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8

A Practical Guide to Feline Deimatology

conditions are optimal, flea populations can grow rapidly within a fortnight.

Larvae are very sensitive to desiccation. Exposure to RH of less than 33% is fatal. Larvae are capable of resisting a temperaturę of 27°C if the RH is greater than 50% '⁹. At 50,55 and 65% RH, the percentage of surviving larvae capable of developing, increases (24, 77 and 91% respectively)²⁰. As humidity is a critical factor for larval survival, the areas where RH is high must be monitored carefully.

Survival and development of fleas is morę hazardous out of doors than indoors. Less than 60% of larvae survive at a temperaturę of 35°C '⁹. They do not develop in areas exposed to sunlight and only survive out of doors if RH is higher than 50% for several consecutive days, or in microhabitats with elevated RH²¹. Areas outside which are favourable for the development of Ctenocephalides felis are characterised by RH higher than 50% and a temperaturę of between 4 and 35°C⁵.

The areas showing maximum larval development are near to sleeping and resting places. W.A. Osbrink (personal communication, 1993) observed that less than 15% of larvae placed in a carpet moved morę than 20 centimetres before pupation. These data are in accord with previous observations which show that the first stages of development do not move far from the point of hatching ²⁽⁾. The dispersion of immature stages is principally linked to the behaviour of the host. Larvae show positive geotropism and negative phototropism, resulting in development at the base of carpet fibres, under fumiture and around skirting boards ^l8. Out of doors, laml activity is restricted to the upper few millimetres of the soil²². Flea larvae are protected from most of the adult insecticides applied indoors, probably because they develop at the base of carpet fibres where the treatment cannot reach them.

Pupae

Following a short pre-pupal period, the lam matures into a pupa, usually within a silk cocoon. The cocoon is ovoid, around 0.5 mm long and whitish (Fig. 8 : 8). The silk fibres are sticky and soon surrounded by debris from the environment. Cocoons can be found in soil, in vegetation, in carpets, under fumiture and in the animals’ sleeping areas.

Lamę which have been disturbed are able to come out of their cocoon and spin a second one or develop as naked pupae ^23,2A. Studies on the formation of cocoons by Ctenocephalides felis larvae show that the lamę need to align themselves along a vertical surface to spin a cocoon successfully ^M. Less than 3% of lamę are able to spin a cocoon at 28°C and 80% RH in an environment without any vertical objects. However, morę than 95% of these lamę develop successfully as naked pupae, showing that the cocoon is not essential for pupation or for moving to the adult stage.

The pupal stagę can last from 6-7 days to several weeks. The pupal stage is the most resistant immature stage to desiccation, with around 80% of pupae emerging as adults at 27°C and 2% RH^l9. At 24.4°C and 78% RH, adult Ctenocephalides felis begin to emerge 8 days after the start of pupation and all the fleas will have emerged withinB days⁸.

The cocoon does not present a barrier to insecticides²⁵. The survival of pupae in houses treated by insecticides is not due to the protection conferred by the cocoon but due to a lack of penetration of the insecticide into carpets. In the wild. the primary function of the cocoon could be to prevent predation by various arthropods²⁶.

Pre-emerged adults in the cocoon

Adult Ctenocephalides felis can stay ąuiescent in the cocoon for 140 days at 11°C and 75% RH^2J. When eggs are laid in the autumn, the emergence of adult fleas can be delayed for 20 to 30 weeks²⁷²⁸. The ability to survive in the cocoon for prolonged periods of time is very important for a species like Ctenocephalides felis, with hosts that are often mobile. Emergence from cocoons is favoured by an increase in temperaturę and mechanical factors. Ctenocephalides felis can complete its development and emerge in a minimum of 13 days and a maximum of 300 days, depending on temperaturę and environmental stimuli. Nevertheless, in most cases, Ctenocephalides felis completes its development and emerges within 3 to 5 weeks.

It is common to see adult fleas reappear, several days after treating carpets with aerosols or sprays. Fleas can continue to emerge from their cocoons for 4-6 weeks after an insecticide and insect growth regulator has been applied to the environment. This resurgence is attributed to pupae and pre-emerged adults being present in their cocoons at the time of treatment. Vacuuming stimulates the emergence of adult fleas and decreases the period of infestation in as much as some newly-emerged fleas are eliminated by the vacuum cleaner.

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