It  looks like infections

I agree, this is probably due to a microbial infection. You can try to isolate and rear healthy individuals to overcome the problem. You can also change the food as it is normally the main source of water contamination.

check the feed and the water in which you are rearing the mosquitoes. That the water looks clear does not imply that its safe. I did experience this once until I fixed these parameters. Again,the  number of death compared to the survivors also matters before conclusion.

It seems due to fungal infection, If you don't change culture water over 5-6 days fungal growth expended and hinder larval respiration, ultimately death occurs. This is happen in Chironomidae and some Ceratopogonidae, so same would be in Culicidae. 

Yes, You have to change the culture water after 5 to 6 days and check the food is not contaminated.

Hi colleague,

You are right anyone rearing mosquitoes in their labs encountered probably this problem and it is probably due to bacterial or fungal infection. Some bacteria (Bacillus thuringiensis isralensis or Bacillus sphaericus) are used as biological tools against mosquitoes and act by destructing larvae’s midgut.

I suggest you to read MR4 manual of procedure (attached), it will be helpful for you work.

Good luck,

Dr El Hadji Amadou Niang

Laboratoire d’Écologie Vectorielle et Parasitaire

UNIVERSITÉ CHEIKH ANTA DIOP DE DAKAR

About 5-10% larval mortality in any culture is ok and should be acceptable. Don't worry much about that. However, In my opinion water of culture need to be changed on alternate days, especially if you are culturing Anopheles species. Could it be an infection of Lambornella (a ciliate protozoan) ? Beware of this infection..it had once wiped off my entire colony of Anopheles

All the recommendations made above are valid. The temperature, proper diet according to species, control of water quality (remove and replace between 1/5 and 1/4 of the volume every 3-4 days in my experience) are essential aspects to consider.

I would add the volume and surface area per larva and the height of the water column. These parameters can be decisive in the success to set a colony of certain species of mosquitoes.

Do a search about the experience of others with breeding of the species of which you are working and / or also make several tests based on the characteristics of the species in question (degree of aggregation, physical and chemical characteristics of the most productive natural breeding , etc.).

This is definitely an infection, however the source or causative agent will be difficult to ascertain. One thing to do for sure is to institute bleach cleaning on the mosquito eggs, this eliminates a large number of external pathogens which build up over time in culture. Eggs can be placed on a fine sieve and then immersed for 10 seconds in a 1% bleach solution and then rinse gently several times before hatching.

Additionally you should soak everything (rearing pans, tools, cups) that can in 5-10% bleach overnight followed by a 24 hour soak in water and then 24-48 hours desiccation as well as throw out any diet used. These two things combined usually resolve a majority of issues.

If the problem continues, you might consider adding antibiotics to the sugar meal (warning, test various concentrations on small populations to determine what is safe before applying it to the larger population.

What species of mosquito? What is your protocol? What is the percentage of mortality?

This question above, asked by Saul wasn't answered. Please Mayke, it will be easy to advise you if the details requested is provided. Even though the rearing is slightly similar for all the species, but it will be easy to give you some tips when the specific species is known. 

I have enjoyed reading the suggestions. They are all good. For egg deposition and larval habitat different species have their own breeding requirements / selection. One of the most choosy  species is Anopheles culicifacies, the primary malaria vector of the Indian sub-continent, which is difficult to colonize and even more difficult to rear. Even for it we never changed water or used antibiotics. Our water source was deep tube-well, which is potable. Liver powder is source of feeding, which is sprinkled daily with a table salt dispenser.  Depending upon the size of bowl, pan or tray, liver powder was sprinkled in a minimum quantity, so that after 24 hours no scum was formed. Every lab  can developed its own standard, like a housewife adding salt to the cooking food. We have used this  "protocol" for many species of Aedes, Anopheles and Culex, without any problem. Good Luck.     

Try to boil all the stuff ( trays, bottles , cups ...) that you using for rearing in your lab. It could keep your colony healthier. Bleaching is another way.

I believe that each species, have different conditions.

1- Temperature between 40-45 ° C in the insectary

2- Humidity between 80-92 %

3. Quality nutrient

4- Water Quality (change every two or three days)

5 High-Number of larvae in the tray

Thanks

There could be the problem of infection with Lambornella spp, a Hymenostome endoparasite, in your colony. This infection had once wiped off my entire colony of Anopheles stephensi......Please beware of

You may wish to consider the comments of Joseph and Anil. If the population of larvae that died is not significant, I think it is normal since we tend to lose larvae during rearing. And if the larvae were already infected before hatching, then it will only take few developmental stages to manifest and they start to die. I sometime see large number of death that stabilizes after in my colony. So if all is set right especially good feed and hygiene, I think you will be fine. Observe other colonies under the same laboratory conditions, then see if it would stabilize however, check your rearing procedures right from the egg stage.