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The German cockroach, Blattella germanica L., is the No. 1 indoor pest in residential buildings in the United States. In addition to being a nuisance pest, German cockroaches pose a significant public health threat, particularly in homes occupied by people with allergies and asthma.
Cockroaches in apartment buildings are usually controlled through the monthly application of baits or insecticide sprays in the U.S. Yet, high infestation rates are frequently reported: 28 percent in apartments in New Brunswick, N.J. (Zha et al. 2018), 30 percent in apartment buildings in four cities in New Jersey (Abbar et al. 2022).
Insecticide resistance, low-quality pest management and lack of cooperation of clients in properly maintaining their premises allow German cockroaches to become a chronic pest in multi-unit dwellings, food handling and processing facilities and similar indoor environments (Wang et al. 2018).
Applying gel baits has been a standard and very effective method for controlling German cockroaches by pest management professionals since the 1990s in the U.S. (Appel 1990, 1992). While behavioral and physiological resistance to gel-based bait formulations has been reported in some German cockroach populations (Silverman and Ross 1994, Wang et al. 2004), development of high levels of resistance to baits among field German cockroach populations appears to be slow (Gondhalekar et al. 2011, Ko et al. 2016).
Baiting remains the primary choice for controlling German cockroach infestations by professionals (Schal 2011). High-level German cockroach reduction was reported in recent studies using baits alone or in combination with other methods (Sever et al. 2007, Wang 2010, Wang et al. 2013, Zha et al. 2018). Baits also are widely available in stores and are used by consumers (Dingha et al. 2013).
Sticky traps are widely used for monitoring cockroaches and other indoor crawling insects. The efficacy of different sticky traps varies significantly (Ballard and Gold 1984, Wang and Bennett 2006).
It is known that sticky traps alone were not able to effectively reduce German cockroach populations in heavily infested homes (Ballard and Gold 1983, Schal and Hamilton 1990). However, it is unknown whether placing large numbers of traps will achieve equivalent results as baiting for controlling low-level infestations.
Most of the natural German cockroach infestations have small numbers. For example, Wang et al (2018) found 74 percent of the 104 infested apartments had 10 or fewer cockroaches based on four sticky traps placed over a two-week period. Another survey of 317 infested apartments found 54 percent had 10 or fewer cockroaches in four sticky traps, per apartment, placed over a two-week period (Abbar et al. 2022).
We consider apartments with trap catches of 10 or fewer cockroaches over a two-week period as low-level infestations. We set out to see if these low-level infestations can be effectively controlled by trapping alone. This information will be valuable for managing cockroaches safely and minimizing insecticide applications. It is especially important to consumers, who often do not know how to properly apply gel baits. Here, we present results of two field studies comparing trapping and baiting for controlling low-level German cockroach infestations in apartments.
MATERIALS, METHODS, SITES AND INITIAL MONITORING. Two field studies were conducted in 2017 and 2019. Study I (2017) included four high-rise apartment buildings located in Irvington and Paterson, N.J. Study II (2019) included two high-rise apartment buildings located in Jersey City, N.J. Each building has 130-156 units. The majority of building occupants were low-income seniors (62 years old or older) or disabled residents. All apartments were one bedroom or studio except for 12 two-bedroom apartments.
In Irvington and Paterson, the existing pest control provider hired by the housing authorities serviced the communities monthly, with the exception of one building in Paterson, which was serviced by Rutgers University researchers prior to this study. In Jersey City, licensed housing staff from the Housing Authority of Jersey City provided pest control. Various gel baits containing fipronil, indoxacarb and dinotefuran were used for cockroach control in these buildings prior to this study.
In study I, building-wide cockroach surveys were conducted in April and May of 2017. In study II, building-wide cockroach surveys were conducted January through April 2019. Four Trapper Monitor & Insect Traps (1/3 of the three-section trap) from Bell Laboratories, Madison, Wis., were placed in every accessible apartment (Figure 1A). The locations of the four monitoring traps were in the kitchen cabinet under the sink, beside the stove, behind or beside the refrigerator and behind the toilet in the bathroom. The traps were collected approximately 14 days later, and the number of trapped cockroaches were recorded.
Based upon the total trap count in each apartment, we selected 44 apartments in study I and 22 apartments in study II. These apartments met the following two criteria: The total cockroach numbers based on 14-day trap count in the apartment were between one and nine, and the resident agreed to participate in the three-month study. They were randomly assigned to baiting and trapping alone treatments.
In study I, the baiting and trapping alone treatments included 21 and 23 apartments, respectively. In study II, the baiting and trapping alone treatments both had 11 apartments.
TREATMENT AND IMPLEMENTATION. Within less than two weeks of being identified as eligible apartments, homes in the baiting alone group were treated with Advion cockroach gel bait (.6 percent indoxacarb) from Syngenta Crop Protection, Greensboro, N.C., by licensed Rutgers University researchers. The mean quantity of bait used in study I was 5 grams per apartment. Approximately 10 grams of bait was applied per apartment in study II.
The higher amount of bait usage in study II was designed to achieve better control results than that in study I, which was completed earlier. Cockroach bait was applied as small beads (approximately .1 grams per bead) in kitchens and bathrooms. Although a small amount of bait was used per apartment, the application was very thorough (approximately 50 spots per apartment). A standard protocol similar to that used in Wang (2010) was followed throughout the treatment to ensure consistency in application patterns.
The trapping alone treatment in studies I and II is similar. Each apartment in the trapping alone group received 10 and 16 Victor-M330 Roach & Insect Glue Trap from Woodstream, Lititz, Pa., respectively (Figure 1B).
Four of the Victor traps were placed in the standard locations as the Trapper traps. The other six Victor traps in study I were placed in the following locations: in the cabinet under the kitchen sink, in the cabinet above the kitchen sink, between the refrigerator and kitchen cabinet, inside the kitchen island cabinet or the bathroom cabinet, in the closet near the kitchen and on the kitchen counter (usually behind the microwave).
In study II, 10 Victor traps were installed in the same locations as in study I. The other six Victor traps were installed in the following locations: in a cabinet above the kitchen sink, behind the toilet in the bathroom, beside the stove, in the bedroom, in the living room next to the furniture and behind the main entrance door.
Residents from all apartments received a one-page handout explaining the public health importance of German cockroach control as well as non-chemical methods to prevent and help control them. Residents were advised not to use any insecticides during our studies.
FOLLOW-UP MONITORING. Starting one month after the treatments in both studies, and over a period of three months, Trapper sticky traps were placed in each apartment (in the same manner as initially) for two weeks every month to evaluate the effectiveness. In study I, no additional bait was applied during subsequent visits regardless of the number of cockroaches. In addition to trap counts, residents were asked if they noticed cockroach activity at the end of the study.
In study II, additional bait applications were made during subsequent visits in apartments with a total trap count of more than three cockroaches. Four apartments had total trap counts of more than three in the first monthly evaluation and received 5 grams of bait. The same four apartments had more than three total trap counts in the second monthly evaluation and received another 5 grams of bait.
In the third monthly evaluation, only one apartment from this group had a total trap count of more than three. However, this apartment did not receive additional bait, as the study was concluded at that point in time. The number of cockroaches in Victor traps was recorded during each visit. If the Victor traps were missing, damaged or had cockroaches during monitoring visits, they were replaced with a new trap.
All cockroach trap counts were adjusted to 14-day counts during analysis. We used the Kruskal-Wallis test to compare the trap counts between the two treatment groups.
RESULTS. What was the effectiveness of the treatments on cockroach suppression? In study I, the percentages of cockroaches caught in trap locations one through four were 22, 26, 36 and 16 percent, respectively. In study II, the percentages of cockroaches caught in trap locations one through four were 22, 34, 41 and 2 percent, respectively.
Thus, traps located at the refrigerator and stove trapped the greatest number of cockroaches, and the trap next to the toilet had the fewest. In study I, trap counts decreased in 76 and 70 percent, respectively, of the apartments after three months in the baiting and trapping alone treatment (Figure 2). Trap counts in 43 and 48 percent of the apartments in the baiting and trapping alone group were reduced to zero.
There were no statistically significant differences in the median trap count between the two treatments at each sampling period (Kruskal-Wallis test). Trap counts in all but one apartment in the baiting group were 10 or fewer at three months. Therefore, a single application of a small quantity of bait alone is similarly effective as trapping alone for suppressing low-level German cockroach populations.
In the 19 apartments that still had cockroach activity after three months, 63 percent of residents interviewed said they did not see cockroaches in their homes. Residents did not report seeing cockroaches in the apartments with zero trap catches.
In study II, trap counts decreased in 91 percent of the apartments after three months in both the baiting and trapping alone treatment (Figure 3, page 60). Trap counts in 73 percent of the apartments in both the baiting and trapping groups reduced to zero. There are no statistically significant differences in the median trap count between the two treatments at each sampling period (Kruskal-Wallis test). Trap counts in all but one apartment in the baiting group were 10 or fewer after three months. Therefore, baiting alone at a higher application rate is similarly effective as trapping alone for suppressing low-level German cockroach populations.
DISCUSSION. Presumably, low-level cockroach infestations should be easily eliminated by bait treatment. However, in study I, only 43 percent of the apartments treated with bait were reduced to zero trap catches.
To determine whether this low percentage was due to an insufficient amount of bait used at the beginning and lack of follow-up treatment, we designed study II where we applied 10 grams of bait at the beginning and provided follow-up treatment to apartments that still had more than three cockroaches per apartment in the first and second months. The trapping treatment groups received 16 traps per apartment instead of 10 traps.
Both baiting treatment and trapping alone did result in a higher percent of apartments with a cockroach count of zero than that in study I (73 versus 43 percent in baiting, and 73 versus 48 percent in trapping alone). These results indicate that relying on one control method may not be sufficient to eliminate German cockroaches rapidly, even when their numbers are low. Incorporating other methods will help achieve faster elimination (Wang and Bennett 2006). This study also shows that follow-up monitoring for several months is necessary to confirm cockroaches are indeed eliminated.
Material cost for each apartment was $1.04 and $2.83 in the baiting group in study I and study II, respectively. In the trapping alone group, material cost for each apartment was $4.07 and $6.22 in study I and study II, respectively, based on product price obtained from the distributor in both studies. The cost does not include the additional Victor traps placed due to damage or those that became dirty during the study. Thus, from a material cost perspective, the trapping alone method is much more expensive compared to baiting.
We did not record the time spent to check traps in each apartment due to logistic difficulties associated with subtracting out the time for counting trap results and recording data. Mass trapping to suppress low-level cockroach infestations provides several advantages: No. 1, it removes cockroaches from the apartments; No. 2, it does not leave insecticide residue in the apartments; No. 3, it does not need as much skill as required for applying cockroach gel bait; No. 4, traps placed by in-house staff can reduce costs by helping to identify apartments with cockroach activity and limiting insecticide treatment to only those apartments in need of treatment; and No. 5, it aids in determining if cockroaches are still present.
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