Sewer Baiting: Another Dimension To Rat Control

Pest management professionals should never discredit the claims of customers who say, “There is a rat in my toilet” (read story at http://www.pctonline.com/articles/article.asp?MagID=1&ID=1740&IssueID=171) or, “They must be getting in through the sewer.” These are not unrealistic possibilities considering the aging infrastructure of older cities and the degradation of their waste systems. These conditions are readily exploited by opportunistic Norway rats (Rattus norvegicus).

Typically, control of Norway rats involves implementing pest management strategies in and around structures. Surface rodent control is largely dependent upon environmental management (sanitation, exclusion and harborage removal) for achieving sustainable results. However, in older cities there may be an entire dimension to controlling this pest that goes totally unexplored or, at best, cursorily addressed — the sewer system. Subsurface control is more dependent upon baiting, since making widespread change to a sewer system is difficult and costly.

In 2001, American Pest Management, Takoma Park, Md., teamed up with the District of Columbia Department of Health, Bureau of Community Hygiene Rodent Control Division to assess one of the major un-addressed problems in the city’s “New Rat Control Program” — the sewer rat population. The problem was sporadically addressed by many of the companies providing rat control services in the District. Their findings typically consisted of rats entering structures through toilets and abandoned and open sewer lines in dilapidated and renovated structures.

Additional areas were identified by the District’s Water and Sewer Authority (WASA) personnel who periodically enter the sewer system and observed rat activity. Furthermore, a comprehensive study of the rat problem in the District was conducted by Dr. Bruce Colvin. He identified potential areas of activity based on his extensive experience with the Boston’s Central Artery Tunnel Project.

Colvin used the following criteria in selecting survey areas: residential areas with brick or clay systems that were less than 24 inches in diameter and had a sloping topography. A total of eight sites were selected for initial surveys (these sites coincided with the eight District wards).

SURVEY. Below is information regarding the surveys:
 • Since rats are not uniformly distributed in the sewer system and structural aspects of the system were not well documented, surveys were conducted to locate centers of activity. The strategy was to identify rat distribution patterns and progressively reduce the population.
•  Baiting was coordinated with the sewer authority. Safety (traffic and personal health) was a major concern, thus, personnel did not enter manholes and all baiting was conducted from the street. Sewer baiting is a two-person operation and depending on traffic, may require assistance from local police or other traffic control. In high traffic areas, work was often more safely and easily performed during early morning hours.
•  The initial survey consisted of monitoring 20 to 25 manholes per geographic area. The sewer systems surveyed were either sanitary or combined (sanitary and storm water). The survey involved three rounds of baiting in each manhole at 10 to 12 day intervals.

SUB-SURFACE MONITORING. Following are the details of our monitoring program:
•  A boxed-shaped loop of stainless steel wire with a plastic straw inserted inside the 1-pound paraffin bait block was tightly secured around the block using a brass ferrule (see photo, pg. 52). Some professionals recommend the use of plastic coated wire, however, over a year’s time we found no significant deterioration of the stainless steel wire and it was much easier to use. Gnaw marks on the plastic straw were easier to interpret than those on plastic-coated wire.
•  Our procedure evolved into the use of 8-ounce blocks for monitoring and in most cases baiting. One-pound blocks were overkill — many were destroyed through deterioration and water flow. The larger blocks were used in sewers when there was significant bait consumption.
•  A key ring was placed on the loop for easy attachment to the suspension wire. The block was hung from the manhole chimney using a stainless steel wire with two small loops (made using brass ferrules) at either end. The wire was attached to the chimney using the ladder or a masonry nail with a washer driven into the wall of the chimney near the rim. The block was suspended about two inches above the bench at the manhole base.
•  All manholes in a geographic area were baited. Manholes that had heavy water flow (white water rapids) and high water marks were not monitored.
•  Baited sewers were identified and referenced to a sewer/utility map. This information was obtained from the sewer authority (WASA) and contained information regarding the type of system, size of pipes and direction of flow. One method of identification involves attaching numbered metal tags to the bait line and using GIS mapping. Hand drawing maps and using street numbers was equally effective and less expensive.
•  Monitoring (bait) placements were inspected in 10 to 14 days. This inspection was repeated at least once to determine if there is any rat activity at the site.
•  Interpretation of “feeding activity” is essential to understanding what is happening in the sewer system. Pulling up the suspension line and finding the block missing does not necessarily indicate a rat ate it. Evaluation criteria recommended by Colvin was used to interpret monitoring/baiting and rat activity:
1. Does remaining bait have gnawed marks? Yes = rat activity. However, do not discount the fact that other animals, e.g., raccoons, occasionally enter combined sewer systems and feed on bait blocks.
2. Are there gnaw marks on the plastic-coated wire (straw insert)? Yes = rat activity.
If 1 and 2 do not apply:
3. Is the bait wire in a square, as if bait were still present? Yes = water damage, insects or decay.
4. Has the bait loop been stretched from its square shape? Yes = assume rats, could be water.
5. Does any remaining bait have a coarse, peppered look? Yes = insects.
•  Date, feeding activity, bait consumed and other observations (high water, bait decay, fungal growth on bait and insects) were recorded for each site.
•  Over a seven-month period, 36 sites (329 manholes) within the District’s eight wards were surveyed (baited).
•  Manholes with feeding activity were categorized as low, medium and high feeding activity.

SEWER BAITING. Further information on the baiting program is as follows:
• Pulsed baiting was used in manholes where feeding activity was noted. The pulsed baiting scheme was determined based on the intensity of feeding activity.
1. Low to medium — Three rounds of pulsed baiting were performed at 14 day intervals until feeding ceased. This process was scheduled to be repeated at six-month intervals.
2. High — Three rounds of pulsed baiting were performed at 14-day intervals until feeding ceased. This process was scheduled to be repeated at three-month intervals.
•  During the contract period, 329 sewers were inspected and were either re-inspected and/or baited approximately four times each.
• We estimate that 723 ounces of bait were consumed during the service period.

OBSERVATIONS. Here is a summary of our thoughts regarding this sewer baiting rodent project:
• Sewers are an extremely harsh environment (high humidity, high temperature, high water and mold/fungus) for extruded wax block baits. Depending on conditions, blocks did not last more than two to four weeks.
• Surface mold on the blocks did not appear to affect palatability. However, some blocks grew “black hair” (fungal mycelium?) and the rats would not feed on them.
•  The techniques used in this program minimized insect consumption of the bait. Less than 5 percent of the manholes had insect activity, which typically were American cockroaches.
•  The depth of the sewer did not appear to be a factor in feeding activity. Rats fed on bait blocks placed as deep as 40 feet and as shallow as 2 to 3 feet.
•  Typically, sewer feeding activity was more common in areas where above-ground sanitation was good and there was minimal harborage and/or burrowing activity. Conversely, when above-ground conditions were conducive to rat activity, there was little feeding in the sewers.
•  Sustained feeding activity occurred in about 15 percent of the manholes surveyed.

CONCLUSION. Sewer baiting is not a standalone control for rodents. It is not designed to resolve the problem of rats entering structures through toilets and/or abandoned sewer lines.
It should be used as a complement to above-ground rodent control strategies, e.g., sanitation, code enforcement, harborage elimination, exclusion, trapping, baiting and burrow treatment. 

Richard Kramer is technical director of American Pest Management, Takoma Park, Md. Mark Greenleaf is a rodent control officer for the Washington, D.C., Department of Health. Colin Ellis is lead service technician, American Pest Management. Kramer can be reached via e-mail at rkramer@pctonline.com.