Fly media, or fly food is one of the most important parts of fly rearing, and has profound impacts on behavior. As my friend Monica Dus writes about, you are what you eat (and so are flies). Nutritional rearing conditions are very important as they can impact the health and size of the fly, which both impact behavior as adults. In my experience, the rearing media can impact the robustness and variability of behavior, how strong your effect is, and pretty much anything having to do with feeding, foraging, taste or odor responsivity, although these effects may be subtle.
In addition, sometimes, it’s not just the food conditions of the actual flies you are using for behavior, but the food conditions of their parents and their grandparents that matter. Basically, it’s really important to maintain happy healthy vials with nutritious food for behavior.
Each lab or institute usually has variations in their fly food, and this is dependent on the supplies you can get for the best prices, how much food you need, and how much the weather changes throughout the year (for example in RI humidity changes throughout the year which means we have a winter and summer recipe which basically just differs in the amount of water added). My experience also suggests the food each institute uses also depends on what food the first fly person at that institute used when they got there.
So the question is, which fly food should you rear your flies in for behavior experiments? Any recipe with enough nutrients is (most likely) OK to use. I’ve used probably 10 different recipes throughout the time I’ve worked on flies: from insta-food to cornmeal to molasses recipes. I’ve always been able to measure the behavior that I’m interested in, with subtle differences in variability in responses and effect size. The most important thing is consistency in food between vials and over time.
Most people will just use the food that is at the institute they are from and that will be fine. If you think your rearing food is affecting your behavior (like if you moved institutes and all of a sudden can’t get your behavior anymore and one of the things that is different is the food - I may be speaking from experience here), my advice is to characterize your behavior with your background strains on the recipe before starting a big experiment.
There is also some lore in the field although I don’t think any of this has been rigorously tested. For example, I’ve heard that molasses food might be better for complex behaviors like memory. That being said - I think it depends what odors you are using because we’ve reproduced a number of types of memory on standard molasses and cornmeal recipes. We use a standard cornmeal media in the lab now.
As I mentioned, consistency is key. Make sure that you take a peek at the food your flies were reared on whenever you collect flies for behavior. Note down any inconsistencies in your lab notebook. Things that I record when it seems off:
Density of larvae / pupae / flies in vial
Size of flies (too big or too small)
Amount of food in vial (sometimes when vials are poured it can include too much or too little)
Wetness / dryness of food (i.e. is the pulling away from the edges or are flies drowning in the food)
Color of the food (especially if you see pink or yellow or green as these are all indicative of contaminants)
Presence of any slime
My general rule of thumb is when you are first characterizing a new behavior assay, take notes on absolutely anything and everything and with experience you’ll figure out which are the important things to record. Below are a few example pictures of 'bad food' we've found to affect behavior.
Contaminated food vials:
Wet food vials:
Contaminated and dry food vials:
