A Fruit Fly Trap

Drosophila melanogaster, the common fruit fly, is probably familiar to anyone who has kept ripening bananas at room temperatures out in the open (and not, for instance, in a refrigerator). These tiny insects will feed on any exposed fruit pulp, which provides a ready energy source in the form of fructose. Fruit flies are undeterred by any spoilage; in fact, they seem to thrive on the mildly acidic pulp of rotting bananas or discarded banana skins (which retain enough pulp to feed a great many of these miniature diners). They are also attracted to vinegar, and consequently are sometimes referred to as "vinegar flies". As we shall see, that attraction can prove most useful in your efforts to eliminate them.

Moreover, once established in your kitchen or any other household location containing fruit, these flies tend to proliferate rapidly. In fact, their quick lifecycle and large number of offspring make them a favorite object of study among biological researchers.

Fruit fly
Figure 1. Fruit fly

Unlike cockroaches, fruit flies are not known for carrying diseases; nonetheless, most people responsible for running a household would prefer not to have their kitchens infested with these creatures — feeding and pooping and reproducing. You can try killing them with a traditional flyswatter, but only if you have the visual acuity to see these tiny insects and you are willing to hit your ripening fruit with a swatter, possibly bruising the fruit. Alternatively, to kill them in flight, you could try using one of those electrified zapping racquets typically used against mosquitoes, but that may not work since fruit flies tend to dart about when airborne and can be difficult to see.

Another disadvantage is that even though whacking these uninvited guests may provide some satisfaction, do you really want to spend your time hovering over the fruit bowl or the trashcan, gripping a flyswatter or zapper racquet, hoping to spot and vanquish one of these fast-moving creatures? It would be wiser to invest a fraction of that time in creating a simple trap, which will work on your behalf, day and night, without any monitoring or other active involvement needed on your part.

Herewith is a passive trap that I concocted based on information gleaned from other articles (most of whose solutions, such as funnel traps, didn't work in my experience). Begin with a glass or cup of any size; a fully transparent glass has the advantage that you can quickly assess the effectiveness of your trap without having to remove the seal and thereby possibly allow the escape of one or more flies that have not yet died. Into this glass, pour a small amount of vinegar; in my case, I used apple cider vinegar, hoping that the apple odor would be more attractive than plain vinegar to the fruit flies (as well as the human residents). Then squirt in about a tablespoon of liquid dish soap, which apparently disrupts the nervous systems of the fruit flies, just as it does the same to dissuade ants from invading an area marked off with soap. The vinegar does the attracting and the soap does the killing.

Fruit fly trap
Figure 2. Fruit fly trap

Cover the top of the glass with clear plastic cling wrap. Using a sharp wood toothpick, puncture several holes through the plastic wrap, being careful to not tear the plastic. The tip of the toothpick will puncture the plastic, but if you use only the tip, then the resultant hole will be too small for most fruit flies to pass through. Therefore you must gently extend the toothpick downward until the plastic reaches the main body of the toothpick, enlarging the hole. Slowly remove the toothpick, and then use the tip to ensure that the rim of the hole is pointing downward. This is critical, because it will allow the fruit flies to enter the glass, but it will prevent them from escaping, because the plastic rim is then pointing toward them and they apparently are unable to fly or walk up through the hole. Yet they were able to move downward, into the glass, possibly due to their strong attraction to the vinegar smell, or the plastic stretching enough to barely allow them inside, or the assistance of gravity, or some combination thereof.

Fruit fly trap seal
Figure 3. Fruit fly trap seal

Set your new trap close to your fruit bowl or trashcan or wherever else the fruit flies appear to be congregating. Within a day or so, you should see at least one victim, floating dead in the apple cider vinegar. In the case of the trap I created, within a few weeks, plenty of fruit flies had visited and met their end.

Fruit flies trapped, side view
Figure 4. Fruit flies trapped, side view

If your trap manages to slay a large quantity of them, it could be difficult to count the exact number, but in this case my trap seems to have caught several dozen.

Fruit flies trapped, top view
Figure 5. Fruit flies trapped, top view

Best wishes in keeping your kitchen free of Drosophila melanogaster!

Copyright © 2024 Michael J. Ross. All rights reserved.
bad bots block