As we perpetuate the fruit fly circle of life, we need to start somewhere and I’m going to assume you already have some large amounts of egg laying flies in a population cage. These population cages are usually fed with yeast paste on some kind of sweet agar (molasses, apple juice, grape juice). Take the trays from the previous night’s feeding and gently wash the embryos into a double layer sieve so that the adult flies get stuck on the first layer and the embryos stay on the second. A boar hair paintbrush can be used to make sure all the embryos are recovered from the tray.
From here there are two ways to proceed: put the embryos in a box with yeast paste or put the embryos in bottles with banana paste and yeast. My fly strains didn’t do so well in the box so I’m going to show the bottle method.
To get an optimum number of flies in the next generation, around 120-150ml, you will need between 3-5 large trays of bottles with 40 bottles per tray. In each bottle tap a small amount of yeast.
Now using a nickel spatula, scrape up a pile of embryos no bigger than a grain of rice. Not those basmati rice grains, more like jasmine rice. Half of a fat sushi rice gain. I like to tap the yeast onto one side and the embryos onto the other so that the yeast doesn’t dehydrate the embryos. I don’t know if that would really happen but it also helps to find the embryo pile when it isn’t blending into the yeast.
Once you have put the embryos into the full tray of bottles, plug them up, move to your incubator, and wait around 8-10 days.
It is hard to estimate the correct amount of embryos to put in each bottle. Too many and the larvae will be stressed, develop slower, and churn up the media so that it is goopy. Goopy bottles are really hard to tap the adults out of so don’t do that. On the other side, underpopulated bottles waste bottles since you need more of them. One tray of full but not too full bottles will yield around 50 ml of adult flies.
To collect the adult flies, I use a 200 ml flask and place it in an ice bucket with a bit of ice. I don’t pack the ice around the flack, it can be loose. The ice will keep the flies from trying to escape. Place a large funnel in the flask and tap each bottle into the flask. Be gentle with the flies in the flask and don’t shake them too much as you tap.
Once you have 120-150 ml of adult flies, dump them into your prepared population cage and get ready to do this again in a week!