Although this is from a popsci article, a biologist says that fruit flies can normally land on liquids and take off from them thereafter:
However, simply leaving a shot glass full of fruit-vinegar will only serve to feed the flies. The surface tension of the liquid [Ed. note: You remember surface tension, right?] allows fruit flies to land on liquids but not drown.
To break the surface tension you’ll need a tiny bit of dish soap. Dish soaps contain surfactants, which are molecules with a head that likes water, and a tail that doesn’t. Because of this property, the surfactants tend to situate at the interface of air and water, forming molecular “cliques” that disrupt the surface tension of water.
By putting a tiny bit of dish soap on your finger, and touching it to the surface of the vinegar in the shot glass, you will add enough surfactant to break the surface tension. (Add too much and the molecules in the dish soap will alert the fruit flies to danger).
Now fruit flies will unknowingly attempt to land on the vinegar for a little sip, then, before they can react, will break through the surface tension and begin to drown.
So if this is true/confirmed, then they do have enough of an evolutionary adaptation to the [surface tension of the] usual liquids of their diet. Just not to [trace] dish-soap contaminated liquids.
And I can confirm from a peer-reviewed paper that dish soap is indeed used even in science trapping experiments:
The cup trap (figured by Beers et al. 2010) is a 946-ml clear plastic
drinking cup with a clear plastic lid (Solo, Urbana,
IL), with four 1-cm-diameter holes in the sides of the
cup near the top. A drowning solution or drowning
solution plus bait was placed within the cup. Forty
grams of boric acid and 0.5-ml liquid dishwashing
soap (Palmolive Pure and Clean Spring Fresh Dishwashing Soap; Colgate-Palmolive Company, New York, NY, USA) were added to a batch of 3.8 l of
water or aqueous bait to make a drowning solution
used in all traps. Boric acid served to inhibit microbial
activity of baits and captured insects, and soap
reduced surface tension to promote capture and submersion of trapped insects. For both types of traps
and all treatments in all experiments, each trap contained either 300 ml of drowning solution or 300 ml
of bait in drowning solution
0.5ml of soap to 3.8l of water is very little, but that's all it takes to drown the little flies. It seems entirely plausible to me to have this level of trace contamination on glasses that have been dishwashed.
And as a complementary survival mechanism, fruit flies can apparently survive 12 hours of drowning at room temperature (or more in lower temperature):
Flies submerged at 23°C did not survive after a submersion time of 12 h whereas flies submerged at 3°C survived up to 72 h of submersion.
@Frieke may be on to something, namely that ethanol lowers the surface tension of the solution (with water) by itself, significantly. At 20-25 degrees C, pure water has a tension of about 72 mN/m. Adding 10-15% ethanol lowers that to about 47-42 mN/m. Also, the trapping science paper does mention that
Very few flies were captured in traps baited with 10% ethanol in water. Traps baited with a combination of acetic acid and ethanol in water captured more SWD flies than traps baited with acetic acid or ethanol solutions alone. These results indicate a synergy of the two materials, and of the two chemicals, as lures for SWD.
Interestingly wine has substantially higher attractive effect than an equivalent ethanol solution though:
Wine is about half as effective a vinegar and about a quarter as effective as a combination of wine and vinegar. On the other hand, a 10% ethanol solution was 20-50 times less effective than alcohol 10% plus acetic acid 2%. So there's more to wine than ethanol that attracts the flies.
So the flies probably do avoid the higher alcohol concentrations... unless there's also vinegar.
Another paper finds that wine's surface tension is mostly determined by its alcohol content.
So it does look like:
- Fruit flies are fairly strongly attracted to wine (on the same order of magnitude as to vinegar).
- The surface tension of wine is substantially lower than that of water.
I'm still not sure what the surface tension of (2%) vinegar might be, in comparison. Pure acetic acid has substantially lower surface tension compared to water, about 27 mN/m at room temperature. And if Wikipedia is correct, 10% acetic acid solution has about 55mN/m around temperature. So acetic acid lower the surface tension less than ethanol, but not by much less. On the other hand, fruit flies are much more attracted to acetic acid than to ethanol (alone). So perhaps that's enough to explain why they drown easier in wine.
Actually it look like Wikipedia is wrong here. From an authoritative source, acetic acid has nearly the same effect as wine on surface tension; at 11.4% it has 43.5 mN/m; just 3.2% is enough to lower the tension to 55.6mN/m; at 1.6% it's 61.7mN/m.
So probably just the fact that acetic acid is much more attractive to flies than ethanol is the only factor (other surfactants excluded). Perhaps they "confuse" a higher concentration of ethanol to a lower concentration of acetic acid, which they would be able to handle as far as surface tension goes.
Yet another source (actually quoted in the previous one, on the next page) gives even lower values for ethanol surface tension.
Apparently determining surface tension is a hard problem, both experimentally and theoretically. So I'm not sure we can properly compare figures reported in different sources. Alas no single experimenter has done both ethanol and acetic acid in the same paper/setup. But what was observed about their relative attractiveness to flies is probably sufficient...
And for ballpark comparisons, a similarly or lower surface tension as that of 10% ethanol (or acetic acid) i.e around 30-40mN/m can be obtained by a adding a few ppm (parts per million) of a surfactant, such as LAS or AE.