Flea Larvae: The Larval, Worm-Like Immature Stage of Fleas

The second stage or cycle of flea development is the worm-like larvae. Fleas go through what is called a complete metamorphosis: egg, larva, pupa, adult.
The female adult flea lays numerous eggs; larvae emerge from the eggs and go through a series of instars before pupating. Inside of the pupae (cocoon) the immature insect is transformed into an adult flea. When transformation is complete, surrounding conditions are right and nature signals that a host might be nearby, the completely formed adult flea will emerge from the pupal casing - ready to jump, feed, mate and continue the cycle. As a group, the egg, larva and pupa are known as the immature stages of the flea.
As you become more familiar with this immature stage of the insect (as well as other stages) you will have more information needed to effectively eliminate fleas from your home and keep them from re-infesting the area. Flea prevention is cheaper and far less time consuming than ridding your home of thousands of hungry fleas.

Larvae hatch from their eggs in as little time as 2 days and up to 14 days from the time they are laid by the adult female. Temperature and humidity play major roles in this timing. Temperatures between 80 and 90 degrees Fahrenheit and a relative humidity of 70% or higher gives optimum conditions for the emergence of the first stage or instar of larvae. Both eggs and larvae have very little protection from drying out and have better survival rates in higher humidity levels. They also avoid light, preferring areas such as beneath bedding and furniture as well as behind baseboards inside the home; shaded areas of the lawn are more likely to far more larvae than the sunny areas of the lawn. (Flea larvae have no eyes but do respond to heat and light; they are easily desiccated, as are flea eggs.) This is important information when understanding, preventing flea larvae infestations or controlling flea populations. Knowing where they usually prefer to hide or feed helps in sanitation procedures as well as which control products to use in most likely areas.

Desiccants such as Drione Dust and Flea Stoppers take advantage of the weaknesses of flea eggs and flea larvae. These products easily dry out and kill soft, moist eggs and larvae. There are no baits used to kill adult fleas but application of Flea Stoppers to carpets does, in a sense, bait and kill their larvae. Fecal matter (larva food) is coated with Flea Stoppers which in turn kills the larvae by ingestion. Simply put, Flea Stoppers destroys flea eggs by drying them out; the material has two modes of action on larvae: it dries them out and is also ingested as they feed. Boric acid products (including borate carpet treatments) have two modes of killing: desiccation and stomach poison. Yet, when used properly, boric acid granules labeled for carpet applications are actually safer than common table salt to mammals!

The larvae break free from their egg shell with a little help from a small egg tooth located on the head of the small worm. Newly hatched flea larvae are usually about 1/4 inch long. This is the first instar or stage of the larval development. Flea larvae go through three such instars between hatching and pupating. Each instar is a little larger than its preceding instar. Depending on availability of food and optimum combination of heat and humidity, flea larvae can take from as little as 6 days and up to 2 weeks or more from egg emergence to the more dormant pupae stage of the life cycle.
As this worm-like stage develops it will molt, leaving behind casings that resemble the larvae. These casings are found in the vicinity of the larvae since the immature stages of the flea are not nearly as mobile as their adult counterparts. Both larvae and their empty casings (castings) are usually found close to areas where flea hosts rest or frequent and are often found in dirty pet bedding materials. They will also crawl under nearby beds, furniture and behind baseboards as they search for debris and food and follow their instinct to avoid light.
Flea larvae have no legs but do have a single row of hair-like bristles around each segment of their body. These bristles aid in maneuvering, as do the anal hairs. There are a total of 13 body segments - 3 thoracic and 10 abdominal sections.
Larvae have no eyes but they can still locate the adult fecal matter (dried blood from the host animal) which flea larvae feed on for survival. Larvae will feed on other types of organic debris but have best survival rate when they feed primarily on dried blood. There are species of fleas whose larvae can feed on certain dead animals for blood meals (instead of droppings of their adult counterpart) but this is a rare occurrence that is not seen with cat fleas. The blood meal of the cat flea larval stage is derived solely from adult flea droppings which are made up entirely of undigested blood.
Coloration of newly hatched larvae is usually a creamy white. This color changes as the larvae feed, changing to darker shades of yellow to brown.
Adult fleas, flea larvae and developed fleas that have not emerged from their pupal casings all respond to vibrations. Vibrations (in combination with movement, carbon dioxide, heat and humidity) will help developed fleas to emerge from their pupal casing and adult fleas jump to find a warm blooded host. The movement of dogs, cats, squirrels (and other hosts) causes flea eggs and adult flea fecal material to fall in various areas close to the original host. Flea larvae also respond to vibrations and movement, but in entirely different ways.
Flea larvae have been known to fake death when they detect movement but their most interesting behavior is clinging to certain when vibrations are felt. As dogs or cats scratch the larvae picks up on the motion, wrapping itself around the animal's hair. Larvae have also been noted to do the same thing in carpet. When vacuum cleaners are close by, the larvae have been seen clinging to carpet fibers - but they are not too successful. Vacuuming usually picks up these larvae, despite their efforts.
As the end of the third instar larval stage draws near, flea larvae will begin to build the pupal casing in which they will develop into fully developed adults. A portion of the casing is made up of a silky material that is used to hold together all other parts of the cocoon. Larvae gather debris from their immediate surroundings with which they build their pupal case. Indoors, the tiny bits of debris include animal hair, human hair, dust, lint and small fibers of furniture, carpet and rugs. The materials used end up as the perfect camouflage as they blend in with their surroundings. If we could plainly see the pupae, they would be easier to locate and remove. Our only choice is to vacuum thoroughly and regularly to remove as many of the pupae as possible.
For more about the pupal stage, go to the flea pupae information page.
By themselves, fleas are not as mobile as one would believe. Adult fleas are designed for jumping (to locate and mount a warm blooded host) and crawling forward through the fur of animals. The eggs easily fall off of an animal as it moves. Flea larvae are capable of movement but do not stray far from where they emerge from the eggs. Their food sources are very close by as are the materials they need to spin and build their pupal casing. Warm blooded, nesting animals not only serve as the food source but also as a source of mobility, with adults, eggs and larvae food dropping off in various locations frequented by their host. This dependence on the host for moving about helps insure that the insect is scattered about an area. In nature's scheme you will find many such examples of this "scattering of the species" which helps insure the propagation of the species. Certain spiders, ticks and fleas are examples of this phenomenon.

Understanding the makeup, feeding and ability to create the perfect pupal casing exhibited by the flea larvae should give you a better feel for the control or prevention of flea populations. Knowing that the larvae are not very mobile helps narrow down specific areas to include in the most important aspects of IPM for fleas: inspection, sanitation and the safe use of proper pest control products.

As you will see in preventing flea infestations, sanitation, the use of an insect growth regulator and a safe desiccant flea powder are your best and safest means of controlling flea larvae.