[Vertebrate Pests] What The Empty Trap Reveals

October 7, 2003

The pest management industry uses perhaps millions of glue traps and "sticky monitors" each month for a wide variety of purposes. The larger tray-style traps are manufactured specifically for rodents (mostly mice), while the smaller sticky cardboard traps are used to monitor insect pest situations, such as confirming whether or not a pest is present, or has occurred since the last service visit. The inexpensive cardboard monitors that are typically folded into "triangle traps," for example, are used to monitor for cockroaches, ants, brown recluse spiders, "mystery bugs" and a wide variety of other crawling insects for both commercial and residential clients.

If inspected casually (e.g., from a distance using only the beam of a flashlight), it is easy for an inspector to interpret a trap lacking any obvious insect or rodent carcass as being "empty." But it’s been my experience that "empty" traps are often not empty at all. In fact, upon close observation (in part, the word "observe" means to look closely and study), "empty" glue traps frequently reveal several pieces of evidence relating to rodent infestations.

Let’s examine the telling "empty glue trap" issue a bit closer.

PRESENCE OF HAIRS. It is not uncommon to find on an empty glue trap one, several or a clump of rodent hairs. Of course, the detection of only one or a couple of hairs requires a close look at the trap. When present, hairs are seen along the perimeter edges of the trap. Obviously, these hairs are from those rodents that have successfully avoided the dangerous surface. Most times, trapped hairs are "guard hairs" from the rodent’s pelage. Guard hairs are long, tactile hairs used by the mammal to help guide them along surfaces as well as guard it from potentially dangerous surfaces.

Within most rodent colonies, some rodents exhibit "cautious" approach behaviors to new objects that suddenly appear in their runways and territories. Presumably, such rodents approach newly appearing glue devices only close enough to get one or a few of their guard hairs from their chest region caught in the glue trap and pull back. How "painful" an experience this is to the rodent, or whether or not any learning occurs with the rodent as a result of this event, is unknown. But even the presence of one hair tells you that a rodent is, or was, in the area. Such a discovery provides insight as to the current pest "profile" of that account.

PRESENCE OF FECES. It is also common to discover only feces on glue traps that are otherwise unoccupied. Finding fecal pellets also requires careful observation to correctly determine what animal was investigating the area and the traps. Mice and several large domestic cockroaches (e.g., American, smokybrown) commonly "perch" on the edges of sticky traps and defecate onto the glue surfaces. Maybe these animals have "learned" to maintain a safe distance from the dangerous surfaces via their tactile sensors (i.e., rodent guard hairs, insect antennae).

Fecal pellets must be identified as to rodent or insect. From a casual view a young mouse’s fecal pellet may appear similar to an adult American cockroach in size and color. Under a hand lens, the pellets of the American cockroach contain ridges and are typically (but not always) rectangular in shape. The fecal pellet of mice on the other hand are typically pointed at one, or both ends, contain hair fragments and lack ridges. (This is one good reason why a hand lens should be standard inspection equipment.)

PAPER, DIRT, WALL INSULATION. Mice and rats will commonly kick or carry pieces of paper, dirt and wall insulation and place them onto the tacky surfaces of the glue trap. Sometimes, the entire trap will be covered in this fashion. Presumably, the rodent performing this task is attempting to eliminate this dangerous surface in its runway. I have seen cases where an entire glue tray trap is covered with pieces of cardboard. Other times, I have seen only a few pieces of wall insulation dropped onto the board accompanied by a mouse fecal pellet or two. The point is, the pest professional can’t always assume such debris on a glue trap has been wind-blown onto the trap.

PIECES AND PARTS. In last month’s column, I discussed how some rodents are adept at eating cockroaches and other insects off of glue traps without themselves becoming trapped. Sometimes, only the legs, the wings, the antennae, or the head of the insect are left behind on an otherwise "empty" trap. Certainly, the insects from which these parts originated did not escape and walk off. These pieces and parts are a strong clue that a rodent is about. Moreover, those mice that can successfully pluck cockroaches and other insects out of the glue may also be adept at avoiding these dangerous surfaces altogether. In other words, such rodents may be gluetrap-shy and/or neophobic rodents, necessitating rodenticide baits or snap traps for their control.

FOOTPRINTS OR PARTIAL PRINTS. Occasionally a footprint or partial print of a rodent’s foot can be seen imprinted in the glue of the traps. The front paw of the mouse and rat contains four digits (i.e., "toes"). Similar to the sequence of events discussed with the mouse approaching, but not committing to the trap, a rodent (mouse or rat) may slowly and cautiously approach, lightly step onto, but then pull off of a glue trap, leaving behind their "fingerprint." Here too, a hand lens is useful to confirm the print.

CONCLUSION. By their nature many structural pests are secretive. And thus, pest management service work is often compared to detective work and solving mysteries. We draw analogies to Sherlock Holmes — or more contemporarily speaking — to the TV series CSI. From my perspective, these analogies are justified.

"Empty" glue traps can provide subtle clues as to what has occurred since your last service or what may be in the early stages of development. That is, they can provide such clues if you are observant and challenge yourself. It is not enough to glance at unoccupied monitor traps from several feet away using only a flashlight and conclude "it’s empty."

Pest monitors should always be retrieved and examined (tip: a 4-inch thin blade spatula is an excellent monitor retrieving tool). Study any and all fragments, hair and other items on the trap and ask, "Is there anything this ‘empty trap’ can tell (teach) me?" Such a detailed look is truly a forensics approach — your own private CSI episode.

The author is president of RMC Pest Management Consulting and can be reached at rcorrigan@pctonline.com or 765/939-2829.