Culturing fruit flies, primarily species of the genus Drosophila, is common for two main purposes. First, they provide a consistent, high-volume food source for small insectivorous pets, such as dart frogs, lizards, or fish, which benefit from the small size and flightless nature of cultured strains. Second, the fruit fly, Drosophila melanogaster, serves as a widely recognized model organism in genetics and developmental biology experiments due to its rapid life cycle. Creating a successful, self-sustaining culture requires careful preparation of the environment and the nutritional medium.
Essential Materials and Environmental Setup
The physical setup begins with the culture container, typically a plastic jar or vial with smooth sides. Containers must have a specialized lid or plug that allows for gas exchange while preventing the tiny flies from escaping. Foam plugs, cotton wool, or fine mesh cloth secured with a rubber band are effective ventilation options.
The container requires a substrate to increase the surface area for the flies to rest and for the larvae to crawl before pupation. Materials like excelsior, fine wood wool, or crumpled coffee filters are added before the food medium is introduced. This substrate prevents adult flies from getting stuck in the moist food and provides a dry space for larvae to undergo metamorphosis.
Maintaining the correct environment is necessary for optimal reproduction and growth. Drosophila species thrive at temperatures between \(70^\circ \text{F}\) and \(75^\circ \text{F}\) (\(21^\circ \text{C}\) and \(24^\circ \text{C}\)), which promotes a faster life cycle. Temperatures above \(80^\circ \text{F}\) can lead to sterility and culture failure. The area should maintain moderate humidity without excessive condensation, which is mitigated by ensuring sufficient air flow through the lid.
Preparing the Nutritional Culture Medium
The foundation of a successful culture is the nutrient-rich medium, which serves as both the food source and the egg-laying substrate. A simple medium often uses dehydrated potato flakes mixed with sugar and a protein supplement. The potato flakes provide carbohydrates and bulk, while brewer’s yeast is added to provide necessary protein and B vitamins, and to stimulate egg laying.
To prepare the medium, one part dehydrated potato flakes is mixed with one part water, a teaspoon of yeast per batch, and a small amount of sugar, such as corn syrup or molasses. A mold inhibitor is necessary to prevent the growth of fungi and bacteria, which can quickly ruin a batch. White vinegar or a chemical like methylparaben or propionic acid is mixed in. Vinegar acts as a preservative and makes the medium slightly acidic, which is favorable for the flies.
Once all components are thoroughly mixed to the consistency of thick oatmeal or mashed potatoes, the mixture is poured into the prepared culture containers. The medium should fill about one to two inches of the container’s bottom. Allowing the medium to cool and set for several hours before adding the flies prevents the adults from sinking into the hot, sticky mixture.
Initiating and Ongoing Colony Care
The colony is initiated by introducing a starter population of 20 to 50 adult flies into the new container once the medium has cooled. This number is sufficient to establish a robust culture. Adult females immediately begin laying eggs on the surface of the medium, with each female capable of laying up to 100 eggs per day at their peak.
The Drosophila melanogaster life cycle is completed in 10 to 14 days at the ideal temperature of \(75^\circ \text{F}\). Eggs hatch into tiny larvae in about a day, and these larvae burrow into the medium to feed for about five days. Mature larvae leave the food source, climbing the container walls or substrate to transform into pupae, which look like small, tan, rice-shaped casings.
Adult flies emerge from the pupal cases after about four days, marking the emergence of the first generation. Daily maintenance involves checking for excessive moisture, which can cause the medium to liquefy and drown the larvae. If the culture becomes too wet, adding a pinch of dry potato flakes or a small piece of paper towel helps absorb the excess liquid. Cultures must also be monitored for common contaminants like grain mites, which appear as tiny white or brown specks and can decimate a colony.
Sustaining Long-Term Culture Production
Sustaining a continuous supply of fruit flies requires starting new cultures before the current one declines. A culture is highly productive for about three to four weeks before the medium is exhausted by feeding larvae and waste buildup. This saturation and aging creates an environment where mold and mite populations are likely to bloom, leading to a colony “crash.”
New cultures should be established just as the first generation of adult flies begins to emerge in the parent container, typically 10 to 14 days after the culture started. This timing ensures a large, healthy population of newly emerged adults is available for transfer. Flies are moved to the fresh medium by tapping the culture container firmly to knock the flies to the bottom. The container is then quickly uncapped, inverted over the fresh container, and tapped again to move the flies into the new medium.
A minimum of two cultures should be maintained at all times, with start dates staggered by about a week, to create a production rotation. Once a culture is four weeks old, or shows signs of a mite infestation or heavy mold growth, it should be promptly discarded. This proactive renewal process prevents the spread of contaminants and ensures a reliable, continuous supply of healthy flies.