The keeping of fruit flies is a subject that has been widely written about in Killi News (the BKA journal) over recent years, however I would like to add a few potentially useful tips for getting the most out of drosophila culture.
I’ve kept flighted fruit flies (drosophila melanogaster) in a laboratory for several years, which forms the basis of these notes. These are under ideal, but expensive conditions so I’ve tried to suggest cheaper options as I’ve gone through.
Success with fruit flies depends on a number of factors, with (in my opinion) the ingredients of the media being fairly inconsequential. After all, fruit flies seem to adapt to survive in white, grindal and microworm cultures without invitation- and what they do to old fruit in your fruit bowl is another story! The key to successful culture is in the maintenance regime employed. I keep both flightless (mainly for my dendrobatid poison dart frogs), and winged which my frogs also eat, but are kept mainly for their larvae, which all killis i've tried seem to enjoy. Flightless fruit flies need more space, and some form of dry walking space- winged can be raised at a higher density, and survive with no climbing space typically associated with flightless flies.
The length of the drosophila life cycle is temperature dependent - at 18° c it takes around 18 days for an egg to develop, pass through its larval stages, and emerge as a fly (enclosure). At 25° c it takes 9 days, and at 29° c only 8. At 25° c we need to collect flies once each week, however if they are kept cooler this can be less often. Flies do not do well above 30° C. These times apply to drosophila melanogaster- other flies including the larger flightless varieties can take twice as long.
The number of vials required and times below obviously depend on the exact conditions, and the number of flies required. These are based on vials 4 inches tall, 1.5 inches diameter at approximately 24 ° C. Vials are filled with about media 1-2cm media, and are kept on trays with kitchen roll and a small amount of water. This keeps the humidity high enough for the flies, and also prevents the spread of mites.
The vials that the flies live in are separated into 3 categories-
1.The present generation of adult flies- laying eggs for future generations (3-4 vials)
2.Vials containing chrysallised larvae, and newly emerged flies.(10-20 vials)
3.Flies or larvae for feeding fish (3-4 vials).
Once per week newly emerged flies are collected from 2. These are housed in fresh vials, and are designated 1 the present generation. After 3-4 days these flies are moved again, onto new media- the old vials (which are loaded with eggs or very young larvae) go back in 2, to make future generations. Once the week has passed, the flies making the present generation can be fed to your fish, and their vials may either go back into 2, or be left for a few days to provide larvae. Of course a new generation of adult flies is again collected from 2 at the same time. There are many advantages of keeping to a strict regime such as this
1 each vial will contain only flies/larvae of approximately the same age, resulting in eclosion over a relatively short period. This reduces the time you have to hang onto older vials-making fly collection easier and lessening the chances of mold,
2. The longer flies are kept on the same media, the more runny it becomes- if the flies are collected after a 3-4 days, it will still be quite solid. Runny media can drown flies, stick them down and goes everywhere when the jar is inverted.
3. All flies will have reached sexual maturity and have reproduced before being fed.
The media I use in the lab is the Sussex media, which is relatively simple to make, though probably not as cheap (or the ingredients as easily available) as some other recipes. The consistency of the media is critical for success. It should be solid enough to allow the vial to be upturned (a smaller diameter vial will hold the media in place considerably better than a large one), yet not too dry for the newly hatched larvae. It should be noted that the media acts as food only for the larvae, and not the adult flies. In the wild they eat yeasts growing upon decaying fruit, and can be fed in a similar manner in the lab. Whenever flies are transferred to new media, the vial is lightly sprinkled with baking yeast (the active granular type, found in most supermarkets). Not only does this provide a source of food for the flies, but also ensures the surface is dry, and the flies will not get stuck down.
Water 1litre
Maize meal 100g
Sugar 95g
Agar 6g
Yeast (dried inactive) 19g
Mix the above ingredients, and bring to the boil. The mixture must boil to activate the agar. Leave to cool for a while and then stir in:
Nipagin 2g
Propionic acid 5mls
When cold cover in clingfilm to store. Before use, mop up any condensed water in the vials, or else the flies will just get stuck. This media will feed the larvae only, and should be sprinkled with bakers yeast to provide food for the flies. The media can be kept for at least a couple of months without any decay.
In the lab we use plastic vials as descibed above. These are straight edged and have tight fitting sponge. or cotton wool balls stoppers. Fine mesh, will keep the flies in but this is less effective at keeping mites out. Small diameter containers are best because the media is less likely to fall out when the vials are inverted, however certain jars (nescafe coffee) have wider bases and this obviously has a similar effect. For flightless flies, I make a walkway of plastic coated wiring screwed up into a loose ball, which holds the media in place as well.
The easiest way to move flies is to first aenethetise them with carbon dioxide. First invert the vial, pump CO2 in through a small tube, and tap gently. The flies fall though down onto the bung unconcious in a second. This is incredibly easy, but unfortunately relies on you having a CO2 tank. Without CO2 it is still possible to move flightless flies without escape. Flies should be emptied into a larger bottle via a funnel. If you keep the bottle moving (gently tapping the sides), they are unable to climb upwards and escape. It is also possible to stun flighted flies, allowing them to be transferred to new media - however this requires practice - and perhaps should be done outside! Another way is to use a pooter- a simple device for collecting insects without damaging them. It is common procedure when feeding reptiles and amphibians to dust them with commercially available vitamin/mineral mixes. The smallest amount of powder is shaken into the flies, coating them, and the container. Aswell as improving the nutritional value of the flies, this also impairs their ability to fly, and to climb steep surfaces. You could potentially use any fine dust that will not poison either the fish or flies- remember you only need the slightest pinch. (cornflour? powdered milk, salt)
Larvae of any size can be fed to killies to suit their needs. Once the eggs hatch, they bury into the media and grow. Eventually they will start to climb the sides of the vial, and when fully grown will settle near the top, and form a chrysalis. It is best to avoid any climbing media for ease of collection. I often add a mixture of fruit, vitamins and colour enhancers (such as astaxanthin powder) to the fly vials, just to increase the nutritional value of the larvae before eating.
Lots of larvae can be scraped from the sides, and if a drip of water is added , the rest will make their way out of the media and up the sides. After this you can leave the remaining larvae to hatch turn into flies, or to be really thorough you can add the contents of the vial to a strong (3M) salt solution,- the food dissolves away or sinks and the larvae will float, where they can be scooped out with a net.
Mites can be a problem when keeping fruit flies. There are two commonly occuring types, the first lives on the fly, and generally is not too much of a problem. The second predates on fly eggs , and can wipe out cultures.
Avoiding mites completely can be difficult, it is essential to have a tight fitting bung, and as much as possible avoid contact with wild flies. Cures include soaking bungs in benzyl benzoate, but this can kill some stocks of flies (it killed one batch of flightless I had) but also by continually changing flies to new media. Eventually they should dissappear. Mites can be seen on the vial sides as tiny light pale brown dots, and should be discarded if found.It is better to start with clean cultures, and avoid infection- this can be done by keeping vials in shallow trays of water or on commercially available mite paper- in effect isolating them from one another. Water is preferable as it keeps cultures at the required humidity, and stops the media from drying. I have always found wild type drosophila melanogaster to be far more resistant to mites than the larger flightless flies, probably due to the much shorter development time of the former.
A typical drosophila vial,
for flighted flies.
A home-made drosophila
vial, for flightless flies.
The 'walkway' is essential.