Most flea species parasitise nest-dwelling animals, and the great majority of flea larvae live in the nest or den of their hosts (Marshall, 1981). Among the nest-inhabiting flea larvae, there is a gradation of dependence on the host and on adult fleas for nutrition (Moser et al. 1991). In cat fleas, both larvae and adults are dependent on the blood of the host, and the larvae can be determined as obligate parasites (Dryden, 1989b).
Newly hatched flea larvae are slender, white, apod (i.e. without feet), sparsely covered with short hair, two to five millimeters in length, and possess a pair of anal struts (Dryden, 1993). Their body consists furthermore of a yellow-to-brownish head (Dryden, 1989a) three thoracal (breast) segments and ten abdominal (belly) segments (Kalvelage and Münster 1991). The larvae have chewing mouth parts (Urquhart et al., 1987) and are free-living (Dryden, 1993) (Fig. 1, Fig. 2). As the larvae have no feet, they move by using their skin muscle tube on dry surface, managing to move quite rapidly. They are only able to stop by using the mouth parts and less effectively the soft push of the last segment.
The larva of the cat and dog flea furthermore passes through two molts, thus having three larval instars, and the third larval instar pupates. All instars have a two-hooked caudal anal process and 13 body segments (Harwood and James 1979). The first larval instar is approximately 2 mm in length, and the third instar can be 4 to 5 mm long (Elbel, 1951; Bacot and Ridewood, 1915).
The larval development occurs in protected microhabitats that combine moderate temperatures, high relative humidity and a source of nutrition in form of adult flea fecal blood (Dryden and Rust, 1994). Cat flea larvae have a minimal nutritional requirement of dried blood to develop (Rust and Dryden, 1997). They feed on adult flea faeces, which under natural circumstances are sufficient for living and development (Strenger, 1973). The so-called debris usually thought to be an important part of larvae nutrition, have no importance whatsoever for nourishment of the larvae (Strenger, 1973).
Künkel first reported in 1873 of this larval form of faecal nutrition, but believed it to be supplemented with other organic material digested by cat flea larvae (and dormouse flea larvae). Feeding trials by Strenger (1973) have proven that organic material is not ingested by the cat flea larvae. Apart from adult flea faecal blood, no organic material with the exception of flea eggs as well as injured flea larvae was proven to be ingested by cat flea larvae, this as an example of a form of cannibalism.
Once the larvae start feeding on adult faeces, the color of their gut which can be seen from outside turns into ruby-red (Strenger, 1973), giving the larvae on the whole a brownish color.
The larvae are negatively phototactic (i.e. they move away from light), positively geotactic (i.e. they follow gravitation) (Byron, 1987) and thigmotactic (i.e. they recognize tactile stimulus and react to mechanical contact) (Strenger, 1973). This allows the larvae to find safe hiding places and protection against desiccation (Strenger, 1973). Last but not least, larvae orient to sources of moisture, suggesting some type of hygrotactic response (Byron, 1987). All these abilities allow the larvae to avoid direct sunlight in their microhabitat and make them actively move deep into carpet fibers or under organic debris (grass, branches, leaves, or soil) (Dryden, 1993). In particular the debris which was formerly thought to be of great importance for larval nourishment plays an important role as substratum to satisfy the tigmotactic abilities of the flea larvae, making them stay in a safe protected place (Strenger, 1973).
About 83% of fleas develop at the base of carpets in the home (Byron, 1987). Larvae are capable of moving at least 46 cm in carpet (Osbrink, personal communication, cited in Rust and Dryden, 1997). Kern (1991) similarly reported that larvae would crawl several inches to avoid light and were observed to burrow an average of 2.36 mm into the sand, reaching a maximum depth of 7.5 mm. But despite all this ability to move, Byron’s (1987) observation is still to be supported that first instars do not move far from the point of eclosion. The dispersion of the immature stages is mainly governed by the habits of the host (Dryden and Rust, 1994).
Larvae feed on adult flea faeces. Adults guarantee sufficient nutrition of the larvae by imbibing much more blood from their host than they can use (Moser et al., 1991). Not only the amount of blood consumed, but also the obligate blood meal before every egg deposition means that nutrition for the offspring is guaranteed (Strenger, 1973). Faasch (1935) reported that the imaginal flea gut can only contain 5mm3, but that several times as much is consumed and excreted as blood faeces.
Faecal nutrients would be more efficiently utilised and nutrient investment selected for, if a female invested nutrients in her own progeny (i.e. more eggs or more nutrient filled eggs) rather than investing equally in the progeny of conspecifics. However if most larvae are related to the adults that are feeding them, there is a strong selective advantage to nutrient excretion, for it is likely, at least in fleas that inhabit the nests of their hosts, that all of the individuals are related closely (Silverman and Appel, 1994).
Silverman and Appel (1994) propose an alternative hypothesis for blood excretion by C. felis: It could be a form of harvesting key nutrients generally found at low levels in blood by the adults, such as B vitamins, but then nevertheless concentrating a limited resource may be accomplished for a selfish reason, the offspring would still profit from it and the outcome may be vital for the offspring.
The incomplete utilisation of host blood by adult fleas, the excretion of protein and iron-rich faeces, and the benefit derived from larvae consuming adult faeces strongly indicates a unique form of parental investment in C. felis (Silverman and Appel, 1994), with haemoglobin providing iron for normal growth and proper sclerotisation as adults, and with serum including all the essential proteins (Moser et al., 1991). Fleas produce about 0.77 mg of faeces per day without any particular temporal pattern (Kern et al., 1992). A spiral of faeces is formed from the anus ten minutes after feeding (Akin, 1984).
The larvae of the cat flea are capable of using blood from different hosts (Linardi et al., 1997; Linardi and Nagem, 1972; Moser et al., 1991). Silverman and Appel (1994) observed unfed larvae dying about three days after eclosion.
Duration of the larval stage
The larval stage of the cat flea usually lasts five to eleven days, depending on the climate and the availability of food (Lyons, 1915; Silverman et al., 1981). In the development and survival of newly hatched larvae relative humidity is a major influencing factor (reported here for C. canis) (Baker and Elharam, 1992). At 24.4°C and 78% RH, pupation of a larval cohort began on day 7 and was complete by day 11 (Dryden, 1988). As the temperature decreases, the length of time for larval development increases (Dryden, 1993).
Bruce (1948) found that cat flea larval survival was >90% at temperatures of 21-32°C, but survival dropped to 34% at 38°C. Furthermore he reported of no larval survival at optimal temperatures, but RH of <45% or >95%. Relative humidities of 65-85% resulted in >90% larval survival. And larvae raised at 50% RH had development times twice as long as larvae raised at 65-85% RH.
Cat flea larvae as well as pupae did not survive temperatures of >35°C for >40 hours/month, when the relative humidity was held constant at 75% (Silverman and Rust, 1983). They also reported that only 7% of larvae survived a 1-day exposure at -1°C (75% RH) and that 43% of third instars exposed to 12% RH survived for 24 hours at 16°C and 97% survived at 10°C. Larvae are more sensitive than eggs to low humidity according to Silverman et al. (1981). In their experiments all larvae died at any temperature if humidity was 33% RH or lower. All of the larvae could tolerate temperatures up to 27°C if the RH was at least 50%. High temperatures (35°C) had similar effects on the larvae as on the eggs with larvae desiccating at low RH and apparently overheating in saturated air (Silverman et al., 1981). An exposition of five respectively 20 days at 3°C respectively 8°C is lethal for cat flea larvae (Silverman and Rust, 1983). Pospischil (1995) reported of a larval development of 45 days at 15°C and first fleas hatching at that temperature after 70 days in contrast to a whole generation cycle of about 18 days at the temperature optimum between 25 and 30°C. Below 15°C no larval development is reported to be possible (Pospischil, 1995).
Concerning RH the same author reported of only 30% of the larvae finishing their development at 70% RH in contrast to >60% reproduction rate at a RH of 80-90%. The minimal RH which allows larval development is stated to be 60% (Pospischil, 1995).
Temperature as mortality factor
Outdoor survival is strongly influenced by temperature and humidity. Very high larval mortality was reported in sun-exposed areas (100%) and inside structures that trapped heat, such as the doghouse (100%) (Kern et al., 1999). Low larval survival has been reported at high temperatures, 37°C (Bruce, 1948) and 35°C (Silverman et al., 1981; Silverman and Rust, 1983). A larval mortality of 100% has also been reported by 100% RH (Silverman et al., 1981) respectively 95-100% RH (Bruce, 1948). Not only the nutritional requirements of larvae greatly limit the sites which are suitable for development, but also the necessity for humidity, with suitable outdoor sites even rarer (Dryden and Rust, 1994). Flea larvae are not likely to survive outdoors in shade-free areas. Outdoor development probably occurs only in areas with shaded, moist ground, where flea-infested pets spend a significant amount of time to allow adult flea faeces to be deposited into the larval environment. Likewise, in the indoor environment, flea larvae probably only survive in the protected environment under a carpet canopy or in cracks between hardwood floors in humid climates (Dryden, 1993).
These developmental conditions verify that areas in close proximity to where pets sleep or rest need to be treated, because of the limited movement of the larvae (Dryden and Rust, 1994). But there are still two reasons why flea larvae escape most adulticide treatments indoors:
- The treatment fails to reach them at the base of carpet fibers where they develop
- about 2.5-times more insecticide per gram body weight is required to kill larvae than adults (Dryden and Rust, 1994)
BOX 2. Cat flea larvae
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