FRUITY FRUITFUL FRUIT FLY FACTS

Derek Petrie

During my search for an easy and cheap way to culture Drosophila (I'm lazy and poor!) I have made a few useful discoveries that should be of use to all who 'grow your own'. 

I do not use bananas as I find that they tend to make culture media that gives off an unpleasant 'acrid' smell and my base mix consists of apples and citrus fruit.  My starting point is one large cooking apple and one grapefruit, or one small apple and one lemon.  The citric acid from the citrus fruit helps to 'set' the pectin in the apple thus making a 'firmer' mix and acts as a preservative inhibiting fungal growth.  I have reached the conclusion that a mixture of fruits greatly increases the harvest of flies and for this reason I add such things as mango (excellent), pineapple, strawberries, pears (used as apple substitute or in addition), cherries, oranges, melon or any other fruit you find on the 'bargain' shelves of your supermarket.  My choice is dictated solely by what is cheap! The more variety in the mix the better, but I always try to keep the ratio of citrus fruit to non-citrus at about one to one. 

I make up as much as possible and freeze the excess to save time and effort.  Using a large mixing bowl I quarter and core the apples then chop in rough chunks.  Sprinkle some sugar over them and place in the microwave for a few minutes until they 'fall' that is go soft.  The sugar helps this process and acts as a natural preservative.  Now add in all the soft parts from the citrus fruits.  Add in any other fruits you have obtained cheaply leaving out only the hard parts like pineapple skin etc.  Re-cook in the microwave for a few more minutes.  Time is of course only dependant on the volume you are making as, after all this is not rocket science!

At this point something is needed to stiffen the mix.  I use porridge oats (cheapest supermarket own brand) or you can use Agar, a natural gelatinous substance from seaweed, obtainable cheaply at Chinese supermarkets.  Other alternatives (like when you realise you haven't got any oats and the shops are shut) are rice or flour. 

You should now have a stiff mixture that will just about stay stuck in the bowl if you where to tip it upside down (not recommended!) Allow this to cool for a short time.  Yeast is then added.  I use fresh bakers yeast, as it is easy to obtain cheaply from my local supermarket.  Excess fresh yeast can be kept in the freezer after separating into suitable size chunks.  I use approximately a heaped teaspoon to one litre of mix or about two teaspoons of dried yeast.  Adding the yeast while the mixture is still warm gets fermentation off to a rapid start and again helps to prevent other fungal growth.  I like to place the media into the culture containers at this point and then allow it to stand overnight to settle down before adding the flies.  I used to use crumpled kitchen roll on top of the mixture to give the larva a place to pupate but have since found that hamster bedding (flocking) or equivalent gives better results.  You will find that if you place a bit on top of the media it will eventually be 'pulled' down and incorporated into the mixture by the action of the larvae.  Simply keep adding small fresh pieces until the culture dies back.  This helps to stiffen the mixture and prevent it falling out when you are shaking out the flies.  I have noticed that I also get a much higher harvest of flies since using 'hamster' bedding. 

Containers for your cultures are a matter of preference.  I use large coffee jars as I obtain them for nothing in sufficient quantities from work.  I cover them with either kitchen roll or net curtain (get them cheap at jumble sales) depending on what's to hand at the time. 

Don't be afraid to experiment.  There are no hard and fast rules to breeding fruit flies.  Recently I have had good success using black treacle instead of sugar, only because I found an old tin of it mouldering in the back of a cupboard.  Other ad-hoc ingredients have included dried fruits left over from making Christmas cakes and a dented can of cheap fruit salad from the cheap shelf in the supermarket. 

Don't overlook windfall apples and other fruits.  As I write this I am looking out the window at some ripening elderberries! One final thought, and something that has troubled me ever since I started keeping poison dart frogs.

Why do all the escaped fruit flies end up stuck to the soap in the bathroom?

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