Chironomidae: The Secret Behind Mass Outbreaks

Chironomidae, which are an important part of the ecosystem, can sometimes swarm and cause confusion among people. (Added this note)

Chironomidae can be found anywhere there is a puddle.

Chironomidae larvae grow in water, from clear streams to dirty water, in rivers, gutters, and ponds, and the adults emerge from the water. They are very familiar insects, with about 2,000 species of various sizes said to exist in Japan. Since the larvae grow by eating organic matter in the water, they are also important creatures that help clean the water.
Although they are close relatives of the "mosquito," many species have no mouthparts and do not suck human blood. The main nuisance is that the adults can swarm in large numbers, forming large "mosquito pillars," covering walls, or piling up in heaps of dead bodies, which is considered "unpleasant."
The lifespan of an adult is very short, and most die within a week.
Furthermore, there are no reports in Japan today of them transmitting pathogens or viruses that cause infectious diseases.

In an ecosystem, where there is a rich variety of species, the number of creatures does not fluctuate significantly because there are many predators and competitors.
However, in places where major environmental changes have occurred or in environments that have been artificially created recently, the variety of species is not rich, so mass outbreaks of specific organisms, not limited to Chironomidae, tend to occur. This is also one of the natural phenomena that occur during the process of ecological succession.
Also, mass outbreaks always have a peak, and the dominance of the same creature does not last forever. Mass outbreaks have an "end."
(Note that the peak of an outbreak may occur again after some time, depending on the species and the environment.)

"Shio-yusurika" (Salt-marsh midge) that emerges in brackish and seawater

A species of Chironomidae called "Shio-yusurika" is currently emerging at the venue of the ongoing Osaka-Kansai Expo. This midge is a species that unusually emerges in brackish water where seawater and freshwater mix, or in shallow seawater such as tide pools.
There are not many reports of mass outbreaks, but there is a record of a mass outbreak in a port area under reclamation in Osaka in 2000.

Shio-yusurika seem to hide in the plants within the Expo venue or in places not exposed to much wind during the day, but they engage in a behavior called "swarming" at dusk. The adult males form a "mosquito pillar" in a group and wait for females to fly into it.
Also, at night after they finish swarming, many may be attracted to artificial lights.
Insects that successfully mate lay jelly-like eggs in the water, and the larvae grow in the mud underwater. Note that they cannot grow without water, so they never increase by laying eggs in soil or on the walls of buildings.
Also, since Shio-yusurika is a species whose larvae grow in salt water such as brackish water or seawater, it is unlikely that this species will breed in large numbers in other freshwater areas such as ponds within the Expo venue.

Many adult Chironomidae have no mouthparts, and some species even have degenerated digestive tracts. Once they emerge, they just need to successfully mate and lay eggs, and then they die immediately; they are insects that are annoying due to mass outbreaks but are also ephemeral.

Postscript 1: Situation of Shio-yusurika at the Osaka-Kansai Expo venue as of May 30

When I observed the site again on May 28, although there was a slight decrease from the observation on the 22nd of last week, the numbers were still very high.
Shio-yusurika larvae and eggs were still seen in the "Water Plaza" and "Sea of Connections," and new insects continued to emerge. However, the number of larvae seemed to have decreased slightly.
Most Shio-yusurika remain stationary on walls and plants during the day, but large swarms occur from around sunset until it gets dark. Also, after dark, many insects fly around, attracted to lights.
Since the emergence and activity of insects change significantly depending on the weather and temperature, there are days when there are few, but until the outbreak subsides, there is a possibility that you will encounter large swarms if you are outdoors near the sea, such as the Water Plaza, after 18:30, from sunset to night, for a while longer.

Postscript 2: Subsequent status of Chironomidae

I continued to conduct surveys of the Chironomidae at the site and observed their trends, but because the range of the outbreak was extremely vast, pest control using chemicals was difficult, and because basic data on Shio-yusurika was scarce, predicting the outbreak was extremely difficult.
Nevertheless, after the major peak in late May, the emergence of Shio-yusurika settled down while repeating fluctuations, and after reaching a medium-scale peak in September, the Expo closed safely without reaching another mass outbreak.
At KINCHO, to prevent Shio-yusurika from entering buildings, we installed commercial "Mushi-yoke Katoris Pro" at the entrances of pavilions, shops, restaurants, etc., and were able to obtain practical intrusion prevention effects. In addition, the commercial "Mushi-konars Spray Type" was also well-received at pavilions and shops.
The water areas at the venue were drained by the end of the year after the closing, and the source of the outbreak was eliminated, so the possibility of another mass outbreak of Shio-yusurika at that location has been eliminated.
This mass outbreak of Shio-yusurika at the Osaka-Kansai Expo has become valuable observation data, which will likely be utilized for pest control in future landfill development and water area management.