Summary
Within cocoons, flea larvae develop into adults in 7-19 days. However, the adults can stay inside their cocoons for up to 5 months. The larvae create cocoons in 6.7 hours. They begin pupating 17.8 hours later. They molt into pupae 1.5 to 4 days after the cocoon is formed. The pupal stage lasts 6-7 days, then they molt once more into mature adults.
Details
Flea larvae usually pupate within cocoons. There, they transition through three stages: U-shaped larvae (prepupae), true exarate pupa, and pre-emerged (pharate) adult. They molt between each stage. There’s a larval-pupal molt, and a pupal-imaginal molt.
How Long it Takes Larvae to Spin Cocoons
Mature flea larvae shed their guts to prepare for pupation. Afterwards, they remain inactive for 11 hours. Once this brief quiescence ends, they begin spinning a cocoon. The cocoon gets finished in 6.7 hours. Then movement ceases again.
How Long it Takes Fleas to Pupate
Within its cocoon, an immature flea develops into an adult over the course of 7-19 days. Males require 14-20% more time to mature than females.
Pupation time is influenced by ambient temperature. As temperature decreases, development time increases. At 60°F (15.5°C), female prepupae become adults in 26.8 days, and males in 32.1 days. At 80°F (26.7°C), females mature in 8 days, and males in 10 days Fig 1.
Fig 1 Days (y-axis) it takes for cocooned larvae to become mature adults (x-axis).
Larval-Pupal Molt
Molt Process
Once the cocoon is formed, larvae begin pupating in 17.8 hours. As it transitions into a pupa, body tissues are broken down and reorganized. Fluid in the last three body segments is absorbed 6-12 hours before molting. The segments then collapse, and within 3-8 hours a pupa emerges from the old larval cuticle.
Fig 2 Days (y-axis) it takes for cocooned larvae to become pupae (x-axis).
Molt Duration
Females become pupae 12 hours earlier than males. At 80.6°F (26.7°C), female and male larvae become pupae in 31.5 and 43.6 hours, respectively. However, the first molt took slightly longer in other studies Fig 2.
Pupal-Imago Molt
Molt Process
After shedding the larval cuticle, the pupa’s body gradually shortens and widens. It remains white for 5-9 days. Then it changes to yellow color for 2-3 days. Finally, it becomes brown. The combs and bristles turn black 1-2 days into the brown stage. These color changes occur on the underlying adult cuticle. The pupal cuticle remains colorless. 2-24 hours before molting, the legs break away from the abdomen.
Fig 3 Days (y-axis) it takes for cocooned pupae to become adults (x-axis).
Molt Duration
The pupal stage is the period between the two molts. It often lasts 6-7 days. One study saw it last 13-18 days. Female pupae become adults 1.6 days earlier than males. At 80.6°F (26.7°C), female and male pupae darken in 5.8 and 7.5 days, respectively. And adulthood is reached in 7.3 and 8.9 days. Cooler temperatures prolong the duration Fig 3.
How Long it Takes Fleas to Emerge from Cocoons
Pre-Emerged State
After becoming adults, fleas can remain quiescent inside their cocoons for varying periods of time. The pre-emerged (pharate) stage can last up to 5 months, with a 60% survival rate. However, they’ll quickly wake up and emerge if a nearby host is detected. While cocooned adults can enter this long sleep-like state, most will emerge from cocoons soon after molting into adults.
The pre-emerged state helps ensure a host is around when the fleas emerge. And it increases the odds of survival in poor conditions, as pharate adults are more robust than emerged adults. In cool, dry air, 100% of pre-emerged adults survive for at least 35 days, while 90% of emerged adults die within 20 days. The cocoon itself isn’t a barrier to adverse conditions. Increased survival is due to a lower metabolism and respiration rate.
Cocooned fleas are the most hardy in adverse external conditions, and are in least danger from predatory attacks. Individuals can accumulate in this stage, until they emerge as adults when conditions become favorable. The quiescent state can help infestations survive short winters. In long winters, it may require a combination of survival as a quiescent adult followed by survival on host animals.
The option for adults to remain within cocoons causes control issues. Fleas can seemingly reappear out of nowhere. This problem duration is called the pupal window.
Emergence with Host Stimuli
When cocooned fleas sense heat or pressure, they’ll emerge within 5 seconds. These cues represent a potential host. If they don’t emerge, the fleas will briefly move around in their cocoons and then become motionless again.
Emergence without Host Stimuli
Without pressure and heat, most adults emerge gradually over several weeks. Females emerge several days prior to males.
Ambient Temperature
Between 70°F and 90°F (21°C and 32°C), nearly all fleas will emerge within four weeks. Emergence is considerably prolonged at 51.8°F (11°C), taking up to 20 weeks. Humidity plays a small role in emergence times.
Larval Nourishment
Partially starved larvae emerge as adults sooner than those which are well nourished. Similarly, when forced to construct two cocoons, larvae develop into lighter pupa and emerge significantly sooner than those which spin a single cocoon. Early emergence may be caused by water and food reserves dropping below a critical level. Alternatively, poorly nourished larvae may produce a weaker cocoons that don’t impede adult emergence.
Emergence in Normal Household Conditions
At 80°F (26.6°C) and 80% relative humidity (RH), adult females begin emerging 5 days after becoming pupae, with most emerging on day 8. Males start emerging after 7 days, with most emerging on days 9 and 10 Fig 4. Emergence took slightly longer in another study. Following the first molt, females emerged in 14-17 days, and males in 16-19 days.
Fig 4 Cumulative percent of emergence (y-axis) recorded in days after pupation (x-axis) at 80°F (26.6°C) and 80% RH.
Death
Mortality is extremely low for cocooned fleas in normal household conditions. One study saw less than 1% dying. In another study, 88% of larvae successfully pupated at 89.6°F (32°C). 82% pupated at 80.6°F (27°C).
Temperature
Temperatures above 95°F (35°C) are lethal to developing fleas. 30% of larvae will form cocoons, but the heat kills them after the pupal-adult molt. And, in low humidity, the pupae die prior to molting. Outdoors, 100% of larvae and pupae die when temperatures exceed 95°F (35°C) for more than 40 hours a month. The lower limit for survival is 55.4°F (13°C). In winter, dry and cold conditions prevent survival. Nearly all cocooned stages die within 5 days when continually exposed to 37.4°F (3°C) Fig 5 & 6.
Fig 5 Percent of pupa that survive (y-axis) at three different ambient temperatures across 20 days (x-axis).
Fig 6 Percent of pre-emerged adults that survive (y-axis) at three different ambient temperatures across 40 days (x-axis).
Humidity
Low humidity can be lethal to cocooned larvae, but has little effect once the pupal stage is reached. Pupae are the most resistant stage to desiccation, with 80% surviving to adulthood at 2% RH. In some experiments, fleas died when cocoons were accidentally exposed to dry conditions. This is likely because larvae were affected before the first molt. The cocoon itself offers no protection from desiccation.
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I had a flea infestation in my home last Spring. Is that why I kept seeing tiny black dots appear on my sheet from out of nowhere? I thought I was loosing it.
It’s possible that those black dots were flea feces (“flea dirt”). Adult fleas live permanently on their animal hosts, and regular defecate. The feces then dries. It gets dislodged when the infested pet grooms itself. If your pet rested on your sheets, then the black dots could have very easily been flea dirt. To be certain, placing the black specks in water or rubbing them on a damp cloth would cause them to smear red, as flea feces is mostly host blood.