Visit any park any day, any time of the year, and you will usually see humans recreating and looking relaxed.

Of course, humans are not the only mammal species that find parks accommodating. Other mammals such as tree squirrels, ground squirrels, deer, bats, rabbits, raccoons, weasels, shrews, fox, and, depending on the size and location of a particular park, perhaps some of the larger and even “wilder” mammals such as wolves, moose, bear and others may also use and benefit from our city green-spaces.

You can probably guess where I’m headed. If the mammals listed above find parks accommodating to their survival, certainly so too do our most adaptable and opportunistic mammals, the commensal rats. But while we welcome the wild birds and animals into our parks, we are unwilling to share our parks with rats. Here’s why our intolerance is justified.
 
THE PROBLEM WITH PARK RATS. Rats in parks (and most other areas as well) frighten and/or repulse people. In fact, it is not uncommon for people to avoid a park entirely if the park is known to have rats. Norway rats can significantly damage or kill ornamental plantings from their burrowing activities as well as undermine slabs, historic statues and garden retaining walls of all types. As predators, rats attack and kill waterfowl and songbirds, as well as raid bird nests, killing chicks and eating eggs.

Ideally city parks should be kept rat-free. In cities that have established rat populations (e.g., New York, Baltimore, Washington, D.C., London, Paris, etc.), this is not easy to achieve. So in cities such as these, the goal is clearly to maintain the numbers as low as possible. But managing park rat infestations poses several challenges. Obviously park visitors often are comprised of families with inquisitive children and pets. And as mentioned previously, parks may also be home to desirable urban mammals and birds. When predator species are among those animals (hawks, owls, fox, wolves, etc.), secondary risks can exist to them should rodenticide baits need to be employed.

So city park rat management programs must be carefully designed to ensure minimizing hazards to people, wildlife, and the park’s environment. Let’s examine some of the very special considerations and approaches for city park rat IPM programs.

ULTRA IPM. The best management of city rats in city parks is to never let rats become established in the first place. This may sound obvious, but many parks around the world in cities with established rat populations fail to have proactive surveillance programs to halt the arrival of any newly arriving rats. Once rats become established, getting the park back to a “rat-free” status can be a considerable task in time and money (and this is especially true in times of stressed municipal budgets).

For park rat infestations, an integrated pest management approach (IPM) is both obvious and paramount for the reasons discussed earlier. Moreover, when sensitive wildlife issues exist for a particular park such as resident hawks, owls, fox, weasels or other predator species, additional precautionary measures and considerations above and beyond the standard IPM efforts become mandatory. Some considerations for such ultra-IPM park programs include :

• If there is relatively little daily food readily available to rats, resident infestations cannot become established. Thus, in parks, refuse management is rat management. Park refuse management must be keyed on reducing the daily food shrapnel load on the park via well-designed litter collection/grounds pickup programs and careful attention to litter basket model choices, their locations, and the basket-to-visitor load ratio.
• Minimization of any removable rat harborages (such as old equipment, pallets, clutter piles, composting and recycling piles, etc.).
• Proper landscaping practices. Cavernous bushes and carpet-style plantings are highly vulnerable to rat invasions. For bushes, pear-shaped varieties are best (open at base, dense up top). If for aesthetic purposes, ground covers and cavernous bushes are desirable, they should be pruned so they allow for a modicum of visibility at their bases, and in neighborhoods where rats are established, such bushes should be inspected on a biweekly basis.
• Inspections to determine specific locations of rat activity to facilitate spot-specific baiting or trapping programs (should baits be required). Such inspections will minimize the amount of pesticides used and minimize the number of bait placements, if any, that may be needed for a particular area.
• For any new rat infestations (i.e., before numbers have gotten large), strong consideration should be given to employing snap trap programs in place of bait programs. Trapping programs, if done by skilled technicians in conjunction with enhanced sanitation, can be highly effective. Snap trap stations can be installed in the same locations as would bait stations. However, trap stations must be checked and serviced frequently enough to remove trapped rats and reset traps. If there is high rodent pressure on the park, trap stations may need to be serviced at least two times a week or even daily until the infestation has been reduced.

RODENTICIDE USE. In general, the use of rodent baits (rodenticides) should be considered secondary to sanitation and harborage reduction.

• In parks and/or park areas where predaceous birds are nesting or frequenting, any existing rodent baiting programs should be suspended either for the entire park (small parks); or for those sections of larger parks where the predaceous animals and birds may be foraging. Trapping programs can be substituted into these sensitive zones.
• If baits are necessary due to large infestations, only second-generation anticoagulant rodenticides that are listed as “low to moderate” non-target animal risk should be employed. Bromadiolone is categorized to birds as a low to moderate primary risk and a moderate secondary risk.
• The anticoagulants brodifacoum and difethialone are categorized by EPA and various studies as posing high primary and secondary risks to wildlife, and although these actives are outstanding for other programs, they are not the appropriate choice for city park rat programs.
• The non-anticoagulant rodenticides cholecalciferol (often referred to as Vitamin D3) and bromethalin are sometimes considered in efforts to minimize raptor threats. However, these baits are also categorized as having a moderate to high primary risk (bromethalin) and a low to moderate primary risk (cholecalciferol) to birds. (Thus, note cholecalciferol is rated the same risk as for the anticoagulant, bromadiolone).
• But there are extenuating disadvantages associated with the use of cholecalciferol and bromethalin should inadvertent poisonings occur with these products:
  
  1. Should there be a case of accidental poisoning to dogs, cats, and other non-target mammals, no antidotes are available for cholecalciferol or bromethalin (versus the antidote Vitamin K1 available for the anticoagulants); and,
  
  2. In the case of accidental acute poisoning with cholecalciferol in dogs and cats, death is via hypercalcemia. Various veterinary reports characterize such death for these animals as prolonged and especially painful.

Given the risk categorizations and the extenuating disadvantages, should a rodent bait be absolutely necessary, bromadiolone bait, although not without some inherent risk itself, may be the most appropriate active choice overall.

SUMMARY. For managing rats in city parks, an IPM partnership is essential. The parks are responsible for the prevention aspect. A park cannot contain established rat populations unless the rats have food that is easily accessible and in sufficient quantities day in, day out. So there must be effective refuse management.

As far as the pest management professional is concerned, if rodenticide applications are determined necessary, all procedures must be in place to ensure hazards are kept to a minimum. These include: 1) careful selection of bait active ingredients; 2) low-hazard bait formulations and application techniques; and 3) inspections for conducive conditions, regular reporting to the park and monitoring for sustainable rodent control. 

The author is president of RMC Consulting, Richmond, Ind.