Pesticides have long been the primary way the industry treats for bed bugs.
While non-chemical measures are important, insecticides are often the most efficient and affordable way to combat infestations. When using these tools, it helps to understand their full effect on pest populations. The most studied and noticeable effect from a pesticide application is death — but insecticide tests in the laboratory rarely consider other non-lethal effects from pesticides which might still be influencing pest biology or behavior. Such effects could include impacts on feeding, mating, egg laying, maturation or mobility. If one or more of these non-lethal, or “sublethal” effects occurred, it might improve overall efficacy and help to curb infestations. In the case of bed bugs, sublethal exposure to insecticides probably occurs often — especially considering that it’s not always possible to find and treat all existing harborages. Incomplete spray coverage, inherent resistance and other factors may further result in some individuals receiving less than lethal exposure.
In two recent studies (Crawley 2017ab), we were interested in whether bed bugs exposed to the popular combination product Temprid® SC (containing ß-cyfluthrin and imidacloprid) experienced other detrimental effects on biology and/or behavior, even if mortality was not achieved. The question arose when it became apparent that the lack of death following insecticide exposure in the lab was not consistent with the high levels of bed bug control being reported in the field when using the same or similar products (Gordon et al. 2014, Potter et al. 2015). Here’s what we found.
SUBLETHALLY EXPOSING BUGS. Before conducting experiments, we needed to produce groups of bed bugs that had received sublethal exposure to Temprid SC. To accomplish this, we confined different populations (strains) of bed bugs on freshly dried deposits for varying lengths of time and calculated the duration of exposure necessary to kill only 10 percent of each population (the “lethal time” or “LT10”). The remaining 90 percent of bugs that survived were considered to have experienced sublethal exposure. Three bed bug strains with differing levels of resistance to pyrethroids were included in each subsequent experiment. These included CIN-1, which is susceptible to pyrethroids; NY-1, which is considered moderately resistant; and LEX-8, which we deemed highly resistant. As expected, the LT10 value for each population differed depending on the inherent level of resistance — 57 minutes to kill 10 percent of CIN-1 bed bugs, 68 minutes to kill 10 percent of NY-1 bed bugs and five hours to kill 10 percent of highly resistant LEX-8 bed bugs. In the following experiments, bugs from each strain were first exposed to either their strain-specific LT10 of Temprid (treated) or water alone (untreated). Impacts on feeding, reproduction and movement were then compared.
1. Impact on feeding. Taking a blood meal is crucial for bed bug development/propagation. Since females require blood to produce eggs, and males require blood to produce sperm, finding and feeding on a host is essential. In this experiment, we tested the ability of bed bugs to feed after sublethal exposure to Temprid. Twenty-four hours following exposure to their strain-specific sublethal exposure time (LT10), or to water alone, five live and apparently healthy female and male bed bugs were randomly selected for feeding (10 sets of feeding trials total). In each trial, treated and untreated bed bugs were placed in a glass cylinder (10 inches tall) provisioned with a reservoir of warmed blood at the top. In order to reach the artificial “host,” the bugs were given a ramp so that they could travel freely between top and bottom of the container. Bed bugs were permitted to feed for 30 minutes, after which the number that fed and how long it took them to find the host were recorded.
Results. Regardless of strain, sublethal exposure to Temprid significantly decreased the percentage of bed bugs that were able to successfully locate the artificial host and take a meal. CIN-1, NY-1 and LEX-8 bed bugs exposed to insecticide were 30 percent, 23 percent and 54 percent less likely to feed, compared to those that were untreated (exposed to water alone) (see Figure 1). Temprid-exposed CIN-1, NY-1 and LEX-8 bugs also took 58 percent, 68 percent and 81 percent longer to find the artificial host than their untreated counterparts. In a separate feeding experiment, where we ensured that female bed bugs found the artificial host and successfully fed, we found that insecticide-exposed bugs also tended to take smaller blood meals. CIN-1, NY-1 and LEX-8 treated bugs imbibed 17 percent, 23 percent and 26 percent less blood than untreated bugs (reductions were statistically significant for NY-1 and LEX-8, but not for CIN-1). Overall, these experiments suggest that bed bugs with sublethal exposure to certain insecticides may feed less effectively and less often, resulting in fewer bites. Reductions in the amount of blood taken in also can influence egg production, the focus of the next experiment.
2. Impact on reproduction. With some insects, exposure to insecticides results in decreased egg output, causing populations to decline over time. To test whether this might be the case for bed bugs, we performed two experiments. In the first, we fed adult males and females and allowed them to mate for three days. After mating, we exposed only the females to their LT10 of Temprid. After 24 hours, survivors were removed and placed individually into small plastic wells lined with filter paper. Once per day for 10 days, the number of eggs laid by each (once fed) female was recorded, along with the percentage of eggs that subsequently hatched. Experiment two was performed in a similar manner, except we a) exposed both males and females to the LT10 (as opposed to only females); b) exposed the bugs to Temprid prior to mating (as opposed to after), and c) counted only the total number of offspring/nymphs produced (as opposed to the number of eggs).
Results. There were significant declines in the number of eggs laid by CIN-1 and NY-1 females exposed to Temprid, but not LEX-8 females (Figure 2). The percentage of eggs that subsequently hatched significantly decreased in treated groups for all three strains evaluated. The percentage of eggs that hatched declined from 98 percent to 76 percent in CIN-1; 100 percent to 86 percent in NY-1; and 99 percent to 91 percent for LEX-8. In the second experiment, the results were similar. There was a significant reduction in the number of offspring hatched — a substantial decrease of anywhere between 34 percent and 73 percent when both the male and female bed bug had sublethal exposure to the label rate of Temprid (Figure 3). Collectively, these results show that sublethal exposure to an insecticide may negatively affect both the quantity and viability of eggs produced by female bed bugs.
3. Impact on movement. Aside from using pyrethrum-based flushing agents, pest managers generally want to avoid dispersing pests when applying insecticides. Sublethal exposure to pyrethroids has been shown to cause increased locomotor activity in several pests, including cockroaches, ants and termites (Haynes 1988). Pyrethroid-induced hyperactivity also has been reported for bed bugs (Romero et al. 2009), and can sometimes result in bugs relocating to other untreated areas. We wanted to test whether sublethal exposure to Temprid, which contains both a pyrethroid and a neonicotinoid, caused bed bugs to increase their daily movements, which might lead to dispersal. Alternatively, we thought that it also was possible that bed bugs may make fewer movements after sublethal exposure. This could be beneficial, in that a decrease in the overall number of movements made by bed bugs could affect their ability to find their harborages, or the host. For this experiment, treated and untreated male and female bed bugs were housed in individual plastic wells on untreated filter paper. We used a camera programmed to take one picture every 10 minutes to track their movements during the day and night for a 24-hour period. The percent of time spent moving by bed bugs during the day as well as the night was determined for both Temprid-treated and untreated bugs. Eleven replicates were conducted (11 insects per treatment per strain).
Results. The percentage of time spent moving was significantly reduced for all three bed bug strains following sublethal exposure to Temprid (Figure 4). Both treated and untreated bugs retained the tendency to move more at night than during the day, but nocturnal movement of pesticide-treated bugs was additionally reduced. The findings suggest that sublethal exposure to Temprid should not increase dispersal of bed bugs within treated dwellings, or to surrounding units. However, there is also the possibility that such reduction in movement could cause small “reservoirs” of treated bed bugs to “hunker down” in some untreated location as they recover. Future studies should address whether or not this occurs. Overall, decreased movement, especially at night, might result in fewer bites sustained by residents.
STUDY IMPLICATIONS. Based on these studies, bed bugs are impacted in subtle but significant ways from sublethal exposure to Temprid (Crawley 2017ab). The adverse effects observed on host finding, feeding, egg laying/hatching and movement should be helpful in constraining populations. Taken together, these sublethal effects may result in fewer bites sustained by customers, as well as a decline in the number of bugs over time — even if they aren’t dying outright. This could explain why people whose homes are being treated for bed bugs often cease to complain, despite continued presence of live bugs (White 2012).
Non-lethal exposure to insecticides probably occurs frequently for bed bugs. For instance, detrimental effects on bed bugs were reported from sublethal exposure to permethrin-impregnated ActiveGuard bed liners (Jones et al. 2013). Although our current study evaluated sublethal effects of Temprid SC, it is clear that other insecticides may have similar characteristics and also should be investigated. This is especially important considering field populations often have high levels of insecticide resistance to a number of products — increasing the odds of sublethal exposure to applications. In addition, hidden aggregations and piecemeal spray coverage might reduce overall exposure to deposits, furthering the odds of sublethal effects in the field that should be explored.
Current findings reinforce that insecticide effects on bed bugs (like other pests) can be subtle, extending beyond mortality alone. Some of the recent discrepancies between laboratory and field results could be due to such factors. Preliminary observations with Temprid suggest sublethal effects on bed bugs may persist several weeks post exposure, although more study is needed to confirm this. If recovery is slow, such effects may indeed aid in restraining populations. While impaired bed bugs aren’t quite as desirable as dead bed bugs, the net effect could be almost the same…
Dr. Sydney Crawley is a post-doctoral research associate at the University of Kentucky. Dr. Kenneth Haynes and Dr. Michael Potter are professors at the same institution. Photos © the authors.