Effects Of Color Contrast On Trap Efficacy

The use of monitors/traps for detection of bed bugs has been shown to be a very important tool in bed bug management to detect infestations and to confirm bed bug elimination (Cooper et al. 2015). Pitfall-style monitors have been proven to be highly effective for detecting bed bugs and can be enhanced by adding a chemical lure or carbon dioxide. In a community-wide survey of bed bug infestations, placing ClimbUp insect interceptors under furniture legs for two weeks detected 89 percent of the bed bug infestations, whereas residents were only aware of 41 percent of the infestations (Wang et al. 2016).

Efficacy of pitfall-style bed bug monitors is affected by the color and texture, making some monitors much more effective than others. Comparison between black and white traps, and red and white traps, revealed that bed bugs were more attracted to the black and red traps. Previous studies show interceptors with black tape were significantly more effective than those with white tape. Monitors with fabric or a similar texture are preferred over wood, metal or plastic (Singh et al. 2015).

In addition to the color of traps, the color contrast between the trap and its background can influence insect movement and subsequent orientation to the object. For example, funnel traps with a white lid on top of the black funnel were significantly more attractive to gravid females of yellow fever mosquito (Aedes aegypti) compared to the same trap with a black lid on the black trap (Prokopy and Owens 1983). The effect of contrasting colors on trap effectiveness with bed bugs is unknown. We examined how trap catch is influenced by the contrast between the color of the trap and trap exterior wall, and between the color of the trap and background color. Information from this study will help in designing more effective bed bug detection tools in the future.

EXPERIMENTAL CONDITIONS. All experiments were conducted in a 9-square- meter walk-in chamber at 30 ± 2°C and a photoperiod of 12:12 hours (light: dark). A fan (13 cm diameter) was placed in the center of the chamber. A red light was placed at center of the room during dark phase. Four white plastic tray arenas (55.5 by 43.5 by 7.5 cm) were lined with black fabric (two arenas) or white fabric (two arenas) (see Figure 1). The interior vertical surface of the arenas was coated with a light layer of fluoropolymer resin (BioQuip products, Rancho Dominguez, Calif.) to prevent bed bugs from escaping. Two red harborages were placed along the sides of each arena. Prior to being used in the assay, the harborages had been used in rearing containers for immature bed bugs, thus each harborage contained numerous bed bug feces but no eggs. These paper harborages were to stimulate movement and subsequent arrestment of bed bugs wandering in the arena (Wang et al. 2017)

ClimbUp Insect Interceptors (15 cm diameter), hereafter referred to as traps, were used to evaluate trap efficacy. Traps with three color schemes were tested: black traps with black tape (BB), black traps with white tape (BW), and white traps with black tape (WB). We did not include white traps with white tape because it was less preferred when compared with white traps with black tape in a prior study (Singh et al. 2015). The black color of the BB and BW traps was achieved by applying Fiebing’s leather dye to either the trap surface and cloth tape around the exterior perimeter (BB) or to the cloth tape around the exterior perimeter (BW).

Two traps of different color patterns were placed in each arena. Among the three-color schemes, there were three possible paired comparisons: BB versus WB, BB versus BW, and BW versus WB. Each pair was tested both in white and black arenas.

EXPERIMENTS. Experiment 1 was a comparison of traps with different color combinations.

BB vs. WB traps. In each arena, BB and WB traps were placed on opposite corners of the arena. In the center of each trap, a lure was placed. The lure consisted of nonanal, L-lactic acid, 1-octen-3-ol and spearmint oil (Singh et al. 2015). A total of 200 µl lure was pipetted onto a piece of cotton in each 1.7 ml centrifuge vial and was sealed with a lid which had a 1.5 mm diameter opening. Approximately 400 bed bugs, consisting of nymphal stages and adults (males and females), along with their harborages, were placed at the center of each arena and confined with a plastic ring (13.3 cm diameter and 3.4 cm height) approximately 16 hours prior to the experiment started. The bed bugs were last fed one to two weeks prior to the experiment. The confinement ring was removed approximately two hours into the dark photophase. Trap catch (bed bugs found in the inner and outer trap wells) was recorded after six to seven hours. Two black arenas and two white arenas were used on each day. They were placed at four corners of the walk-in chamber. This experiment was repeated four times over four days, yielding eight replications for each type of arena. The arenas were rotated in a clockwise fashion each day to eliminate any potential location effect on the mean trap catch.

BW vs. WB traps. In each arena, a BW and WB trap were placed on opposite corners of the arena. The procedures and experimental conditions were the same as those in the last experiment. The bed bugs in inner and outer well of the traps were recorded at the end of the experiment.

BB vs. BW traps. In each arena, BB and BW trap were placed on opposite corners of the arena. The procedures and experimental conditions were the same as those in the other two experiments. Bed bugs in the inner and outer well of the traps were recorded at the end of the experiment.

Experiment 2 was an escape test experiment designed to evaluate if there are any differences among the traps in their ability to retain trapped bed bugs. The results would allow us to determine whether any observed differences in trap catch were indeed due to differences in color scheme rather than the differences in the traps’ ability to retain trapped bed bugs. We hypothesized that a very low percentage of trapped bed bugs could enter the inner well of the traps or escape from the traps. The procedures/conditions were the same as experiment 1. Each trap was placed on an inverted dog bowl (600 ml volume, 18 cm diameter, 6.4 cm depth) (see Figure 2 on). Five sets of traps were prepared, yielding five replicates. The inside surfaces of the dog bowls were coated with a light layer of talcum powder to make the traps slippery and prevent trapped bed bugs from escaping. For each trap, 40 bed bugs (20 small nymphs, 10 large nymphs and 10 adults) were placed into the outer well. If bed bugs escaped from a trap, they would be found in the dog bowl. After six to seven hours, the number of bed bugs in the inner well, the outer well and outside of the trap were recorded.

RESULTS/DISCUSSION. Following are results and discussion of the experiments.

BB vs. WB traps. In this experiment, the median number of bed bugs caught in the two traps in each arena was 41.5 (range: 15-116). In each arena, the proportion of bed bugs caught in BB trap was significantly higher than that in the WB trap (see Figure 3). The mean difference in the proportion of bed bugs caught between BB and WB traps in the black arena and white arena was not significantly different. Therefore, regardless of the color of the background, black traps are more effective than white traps with black tape.

BW vs. WB traps. In this experiment, the median number of bed bugs caught in the two traps in each arena was 124 (range: 42-182). In each arena, the proportion of bed bugs caught in BW was higher than that in the WB in black arenas (see Figure 4). The mean difference in the proportion of bed bugs between BW and WB traps in black arena and white arena was not significantly different.

BB vs. BW traps. In this experiment, the median number of bed bugs caught in the two traps in each arena was 52.5 (range: 15-144). In each arena, the proportion of bed bugs caught in the BB and BW traps was similar in both arenas (see Figure 5 on ). Therefore, for black traps, the color of the tape placed on exterior wall does not affect the trap efficacy.

The escape test. At seven hours after releasing bed bugs, the mean percentage of bed bugs that escaped from BB, BW and WB traps into dog bowls was 9 ± 1, 4 ± 2, and 3 ± 1%, respectively (see Figure 6). There were significant differences in the traps’ ability of retaining trapped bed bugs. Significantly more bed bugs escaped from BB traps than from WB traps. Among the total of 30 bed bugs that escaped, 87 percent of them were adult males. Adults were more likely to escape than nymphs. Bed bugs also went to the inner well of the traps. The mean percentage of bed bugs that moved from the outer well to the inner well was not significantly different. Among the total of 34 bed bugs moving into the inner well, 79 percent of them were adult males. Adults were more likely to move into inner well than nymphs.

Previous studies have shown interceptors to be more effective than trained bed bug sniffing dogs or visual inspection (Wang et al. 2010, Bennett et al. 2016) for detecting bed bugs present in low numbers. Better monitoring tools would allow for more efficient detection of bed bugs which, in turn, enables earlier detection and more effective evaluation of the effectiveness of bed bug management programs. Previous studies also have shown that black traps are more effective than white traps. Based upon the results of our study, traps that are entirely black (BB) traps are superior to traps with an outer white surface (WB) in catching bed bugs, however color contrast between the trap and the background did not affect trap catch.

From the escape test results, more bed bugs escaped from BB traps than WB traps. This difference could be due to two reasons: 1) the black dye altered the surface texture which made it easier for bed bugs to escape or 2) bed bugs on a black substrate are more active than those on white substrates, making the escape rate higher. Since the traps were lubricated with talcum, the first factor is probably negligible. Despite its higher escape rate, BB traps caught more bed bugs than WB traps as shown in Figure 3, regardless of whether the color of the arena floor. Thus, the higher catch in BB traps compared to WB traps was due to bed bugs being attracted to the black color of the exterior wall, rather than texture difference. The finding further proves that black color is preferred by bed bugs compared to white, whereas black and white color contrast is not important.

CONCLUSION. All previous studies used white interceptors for bed bug detection and treatment evaluation. This study demonstrates that: 1) black traps are more effective than white traps and 2) color contrast does not influence the effectiveness of traps. Using the more effective black interceptors as shown in this study, greater detection or control efficacy can be achieved. Additional field studies on the effectiveness of various trap designs will be helpful to provide further evidence on the cost-effectiveness of different monitors for monitoring and controlling bed bugs.

The authors are with the Department of Entomology, Rutgers University, New Brunswick, N.J.

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