Flea Larvae:
They Lie In Wait In The Dark To (re)Infest Your Pet

All insects develop by undergoing several molts.

In some insects, the process is limited. Each molt corresponds to an increase in size without any significant change in lifestyle or feeding habits. This is the case, for example, for grasshoppers, lice, and true bugs.

Other insects are holometabolous: they undergo a complete metamorphosis during their last molt from pupae to adults.

Everybody knows how caterpillars crawling on leaves become wonderful butterflies or how aquatic mosquito larvae become harassing flying pests seeking to take our blood.

Fleas belong to the second category. Immature fleas are dramatically different from adults. While adults can jump very high to reach the host on which they will stay for the rest of their life, larvae crawl on the ground and try to hide from the light as much as possible.

You’ll hardly notice them. But they are much more numerous than adults, about 7 times more, on average.

Larvae are a major part of the flea problem: they are the invisible army that makes flea infestations persistent. Eliminating the flea larvae population should be considered as a way to help control fleas on your pet.

Flea Life Cycle

The flea lifecycle consists of 6 different stages: the flea egg, the 3 larvae instars, the pupa, and the adult, male or female.

Adult female fleas lay up 25 to 40 eggs per day^[1] on the pet’s coat. From there, the eggs eventually fall on the ground and are found where the pets spend the most time.

Depending on temperature and humidity conditions, the eggs hatch after 2 to 10 days into the first of the three larval instars.

At the end of its development, the stage 3 larva secretes a film of silk which it wraps around itself to begin its metamorphosis and becomes a pupa. The pupa is the quiescent stage of the flea. Once the metamorphosis is complete, protected by its cocoon, it can wait for weeks or months for the arrival of a potential host, most often a dog or a cat.

In the protective conditions of a domestic environment, the cycle typically needs around 3-4 weeks to be completed. But it may last much longer if the pupae do not receive the signs of the presence of mammals nearby.

What Does Flea Larvae Look Like

You may find 2 different types of fleas on dogs and cats: Ctenocephalides felis commonly called the cat flea, and Ctenocephalides canis, the dog flea. The cat flea is by far dominant in both dogs and cats.

The adults of these two species do not differ much. The head of the cat flea is shorter and less rounded. The cat flea also has more notches on the hind legs. This is a matter for the experts.

There is no way to distinguish between the larvae of the two species.

Flea larvae look like slender maggots. They are white, almost transparent when they emerge from their eggs. They become darker as they feed. During their development their size increases from 1-1.5 mm long (1^st instar) to 5-8 mm (3^rd and last instar). They are segmented and have large chewing mandibles that help them satisfy their voracious appetite. They have many setae all around their body, but not plenty enough to be called fur.

Flea larvae have no legs or eyes.

Where Do Flea Larvae Live

Adult cat fleas live, feed, and reproduce on their host, dog, or cat. The females lay eggs when the host is resting^[2] or sleeping. The eggs fall quickly from the coat to the ground. That’s why most eggs are found in places where the pet is used to staying.

First stage flea larvae tend to live close to the eggs they have hatched from. But they quickly seek to hide from the light. This is why you may never notice them.

They’ll move deep into carpets, down in wooden floor cracks or under the soil, outside the house.

Once they have molted to the next larval stage, they can move further away, up to 40 cm from their initial landing point.

Flea larvae are the weak link in the flea’s life cycle. They are very sensitive to environmental conditions. They need:

• At least 33% relative humidity. More than 50% is the temperature is higher than 25°C (77°F). Flea larvae are very susceptible to desiccation
• An ambient temperature comprised between 8 and 35°C (46 – 95°F)
• To be protected from direct light

Otherwise, they die.

Indoor, less than 25% of the flea larvae population survives.

Outdoors, fleas can develop in moist and shaded areas.

It is important to understand that, because homes are maintained at temperatures favorable to the development of fleas, they can infest your pet all year long!

On the other hand, contamination is rare outside the house during the winter.

How Long Do Flea Larvae Live

In ideal external conditions i.e. 25°C and 75% humidity, and if enough food is available in their environment, the 3 larval stages take 5 days in total.

In case temperature and/or relative humidity is lower, or if the larva has a hard time finding food, it takes longer, up to 11 days^[3].

What Do Flea Larvae Feed On

Flea larvae size increases 3 to 5-fold in about one week. Needless to say, they must eat a lot.

And this is what they do: they ingest all kinds of organic debris they can find.

The primary food source is flea feces in the form of fecal pellets made of the poorly digested blood meals of the adult flea^[4]. They fall out of the pet’s coat in just the same way as the eggs. Fecal pellets can be found in the same areas that infested cats or dogs regularly frequent. These pellets are often referred to as flea dirt. Studies have shown that flea larvae cannot survive if flea dirt is not part of their diet.

But flea larvae can also be carnivorous, if they find prey within their reach, other insects or mites. They often are cannibals. They do not hesitate to eat smaller flea larvae or pupae.

Do You Need to Treat Flea Larvae

In theory, to eradicate fleas on your dog or cat, it should be sufficient to break the flea life cycle at one of its stages only.

This would mean eliminating either all adults, all eggs, or all larvae. It is not possible to eliminate pupae because they are protected from chemical products by their cocoon and the debris that surrounds them.

Most often, it is recommended to treat adult fleas on the pet.

This strategy has some limitations, however:

• You have to treat all your pets at the same time, even those where you didn’t find fleas
• You need to use very effective and rapid-acting flea medicines, to prevent adult females from laying eggs
• You must renew the treatment in time, in compliance with the legal recommendations mentioned in the product leaflet
• You should not bathe your dog too often in case she has been treated with a topical product (i.e. applied to the skin), as this may result in a reduction of the duration of protection

To get around these constraints, you may be advised to seek to eliminate immature forms of fleas infesting your home.

Remember larvae, eggs, and pupae together account for 95% of the total flea population

How to Control Flea Larvae Infestation

Removing flea larvae from home

It is important to remember that, because homes are maintained at favorable conditions (temperature, humidity, presence of shadowy areas) for flea development, fleas may be present throughout the year. Infestation of your pet outside the house is rare in winter times.

Flea larvae try to escape the light. But they don’t go very far from the egg they came, 40 cm at the most for stage 3 larvae.

In addition, they have to find the fecal pellets they absolutely need to feed on. These are found in the same places as the eggs.

This means that flea larvae live near the places where your dog or cat spends the most time. They will find favorable conditions in carpets, rugs, between floorboards, or in the deep fabrics of sofas and armchairs: these are places where they are hidden from light and where the humidity level is a little higher.

These are the areas you should target. Spy on your dogs or cats and try to find out the spots where they usually spend time, play, sleep, or rest.

Cleaning and vacuuming

You’ll start by washing the pet bedding often.

It is also recommended to regularly vacuum carpets, rugs and wooden floors. Machines equipped with a beater are more effective. They are active on the ecosystem flea larvae rely on:

• Flea eggs, flea larvae, and flea pupae
• Organic debris used as a source of food
• Flea dirt mandatory for flea larvae to grow and develop

Although necessary, vacuuming is not enough to control the flea larvae population.

Vacuum cleaners can’t reach the deepest parts of carpets. Not all larvae will be removed, even with the most careful cleaning.

One study showed that thorough cleaning eliminated between 40 and 80% of the eggs, but less than 5% of the larvae. Vacuuming is more effective on short-pile carpets. The power of the vacuum cleaner makes no difference^[5].

Be careful with steam cleaners: the steam is often not hot enough to kill the larvae but creates a lot of moisture that will greatly benefit their development.

Treating

Methoprene and pyriproxyfen are IGRs (Insect Growth Regulator). These are the molecules of choice for treating your home. They are the main ingredients of the specialized products you’ll find on the market^[6]. They imitate the action of the juvenile hormones in insects. They block the development of larvae to the pupal stage. They also kill flea eggs.

These molecules are very safe for humans and pets.

They are often combined with low toxicity insecticides such as permethrin or natural pyrethrins.

But there are some limitations (again).

Firstly, the use of foggers is not recommended as they treat a room uniformly and cannot reach hidden areas: behind or under furniture.

You should use a conventional spray that can be used to precisely target the areas you have identified as the sources of infestation.

Secondly, there is no data on the effectiveness of these products under real conditions of use. There is no way to know the level of efficiency you can expect in your home.

In comparison, products applied to dogs or cats benefit from numerous, on the field, and documented data on their actual effectiveness.

The effectiveness of these treatments depends largely on you, especially on your ability to identify hot spots and treat them regularly and thoroughly.

Removing flea larvae from the yard

An infested animal will lay flea eggs in your yard, just as she does in your home, except that conditions are less favorable for flea larvae. They will not survive if the temperature drops below 8°C (46°F). They can’t stand direct sunlight either.

But your yard may also be contaminated by other animals that you haven’t noticed: stray cats, raccoons, opossums…

This is more of a seasonal issue since flea larvae most likely won’t survive in winter.

The first measure is to keep your pet away from trouble spots. That is places that never see the sunlight: under a bridge, under hedges, in the woods, or in the shelter of a thick lawn. Instead, you’ll point him to areas that are exposed to sunlight and short-cut lawns^[7].

In case of a real and persistent flea infestation problem, you may consider treating your yard.

You should be aware this is a difficult and expensive method. You will need the assistance of a specialist. But, again, although treating the yard makes sense, there is no guarantee of effectiveness. We couldn’t find any data exemplifying the results you may get.

Preventing Flea Eggs Shedding

Preventing adult fleas from shedding eggs definitely breaks the flea life cycle.

Many highly effective treatments available on the market can solve the problem. They kill the fleas that live on the animal. Some of them, as an additional benefit, make the eggs the females still manage to lay, non-viable.

There is a short-term benefit to seeing fleas disappear from your pet’s fur.

Over the long term, treating the pet dries up the reservoir of eggs, larvae, and pupae in your home and possibly in your yard.

But, still (sorry!), there are some limitations to the long-term effectiveness of the method:

• It takes time. Even when this method is fully effective, you need to wait at least one month before your pet gets rid of all the fleas.
• All pets in the household need to be treated, and at the same time
• None of your pets should be infested outside your house. If it is the case, you should continue the treatment

Frequently Asked Questions

Flea Larvae Conclusion

Regarding the flea life cycle, we can consider larvae as “the invisible part of the iceberg”. You don’t see them and yet they are much more numerous than the adults. They are an important reservoir for the reinfestation of your pet.

Controlling flea larvae should not be overlooked. Not considering them is the main reason why flea treatments so often fail.

Flea control requires a high level of commitment on your part. You must take the time to understand how the flea life cycle applies in your home and on your pet.

Gilles Ventejol

Gilles Ventejol is a professional in Animal Health. He holds a master's degree in life sciences. He has worked for more than 20 years in veterinary pharmaceutical companies, where he contributed to the development and promotion of many veterinary drugs. In this context, he has organized numerous conferences for the veterinary profession. His areas of expertise are parasitology, cardiology, nephrology, vaccines, and reproduction.