Are Blacklights Fading?

One of the most important steps in any Integrated Pest Management program at a food plant is to minimize pest activity on the property surrounding the plant and especially, to deny access to the plant itself. In reality, pests enter a food plant in one of two ways:

1. Pests can enter the plant on their own via structural faults such as holes in walls, doors that are left open, damaged screens, etc. At the same time, large populations of insects, birds and rodents on the outside grounds provide increased opportunities for invasion of the plant. Consequently, it is important to a) maintain low populations around the facility by minimizing pest supporting resources such as ground spillage, open ditches, fruit-bearing or flowering vegetation and equipment stored directly on the ground (food, water and harborage), and b) to ensure the structural integrity of the plant (pest-proofing).
2. Pests can also gain access to the plant on incoming goods. As such, it is important to establish a viable and documented incoming goods inspection program. Production constraints make it difficult (impossible?) to thoroughly inspect every item coming into a plant, so random sampling with user-friendly techniques must be used. After all, risk cannot be eliminated; it can only be minimized.

THE BEST OPTION? An inspection tool often used to disclose rodent activity on incoming goods is the portable blacklight. It is a commonly held belief among sanitarians that “If it works for the regulatory inspectors, then it’s good enough for me.” I do not want to discourage the use of any tool or technique that will prevent infestations, however, blind allegiance such as this cannot be justified. While rodent urine will glow or fluoresce under longwave ultraviolet light, the use of such a detection device under real world conditions can be cumbersome and problematic. Let’s consider some of the peculiarities of this inspection tool.

Several common, everyday materials found in food plants produce dry salt residues that will fluoresce under ultraviolet light, including lubricating oils, ink, glue and detergents. Inspectors must be trained to distinguish between such offending chemicals and rodent urine stains. When a suspect material glows, the shape of the residue and the pattern by which it is deposited will provide clues to its source. Since rodents have little control over their bladders and urinate frequently as they travel, rodent urine stains are most often seen as a series of nearly round droplets strung out in a more or less straight line. Except in nesting situations, the amount of urine observed in one location (i.e., on a bag of goods) is usually small since mice only produce 1 to 2 ml of urine per day, while rats produce only about 8 ml per day. Inspectors must be able to recognize the basic geometry of rodent urine stains: trails of closely spaced, small droplets.

Rodents are ureotelic animals: nitrogenous kidney wastes are excreted from the body mainly as urea. It is interesting to note that rodent urine (as well as urine produced by cats — another common pest around food plants and warehouses) contains a lot of water, which can easily soak into food packaging materials. Fluorescent evidence as described previously is only one clue to rodent activity; relying heavily on the presence of glowing stains on incoming goods could provide misleading results. In order to verify rodent activity, once a suspicious stain is encountered, a sample must be removed to the laboratory for chemical analysis. This is a standard operating procedure for regulatory inspectors whose job is to build a case (gather evidence) that could be used in a court of law. Unlike regulatory inspectors, incoming goods inspectors are simply trying to identify potential problems that could negatively affect their facilities and products. Fortunately, in-house inspectors or sanitarians do not have to wait for such chemical verification before making an “accept or reject” determination since in addition to urinating on the run, rodents also defecate, producing numerous, easily recognized droppings. Their constant gnawing behavior and nesting habits provide more easily recognized evidence of rodent activity.

Fluorescing rodent urine stains are faint; under the strong competing lights often found in receiving docks this evidence is difficult to see even by experienced inspectors. In order to view rodent stains with a handheld light, the inspection must be done slowly and methodically and should be conducted under low light conditions. Receiving docks do not lend themselves to this level of intense scrutiny.

SUMMARY. Identifying rodent urine with a blacklight is difficult to accomplish even under ideal conditions. Under the warehousing and receiving conditions normally encountered in food-processing plants, reliance on blacklights to determine the presence of rodents can be a delicate undertaking, since rodent urine stains are often confused with other chemicals commonly found in food plants; faintly glowing, small urine droplets are difficult to view in well-lit environments; and incoming goods must be inspected quickly, allowing little time for an on-the-fly discovery of rodent urine, especially when more reliably visible evidence always accompanies such staining.

Incoming goods must be inspected for rodent activity. But in my opinion, the use of blacklights to detect rodent urine is just not practical for incoming goods. If used at all, this tool should be relegated to static product stored for extended periods in poorly lit warehouses. But even under these storage conditions, rodent evidence such as droppings, gnaw marks, nesting and trapping results provide more useful evidence of rodent activity.

The author is an entomologist and president of Technical Directions, Decatur, Ala. He can be reached at mholcomb@pctonline.com.