Preemerges AdultsDepending on the temperature inside the cocoon the flea develops via the stages prepupa and pupa within seven to 19 days into an adult which at first rests inside the cocoon (Karandikar and Munshi 1950; Silverman et al. 1981b). The observation of adult fleas remaining quiescent for prolonged periods within the pupal cocoon before emergence has been made by several researchers, including Bacot (1914), Karandikar and Munshi (1950) and Silverman et al. (1981b) and characterizes the so-called preemerged adult. Lower respiratory demand and longer survival under dry conditionsThe preemerged adult has a lower respiratory demand than the emerged adult and its survival is considerably longer under low humidity conditions (Silverman and Rust 1985). By that stage, prolonged adult survival within the cocoon, particularly under desiccating conditions, is possible and mainly due to quiescent periods of low metabolic activity rather than restriction of water loss through the cocoon wall (Silverman and Rust 1985). It can be suggested that the preemerged stage is ideal for prolonged survival during the absence of hosts or during unfavorable environmental conditions such as in winter or midsummer (Metzger and Rust 1997). About 60% of adult fleas successfully emerge from cocoons held at 13°C by day 140 after eggs are collected (Silverman et al. 1981b). At 15.5°C, some adults emerge as late as 155 days (Metzger 1995). When held at 11°C and 75% RH adult C. felis may remain quiescent in the cocoon for up to 140 days (Silverman and Rust 1985). Exposure at 3°C for ten days and -1°C for five days was lethal to preemerged adults. Exposure at 8°C for 40 days resulted in 72% mortality. All preemerged adults held at 2% RH and 16°C could survive for >35 days, whereas 90% of the emerged adults would die (Silverman and Rust 1985). At 16°C in saturated air, 92% of the preemerged adults survived for 70 days compared with 62% of emerged adults (Silverman and Rust 1985). According to Rust and Dryden (1997) the decreased metabolic activity may in part explain the increased longevity of preemerged adults. The option that the pupal stage can be as short as ten days, but the preemerged adults might remain in the cocoons for up to six months is meant by the term ‘pupal window’, which may cause problems in control measures and has to be understood by pet owners (Dryden 1996). The ability to survive for extended periods in the cocoon is especially important for species such as C. felis, which infest mobile hosts that may not frequently return to a nest or burrow (Dryden and Rust 1994). Fleas may continue to emerge from cocoons for up to four weeks after insecticide and insect growth regulator application to the environment (Dryden and Prestwood 1993). These resurgences are attributed to the presence of pupae and preemerged adults in cocoons at the time of treatment. Rust and Reierson (1989) noticed that preemerged adults in cocoons placed at the base of the carpet were not killed by sprays. Dryden and Rust (1994) believe the survival of pupae not to be due to any protection afforded by cocoons, but the lack of penetration of the carpet canopy by the insecticide, nevertheless causing the same problem of the pupal window in control measures. Two main stimuli for emergency rate: pressure and heatEven though there is a direct relationship between temperature and rate of adult emergence of C. felis, at a given temperature there has been a proportion of the flea population observed by Silverman and Rust (1985) that remained in the cocoon for extended periods. Different emergence periods are thought to be caused by different environmental conditions and emergence stimuli, but under similar environmental conditions Silverman and Rust (1985) believe them to result at least partly from larval competition for food, with adults developed from light prepupae remaining for a shorter time in their cocoons. Water and food reserves dropping below a critical level, triggering emergence mechanisms, or a weaker cocoon produced by an incompletely nourished prepupa not impeding adult emergence could be possible reasons. But the primary factor responsible for initiating adult emergence and reducing the randomness of host location will be host-produced stimuli (Silverman and Rust 1985). Pressure and heat are the two main stimuli inducing rapid emergence from the cocoon, in detail pressure of 13-254 g/cm2 and temperature between 32-38°C (Silverman and Rust 1985). Since the combination of warmth and pressure provide higher emergence rates than either warmth or pressure alone, it is likely that an endothermic animal resting on a cocoon increases the chance that the adult flea would emerge and successfully attack the host (Silverman and Rust 1985). A man with a body weight of >75 kg walking over a carpet containing cocoons induces the emergence of 31% of the cocoons’ population after the first walk, 97% after the fourth and all the imagines emerge after the fifth time (Silverman and Rust 1985). In the absence of stimuli adults emerging gradually over several weeks, depending on ambient temperature, with the length of time spent in the cocoon related to prepupal weight (Silverman and Rust 1985). Frequent hosts of cat fleas such as domestic and feral cats and dogs, mustelids, and opossums do not necessarily return to flea-infested lairs, so that successful attack of a mobile host necessitates immediate emergence and host-seeking behavior (Silverman and Rust 1985), making the emergence stimulated by warmth and pressure understandable. To summarize, the preemerged adult represents a developmental stage of the cat flea offering the possibility to survive non-parasitic periods without being harmed (Silverman and Rust 1985). Summary see Box 4.
References
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