Behavioural and physiological state dependency of host seeking in the blood-sucking insect Rhodnius prolixus.

Vertebrate blood is essential for the growth and the reproduction of haematophagous insects. Provided that hosts play the double role of food sources and predators, feeding on their blood exposes these insects to a high predation risk. Therefore, it is expected that host seeking occurs only when insects need to feed. In the present study, we analyse how the feeding status affects the response to host-associated cues in the blood-sucking insect Rhodnius prolixus. We show that the responsiveness to host-associated cues, such as CO(2) and heat, and that the motivation to feed depend on the time elapsed since a blood meal. Depending on the time elapsed after feeding, the same concentration of CO(2) may attract or repel the insects. As far as we know, this is the first time that a host signal has been shown to be repellent for a haematophagous insect. The response to heat is also modulated but no repellence was observed. When blood was replaced by saline solution as food, a significant reduction of the response to both signals was evinced but repellence was not observed. The injection of haemolymph from fed insects into starved insects inhibited the response of the receiver insects to both signals but this was not observed after the injection of saline solution or of haemolymph from non-fed insects. This is the first time that the modulation of feeding behaviour by the feeding status has been analysed in a hemimetabolous blood-sucking insect, fully excluding any effect of other processes, such as reproduction.

State-dependency of host-seeking in Rhodnius prolixus: the post-ecdysis time.

The source of blood of most haematophagous insects plays at the same time the double role of host and potential predator. Feeding behaviour should be triggered only when necessary and should be completed as quickly as possible. From an epidemiological point of view, this modulation has an impact on the feeding frequency of disease vectors and, as a consequence, on the transmission of parasites. At present, not many data are available on the influence of the physiological state on the motivation to feed, and mostly limited to a few mosquito species. We analyzed the host-seeking behaviour of Rhodnius prolixus as a function of the time elapsed since the ecdysis, by testing the response of larvae to a blood source, and long- (CO2) and short-range (heat) orientation cues associated to their vertebrate hosts. Our experiments demonstrated that during the first days following the ecdysis insects do not respond to any stimuli. The ability to follow chemical and physical cues increases either gradually (heat) or step-wise (CO2) with post-ecdysis time. A few insects started to feed on day 2, but only at day 7 following the ecdysis 50% of them took a bloodmeal, to reach the highest motivation to feed on day 10. The reasons for the "maturation period" in feeding behaviour of R. prolixus are discussed.

Reproduction in a variable environment: How does Eupelmus vuilleti, a parasitoid wasp, adjust oogenesis to host availability?

Oogenesis of the parasitoid wasp Eupelmus vuilleti is known to be dependent on host availability. However, examination of ovarian dynamics by microscopy showed that oogenesis and vitellogenesis are initiated before female eclosion and proceed 1-2 days after, independent of host presence. Oogenesis continued beyond the 2nd day only in the presence of hosts, otherwise it was replaced by egg resorption. It is thus possible to distinguish between host-independent and host-dependent periods of oogenesis. In the presence of host, each ovariole (three per ovary) contained generally three oocytes: a fully mature oocyte, a nearly mature one and an immature one. However, host deprived-females resorbed their most mature and their smallest oocytes, but kept one almost mature oocyte per ovariole. Comparison of zero, short and long host deprivation periods showed that females always had the ability to quickly lay eggs to exploit any new host. However, increased deprivation led to a reduction in the number and the viability of eggs. Enzymo-immunological measurements of ecdysteroids were made in whole females, in dissected ovaries and in newly laid eggs. Our results indicated that ecdysteroids play a major role as circulating hormones involved in the regulation of oogenesis.

Temporal modulation and adaptive control of the behavioural response to odours in Rhodnius prolixus.

It has been demonstrated in several insect species that a circadian clock makes the whole of antennal chemoreceptors more sensitive during a particular temporal window every day. This assessment raises the question about how insects exhibiting bimodal activity handle their sensitivity to odours which are relevant at different moments of the day. To shed some light on this problem, we studied in Rhodnius prolixus the daily dynamics of their responsiveness to CO(2) (host-associated cue) and aggregation cues (refuge-associated), which are relevant at dusk and dawn, respectively. We analysed: (1) whether a temporal modulation of the responsiveness to odours does exist in R. prolixus, (2) if this modulation is a general one or it is specific for each type of volatile, and (3) if it is controlled by exogenous or endogenous mechanisms. We found that the responsiveness to CO(2) only occurs at dusk and that to assembling odours is restricted to dawn. Experiments under free-running conditions revealed that only the responsiveness to CO(2) is controlled by a circadian clock, but not that to assembling signals. Thus, by combining endogenous and exogenous mechanisms, sensitivities to different odours are adjusted according to their associated behavioural context and moment of the day.

Host-induced ecdysteroids in the stop-and-go oogenesis in a synovigenic parasitoid wasp.

Eupelmus vuilleti (Hymenoptera; Eupelmidae) is a solitary ectoparasitoid producing yolk-rich eggs. The female oviposits mainly on the fourth larval instar of Callosobruchus maculatus (Coleoptera; Bruchidae), which develop within pods and seeds of Vigna unguiculata (Fabacae). Parasitoid females are synovigenic, i.e., they are born with immature eggs and need to feed from the host to sustain egg production during their entire lifetime. However, eggs are rapidly resorbed in unfavourable conditions and an efficient stop-and-go mechanism controls oogenesis in such animals. In this study, the possible involvement of ecdysteroids in the regulation of parasitoid oogenesis is examined. In a first step, the identity and titre of ecdysteroids in reproductively active and inactive female parasitoids were investigated by high performance liquid chromatography followed by enzyme immuno-assay (EIA/HPLC). A larger secretion of ecdysone was found in female during their reproductive period compared with inactive females. In a second step, both the secretion of ecdysteroids into the medium of in vitro incubated ovaries and the ecdysteroid content of females reared with or without host were measured (EIA). The presence of the host, which represents both the oviposition site and the nutritional source, induced an active biosynthesis of ecdysone. This synthesis started at a slow rate after host introduction and reached a maximum after 48 h. When hosts were available, this synthesis was cyclic and continuous during the entire female lifetime. These results showed that host presence triggered ovarian synthesis of ecdysteroids, which are involved in a stop-and-go regulation of egg production linked to host availability.

Importance of sterols acquired through host feeding in synovigenic parasitoid oogenesis.

Eupelmus vuilleti (Hymenoptera; Eupelmidae) is a host feeding ectoparasitoid of fourth-instar larvae or pupae of Callosobruchus maculatus (Coleoptera; Bruchidae) infecting Vigna unguiculata seed and pods (Fabacae). Parasitoid females are synovigenic, i.e. they are born with immature eggs and need to feed from the host in order to sustain egg production. In this study, the role of sterols obtained through host feeding in parasitoid oogenesis are examined. Quantitative and qualitative analyses of the sterol contents in each partner of the tritrophic interaction show that a parasitoid female's larval sterol contents is sufficient to produce only 30% of the total number of eggs laid throughout a female's life cycle. In a second step, by manipulating the composition of the sterols hemolymph in the host, it is shown that cholesterol obtained through adult nutrition plays a crucial role in the eggs viability but does not affect the egg production quantitatively. This result has important implications for understanding both the nutrient allocation strategy in this species and the impact of cholesterol in parasitoid reproduction.