A Pomace Fly Invasion

January 12, 2016

The beautifully-striped African fig fly, Zaprionus indianus. Photo by Kelsey Bailey. We always say that biodiversity is constantly changing in the Los Angeles area, but few groups of insects show this as blatantly as "pomace flies" do. This group, more formally known as Drosophilidae, includes the famous laboratory fly, Drosophila melanogaster, whose genetics have been the source of many of our advances in medicine and cell biology. Most of us know these flies because they "magically" appear when bananas become overripe on the kitchen counter, or they suddenly appear when a bottle of wine is opened. Their attraction to fermentation is also historical, with the first records of these flies in the literature noting that they are found in wine cellars. Growing up, we always called them "fruit flies", but that name is more properly reserved for another fly family, the Tephritidae, which includes the famous med fly.  Thus, the common names "pomace flies", or "vinegar flies" are more appropriate and less confusing (once you know why). Of course, like many other insects, the association of one species, in this case Drosophila melanogaster, with fermentation is just the tip of the iceberg in terms of life histories and diversity of species. Some drosophilids (the way we refer to members of a family like Drosophilidae is to call them "drosophilids") are associated with fungi, and can be seen in clouds over mushrooms on damp logs. Others are parasitoids, whose larvae attack and kill spittle bug larvae (a type of bug that produces a frothy mass to live in–they are often seen on Rosemary plants) . Still others attack plants, as leaf miners (literally living under the surface of the leaf and burrowing through the cells) or flower feeders, and one tropical group feeds on the embryos of frogs! According to my colleague, and world expert on the family, Dr. David Grimaldi of the American Museum of Natural History in New York, most pomace flies are not associated with fruits. Last year (2015), we reported on two unusual drosophilids from the BioSCAN project: one was a species previously known only from a handful of specimens from Central America, the other previously known only from Australia. Because of this, Lisa Gonzalez (one of the collection managers working on the BioSCAN project) keeps a close watch on the drosophilids from our samples. When I asked her a couple of years ago to watch out for the newly recorded Asian species Drosophila suzukii (the spotted-winged pomace fly), she quickly returned with specimens. More recently, last year, we received a bulletin from the Los Angeles County Agricultural Commissioners office about yet another newly recorded pomace fly, Zaprionis indianus, a beautiful orange colored fly with a couple of white stripes through its body. Although present in low numbers in the past, Z. indianus populations seem to have exploded in the last 6 months. The bulletin from Thursday, August 20, 2015, in part, read: The African fig fly Zaprionus indianus was found in backyard figs in Downey. It is a generalist drosophilid that breeds on fallen fruit and fruit on the tree. It is known to infest fruits of 70+ species of plants. Can possibly become a problematic pest for our fig industry. I brought this bulletin to Lisa's attention, and she relatively quickly found one from a Malaise trap sample from L.A. City Hall.

Z. indianus range as of 2010. Photo from Wikimedia Commons. Fast forward to today, with our initiation of phase 2 of the project, and suddenly Lisa finds these flies in virtually every Malaise trap sample in our "ocean to desert" transect! It is incredible how quickly this fly has gone from first recognition to complete colonization of the Los Angeles area. Because we've been looking for pomace flies in hundreds of samples over the last few years, we are able to track and recognize this explosive range expansion. It is sobering to think about how many other insects are being introduced, and rapidly spreading throughout the Los Angeles area, without anyone noticing. How large is the insect fauna of Los Angeles? Does the fact that this fauna is highly modified, with many native species negatively affected by urbanization, make it more susceptible to invasions like that of the African fig fly? How much turnover in species occurs among these tiny, and inconspicuous insects? Does the introduction of species like the African fig fly affect populations of other native or introduced pomace flies here? These are all questions that we hope to begin to address with our ongoing study 

(Posted by: Brian Brown)

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