The bed bug Cimex lectularius is an obligate ectoparasite that engages in recurrent blood-feeding events throughout its lifetime, punctuated by sheltering some distance from the host. This complex lifestyle requires coordination of diverse on-host and off-host behavioral repertoires, including host-seeking, blood-feeding, mating, oviposition, and aggregation to sustain development, reproduction, and survival11,13. Adult female bed bugs are expected to monitor their changing physiological state and nutritional and reproductive needs, as well as environmental cues such as host availability, and express specific behaviors in an adaptive coordinated manner that supports reproductive success while minimizing fitness costs. While developing and validating a vertical Y-olfactometer for bed bugs, we observed a mating-dependent behavioral shift where 47% of mated females responded to human skin odor, but none of 20 unmated females responded to the same olfactory stimuli23. In the present report, we followed-up on this observation because, to our knowledge, no experimental studies have investigated whether bed bugs modulate host-seeking and blood-feeding behaviors with changes in their mating and nutritional states.
Consistent with our previous results 23, we found that 64–69% of mated females responded to human skin odor 8 days after their last blood meal (Fig. 3). In contrast, only 24% of unmated females responded, revealing again that host-seeking behavior is profoundly modulated by the mating state of the bed bug. We observed higher host-seeking response rates in this study than in the previous report23, possibly because we used slightly different durations of starvation and olfactometer conditions.
We speculated that the behavioral differences between unmated and mated females were related to the rate of processing of the blood meal, which would be affected in turn by the rate of oocyte maturation and oviposition. Specifically, we hypothesized that because unmated females resorb their eggs24, they have lower nutritional requirements and reduced metabolic rate25, and therefore engage in less host-seeking and blood-feeding. To test this hypothesis, we extended the starvation period for both mated and unmated females. We found that the length of starvation had different effects on the host-seeking and feeding responses of mated and unmated females. Whereas host-seeking gradually increased in unmated females (24 to 60%) through 40 days of starvation, mated females became less responsive to host cues with longer starvation. These observations were consistent with the blood-feeding assays—with longer starvation, more unmated females fed and they took larger blood meals, even though they did not fully process some of their previous blood meal, as evidenced by their unfed body mass (Fig. 1b).
These results are consistent with the hypothesis that the differences between mated and unmated females are driven by the accelerated egg maturation and oviposition cycle of mated females. Mated females initiated egg laying 4 days after ingesting a blood meal, oviposited on average 15.3 eggs per female, and ceased oviposition 6 days later, 10 days after ingesting a blood meal (Fig. 4). Thus, 8 days after a blood meal, mated females digested most of the blood (Fig. 1b), oviposited most of their eggs, and became highly motivated to host-seek to support a second oviposition cycle. Indeed, both laboratory and field observations showed that, given the opportunity, mated females accept a second blood meal while the first blood meal is still being digested and females feed every 2.5 days on average11,13,26. This is in contrast with other hematophagous insects, such as mosquitoes, where female host-seeking and feeding are suppressed for several days after a blood meal, until she completes laying a batch of eggs18,27.
The strategy of unmated females was to feed little when the host is available, but with longer starvation periods, they became more stimulated to host-seek and ingest increasingly larger blood meals. This strategy is likely driven by much-reduced nutritional demands related to resorption of oocytes, which allow unmated females to digest the blood meal more slowly and use it for somatic maintenance rather than egg maturation. As stated by Davis in24: “If the female has fed but has not been inseminated, egg maturation will proceed to an early stage and then the yolk material will be resorbed; thus virgin females avoid the waste of producing unfertilized eggs.” Metabolic differences between mated and unmated females also support the idea that unmated females avoid wasting resources—unmated females have reduced metabolic rates after feeding compared with mated females25,28. Overall, this strategy would result in fewer host-seeking forays and less blood ingested by unmated females (Fig. 1b,c), until they mate and are stimulated to mature and oviposit fertile eggs. A similar strategy appears to operate in the closely related kissing bug R. prolixus, where virgin females remained unresponsive or even repelled by host-associated cues (CO2 and heat) until 20 days after ingesting a blood meal19. Rhodnius is a much larger hemipteran than C. lectularius, it takes larger blood meals, and likely requires more time to digest the blood.
Surprisingly, host-seeking declined in mated females that were starved for 30 or 40 days, and this was especially apparent in females housed with fertile males (mated-long treatment) (Fig. 3). Two factors might account for this observation. The first is female aging and senescence, as 40 days of starvation in these females was beyond the 35-day median survival of females in this treatment group, and 100% of these females died by day 49 (Fig. 5). Thus, the females that we bioassayed 40 days after they ingested a blood meal were likely weak and less responsive to olfactory stimuli. This reduction in the host-seeking and blood-feeding responses of older mated females might be associated with their higher metabolic rate, senescence, or aging that could negatively affect olfactory responses, as shown in the D. melanogaster29.
The second factor that likely underlies their early senescence is the unusual extra-genitalic, hemocoelic (traumatic) insemination in C. lectularius. Females housed with fertile males would receive multiple copulations that represent constant harassment, stress, and physical damage. These interactions with sexually aggressive males reduced their median adult lifespan by 63% (mated-long treatment) relative to females that were housed with fertile males for only 24 h and then with another female (mated-short treatment) (Fig. 5). These findings are consistent with previous studies in bed bugs on the adverse effects of multiple copulations on female lifespan13,14,17,30,31. We also observed that mated-long females assumed a “refusal” posture, protecting the ectospermalege from the males (Fig. 6a, Supplementary Video S1). It is possible, however, that males might circumvent these defensive postures by puncturing the intersegmental membranes away from the specialized ectospermalege, as shown in the closely related Cimex hemipterus32, and thus cause substantial damage to the female. This injurious effect of males on females might shape female behavioral responses in the field, but these responses were obviously constrained under our experimental conditions. For example, mated females might leave aggregations to avoid males, as demonstrated experimentally in laboratory assays33. Mated females might also seek refugia that are too narrow to accommodate themselves as well as courting males. Whether these evasive behaviors occur under field conditions will require further observations.
We found no significant difference in female fecundity in the first oviposition cycle (~ 10 days post blood meal) between two treatment groups—females with long- and short-term presence of fertile males that represented high and low mating rates, respectively (Fig. 4). It is important to emphasize, however, that this experiment was limited to a single feeding and only one oviposition cycle. The lifespan of mated females was dramatically reduced by both the high and low mating rates, but significantly more so by high mating rate (Fig. 5), consistent with previous observations30. Morrow and Arnqvist30 also found that while elevated mating rate shortened female lifespan, it did not affect lifetime egg production. Although we used a different experimental design and did not measure lifetime fecundity, these findings suggest that mated females preferentially direct resources to egg maturation at the risk of significantly reduced lifespan, a strategy that requires close monitoring of physiological state and environmental conditions34.
Remarkably, we also detected a significant effect on females of non-copulatory harassment by males. Females housed with a male that could not copulate (intromittent organ ablated) for only 24 h and then with another female (unmated-short treatment) lived to a median age of 111 days (100% dead by day 142), whereas females housed continuously with an infertile male (unmated-long treatment) lived to a median age of only 73 days (100% died by day 104). This 34% decline in expected lifespan, independent of copulation and egg production, can be attributed to male-specific harassment (Fig. 5). Males engage in a stereotyped behavior where the male repeatedly mounts the female’s dorsum, bends his abdomen to her ventral surface, and probes the female’s sternites with the paramere. We observed that 50% of the unmated-long and 62.5% of mated-long females exhibited a ‘refusal’ posture at least once in their lifetime thereby making the ectospermalege inaccessible to males, while none of the unmated-short or mated-short females displayed this behavior (Fig. 6b). This behavior may be similar to a behavior noted by N. Davis [in11, p. 171], but not described, that “starved females exhibit a distinct avoidance of mating”. The expression of this refusal behavior in virgin females that obviously need to mate is particularly surprising, suggesting that male harassment may be especially detrimental to the metabolic budget of starved females that feed less and endeavor to conserve energy. Unfortunately, our experimental design did not enable us to determine whether co-habitation with a female also would decrease survivorship of starved females relative to solitary females. It is possible that general disturbance of the starved female causes her to move and expend energy, which in turn reduces her lifespan. In this context, it is worth noting that by adding a female to the mated-short treatment, after the male was removed, the presence of the extra female might have decreased survivorship and thus resulted in underestimating the difference between the mated-long and mated-short treatments.
Source: Ecology - nature.com
