We have recently had serious problems with some of our fly stock
cultures, and would really like some input into what could be the
problem, and how to solve it.
We use a standard maize/dextrose/yeast/agar recipe. cooking is done in
a large food cooker, typically 40-50 litres per batch. We add propionic
acid and nipagen (dissolved in ethanol) just before dispensing. We cook
every 3-4 weeks. After dispensing we leave the vials/bottles overnight
before plugging. We store cooked food wrapped in autoclave bags at 4C.
We leave the food to come to room temperature before use. Fly stocks
are cultured in an 18C room, or at room temperature.
Last year (september) we bought a new batch of maize. Initially the
food was good and all the flies were happy. Over the next few months
the quality of the food declined, and stocks began to suffer. By March
this year we had big problems. In the most seriously affected stocks
the adults died after being put on the new food. In other stocks the
flies were alive, laid eggs, but no eggs hatched. In many stocks eggs
hatched (at least some did), but it would be 4 weeks or more before
adults eclosed (rather than the expected 3 weeks). Some stocks were
We bought new maize (March 09), and the problem went away, BUT, over the
next few months (june/july) it came back... So, we thought it was due
to maize going off, and switched to another batch, this one had been
bought a while ago, used with no problems and stored at -20, so should
stay fresh. Again, initially (july) everything was fine, but now the
stocks are slowing their generation times / dying off again.
We have had the first batch of maize tested for pesticide residue but it
came back negative. We have also tested it for Bt spore contamination
(yes it is organic...), by heating a maize /water mix to 80C for 15
minutes and plating on media suitable for Bt. we do not get Bt growth,
however there is growth of something (fungal). we have tested this with
flies in a laying bottle and it does not seem to retard egg hatching,
neither is it toxic to adults or larvae (indeed the larvae love it...).
So, does anyone out there have any ideas what could be causing the
problem, and most importantly, how to make it go away? The only things
we can think of are the temperature the food gets to after adding the
maize/ sugar/ yeast mixture (might it have to get up to 85C?, ours
probably only gets to 80C), should we add the nipagen earlier (could the
ethanol be causing the problem?).
Yours in desperation
Dr Helen White-Cooper
School of Biosciences
Tel 029 20875492