Hi Colleen,

If you haven't, you should check out the "how to" article by John on this forum.
Frog Forum - Fruit Fly Culturing, Care and Feeding - Everything you need to know about Drosophila


Temps:
Your cultures should be kept between 70-80 F.
Lower than this will slow production/growth rate.
Higher than this can cause cultures to dry out/ stress the culture/ sterility if temps are very high/ can cause some selected strains of flies to hatch with the ability to fly.

Generation time (from egg to adult) is temperature dependent. For D. melanogaster as an example, generation time is approximately: 7 days at 84°F, 9 days at 77°F, 11 days at 72°F, 19 days at 64°F (info from Bloomington Stock Center). These generation times are longer for hydei.


Moisture in media:
Your temps and humidity level can change the water/mix ratio that you need to use.

Your media should look like thick mashed potatoes after being made. Later as the larvae develop it will liquify somewhat.
Depending on the mix and your conditions, you may need to adjust the water/mix ratio.

Media that is too dry will cause the culture to slow/stop. If cracks develop in the media then it is too dry. As a quick fix spay a small amount of water at a time into the culture to adjust.

Media that is too wet will result in a mess when feeding and will only get worse when as the culture gets older. A quick fix is to drain out excess liquid or to stuff in a coffee filter or paper towel piece to absorb excess.

The permanent fix is to adjust your water/mix ratio when making your next batch.


Health of culture:
In my opinion, one of the most important aspects to successfully culturing fruit flies is to maintain healthy cultures by keeping them actively growing.
This means starting new cultures as soon as the cultures start producing.

Many of us start new cultures every week. It's better to have more flies than needed and backup cultures on the way, rather than to have too few flies or be stuck with a failed culture.

Only use a newly producing culture to start a new culture. Do not use an old culture unless absolutely necessary to restart.
The old cultures are not as healthy and often are prone to mites (see below).

Using new / actively growing cultures to start new ones will continually select for a fly population that is healthy and that will culture reliably.


Mites:
Mites can cause cultures to crash.
Mites can be carried on flies. Mites can also climb into containers.

Luckily the mite life cycle is longer than that of the fruit fly, so if you only start new cultures from newly producing cultures then you can use this to your advantage to keep mites away (no mites in cultures and no mites carried on flies into new cultures).

I also keep my producing / feeding /older cultures in another location from the new cultures that have not yet started to produce. That way, any potential mites are kept away from the stocks that are used to start new cultures.

You can use mite paper, diatomaceous earth, etc. in your trays/drawers/areas where you culture your flies as a mite preventative if needed. I have found that I do not need these extra precautions if I follow the procedures listed above.


Hydei vs. melanogaster, and number of flies to start a culture:
Hydei has a longer cycle time (~20days) vs. melanogaster (~10days) and this potentially/theoretically makes the cultures more susceptible to mites.

Hydei cultures also hatch in a sex skewed manner, with females hatching first.

To conterract both these issues, start new hydei cultures ~48hrs after the older culture starts to produce. Use more (~100 or so) flies to start the new culture to make sure that enough males and females are added to successfully start the culture.

Melanogaster does not hatch in a skewed manner so only ~20 flies are used to start a new culture.


Too few flies added will not start the culture effectively as well and it will not be vigorous.
A large excess of flies is also not good as the larvae compete with each other and inhibit larval development due to build up of urea/ammonia. These culture may also "boom" but then not continually produce.

Don't worry about being exact with numbers however, as what I mentioned above is only found at the extremes and there is a large range of numbers added that are OK.


As long as you are consistent with how many flies you add, are using the "healthy culture" practices, optimize your media water/mix ratio, and keep the temps between 70-80F then you should not have any problems with keeping your fly cultures reliably producing in a consistent manner.

If you can't "revive" your current culture, then I would suggest replacing it with a new one.

Hope this helps!

James