Life History and Habits of Fleas

Of the 2,237 flea species described as of 1979, less than a dozen have been widely reported as urban pests and only a few pose a serious medical or veterinary problem. About 94 percent of the described species feed on mammals, the remaining percent on birds. The vast majority of the species (approximately 74 percent) are associated with rodents (Marshall, 1981), highlighting the importance of a protected microhabitat such as a burrow or nest for development.

PHYSICAL CHARACTERISTICS. The body of the flea is well adapted to its ectoparasitic lifestyle. Dark colored adults are flattened from side to side with many bristles that point backwards, facilitating forward movement through fur, hair or fathers, and attachment to the host. The presence of genal combs also serves to help anchor the flea to the host, preventing its dislodgement (Traub, 1980, 1985).

Fleas are wingless creatures, with strongly developed legs, and hind legs that are especially adapted for jumping. They have sucking mouthparts designed to feed on the blood of mammals and birds. Their life cycle undergoes what is known as a "complete metamorphosis"; that is, they have an egg, larval, pupal and adult stage. The pupal stage is characterized by a cocoon constructed of silk and various bits of debris. The adult flea may remain in the cocoon for months before it emerges.

LIFE HISTORY AND HABITS. The following information deals primarily with the cat flea, unless otherwise stated, because it is the species most frequently encountered by PCOs.

Egg. The eggs are sufficiently large to be seen with the naked eye. They are about 1/50-of-an-inch long, smooth, translucent, glistening and oval. The eggs are not attached to the body of the host and they fall from the body or are shaken and scratched off. Byron (1987) reports that in a survey of a flea-infested home, the distribution of flea eggs is directly associated with the habits of the pet or host. Places where the pet frequents or rests have the greatest number. There are fewer eggs in areas adjacent to windows or commonly traveled areas.

Most eggs hatch in 1.5 to six days at 90 degrees Fahrenheit and 55 degrees Fahrenheit, respectively (Silverman et al., 1981). Exposure to less than 50 percent relative humidity (RH) results in about 20 to 60 percent reduction in egg hatch. Eggs are killed when exposed to temperatures of 46 degrees Fahrenheit for 10 days or 37 degrees Fahrenheit for five days (Silverman and Rust, 1983).

Larva. The emerging larva cracks the egg shell by means of a tooth on its head, which disappears with the first molt (Kunckel, 1873). The eyeless and legless larva is maggot-like, whitish, with a single row of bristles around each segment. The larva has a distinct head with a pair of short cylindrical antennae, three thoracic segments and 10 abdominal segments. The active larva has no legs and moves by means of the bristles which encircle each segment. The last terminal segment has a pair of anal struts which are used for vigorous movements (Bacot and Ridgewood, 1914).

There are three larval instars lasting from six to 36 days after hatching. The larvae are very susceptible to desiccation and high relative humidities are important for development. Bruce (1948) states that relative humidities below 45 percent and greater than 95 percent resulted in complete mortality. At 50 percent RH larval development required 10 days, whereas at 90 percent RH larvae develop in five days (Silverman et al., 1981).

Exposure of cat flea larvae in unprotected microhabitats such as the lawn or under trees where the RH may be less than 45 percent kills flea larvae (Silverman and Rust, 1983). Outdoor areas where flea larvae may develop are most likely to be in shaded and wind-protected areas with moist soils that are frequently visited by the host.

Flea larvae fail to develop at temperatures below 55 degrees Fahrenheit and at or above 95 degrees Fahrenheit (Silverman et al., 1981) At 80 to 90 degrees Fahrenheit, larvae mature in four to eight days whereas at 60 degrees Fahrenheit development requires about 26 to 36 days.

The small whitish larvae primarily feed on dried fecal blood produced by adult fleas feeding on the host (Bacot and Ridgewood, 1914). Stewart (1939) has this to say concerning the feeding habits of the larvae: "As a whole, hosts with nests are more commonly preferred to those without them. This is particularly true in the case of those species such as the European rat flea, N. fasciatus, whose larvae appear to require a meal of dried blood derived from the excreta of the adult flea for their development. Also in such cases, we observe that the adults spend a great deal more time in the nests than on the hosts, which is a provision of nature to supply the larvae with the necessary food."

Surveys of a flea-infested home for larval cast skins reveal that cat flea larvae do not move much, being found in areas where the pet rests and in protected microhabitats (Byron, 1987). The larvae readily sham death upon being disturbed.

Pupa. Just prior to the termination of the larval stage, the larva commences to weave a silken cocoon, which is spun from its own saliva. This cocoon incorporates small pieces of debris and organic sediment, camouflaging the cocoon in its natural surroundings.

The pupal stage lasts about seven to 10 days (Joseph, 1981) and is the most resistant immature stage to desiccation (Silverman et al., 1981). Similarly, Mellanby (1933) reports that the pupa of X. cheopis is resistant to desiccation whereas the prepupa fails to develop at 50 percent RH.

Adult. The adult flea may remain quiescent inside the cocoon for up to 20 weeks depending upon the temperature (Silverman and Rust, 1985). All fleas emerge within four weeks at 90 degrees Fahrenheit and within 20 weeks at 52 degrees Fahrenheit. At 95 degrees Fahrenheit only 10 percent of the emerged unfed adults will survive 10 days even at 100 percent RH, whereas at 60 degrees Fahrenheit, they will survive about 40 days (Silverman et al., 1981).

Waterson (1916) notes: "Persons entering a long-deserted house sometimes have cause to complain of hordes of fleas appearing, ‘suddenly’ after a short time. It is probable that in such cases, fleas resting in the cocoon, beneath floors, in cracks, etc., have come out in response to the vibrations caused by people moving in their proximity." Studies by Silverman and Rust (1985) show that direct pressure on the cocoon or combinations of increased temperature and direct pressure stimulate emergence. Exposure to carbon dioxide does not stimulate emergence.

Visual and heat cues are the primary stimuli that attract adult cat fleas to the host (Osbrink and Rust, 1985a). Moving dark objects on light backgrounds, or combinations of warmth and dark colors are attractive to fleas, adults being unable to orient in the dark.

The males usually emerge first and are less numerous than the females. Even though mated, the female flea will not lay eggs unless she has obtained a blood meal. The adults of many species of rodent or bird fleas visit their hosts for only a short time in order to obtain their blood meal. Cat fleas probably remain on a suitable host for their adult life span. Most species of fleas have definite host preferences, but do not necessarily feed exclusively on these hosts. This point is of prime importance from a health viewpoint, since the tendency of the flea to feed on several different species of animals enables it to carry diseases. In studies in which grooming was unrestricted, only 27 percent of the fleas were alive after 22 days (Hudson and Prince, 1958) and 47 percent of the fleas were alive after 15 days (Wade and Geogi, 1988). According to Bishopp (1921), in hot weather and with no animal to feed upon, adult fleas may live but from two to five days, whereas when they feed upon blood, they may live from a month to almost a year. "During the summer, probably the average longevity of the man flea without food is about two months, of the dog flea somewhat less, and of the sticktight flea still less." Cat fleas may be found on the host year round. In winter months the average number of fleas and percentages of cats infested are considerably less than in the summer (Lyon,1915 and Osbrink and Rust, 985b)

Dr. Michael Rust is professor and vice-chairman of he Department of entomology t the University of California at Riverside.