• Chelle Hartzer

Oh Karst! (AKA: What the phorid?)

I once had a situation of phorid flies in a home. Since phorid flies are attracted to wet, decaying material, the first thing to do was look for the food source. Since most of the flies were being found in the kitchen area, it made sense to start there. The whole kitchen was thoroughly inspected. The garage was right next to it and since food (pet and bird food) is often stored in garages, that was looked at as well. There were a few windows that led directly outside, so the outside was inspected for wet areas with decaying vegetation. No infested food was found, and no outside sources could be identified. No carrion (phorid flies like dead stuff!) was nearby.

Karst is “a specific type of terrain formed by the dissolution of carbonate rocks and is characterized by the presence of sinkholes, caves, springs, and sitting streams.” Urban karst is a little less defined but is basically those basement, underground, sewer, and other man-made “caves”. Because these are below ground and range from damp to holding water, they are a perfect spot for many pest insects.

Ancient societies knew that caves provided relatively constant temperatures and high humidity. This is why food and wine were stored in them and eventually, caves were specifically dug for the purpose of wine storage. Just like providing a good habitat for bottles of wine, underground storage sites are the right habitat for pests – few predators, steady temperatures, and high humidity.


For our insects, pests, (in the case of this study, mosquitoes), the breakdown of our man-made concrete karsts provides an even more protected area. Water pools in and under cracks in the concrete, filling small pools of water for mosquito development. Those same cracks and crevices can accumulate decomposing organic material for other flies and cockroaches. This urban karst – our man-made caves – is contributing to our pest problems, allowing our pest problems to spread to areas that did not have the right climate for them, and making them harder to find.

The source of the phorid flies was eventually found, though it took a lot of time and effort. This house had a storm shelter built into the basement. The owner had stored some food and water in this space which makes sense: if you are going to be stuck in a storm shelter, food and water are important! However, one of the cans of soup had burst. Constant temps, high humidity, and a decomposing food source had created the conditions for the phorid flies to flourish. A nearby air vent was allowing them to travel up to the kitchen.


Man made structures provide habits for pests, and these habitats are often challenging to find. The urban karst in cities has provided additional safe habitats for pests to expand into areas that were previously uninhabitable because of temperature fluctuations.


A good pest control program can identify and mitigate these issues, though it isn’t easy. Does your program look at all the areas that are a potential issue and address them? We can help!


Lagniappe – there are an estimated 4,000 species of phorid flies in the world. Not all of them are pests, in fact, some can be beneficial!

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