Shizuoka University encountered outbreaks of H. breviantenna in approximately 200411. Many individuals were trapped in 2007 but not in 2008 (Fig. 1). We did not record any individuals after 2009, and there were no outbreaks at the university. Additionally, the abundance of this species gradually decreased until 2010 and became stable at a low density (Tagami, personal observation). No significant differences in precipitation and temperature could be detected between 2006 and 200712. We began our investigations on this species in Tajimi City in 2020, as outbreaks have been reported since 2010. In the city, outbreaks have occurred during the rainy season, similar to Shizuoka University (Figs. 5–8: Tagami, personal observation). The seasonal abundance in Tajimi city must be similar to that in Shizuoka, but annual differences in the outbreaks between the two distant localities remain unclear.
When we investigated the occurrence time of H. breviantenna at one day in 2007 at Shizuoka University, all case outbreaks occurred in the morning, especially from 9 to 10 AM8,11(Fig. 3). In Tajimi City, we collected samples until 10:00 AM because the adults almost disappeared after that time. We usually observed many individuals until 10:00 AM, and it became difficult to observe any adults flying after that. Mayflies are well-known insects that experience mass swarming in the evening13. In the case of mayflies, they practice mass swarming as a mate-seeking behavior14. Compared to this example, however, the H. breviantenna adults that appeared were almost exclusively females, and it was difficult to find males (Figs. 1 and 4). Thus, the outbreaks of this species cannot be sex-seeking behavior, as males were collected at only some limited numbers of the traps set at the forest edge (Figs. 5–7), but these males did not disperse. Future research is needed to determine why males occur and disperse in the morning.
We confirmed that H. breviantenna adults emerged from the forest floor (Fig. 4) and especially inhabited the forest edge (Figs. 5 and 6). It is possible that the females oviposited at the forest edges and then flew to open areas. This could be one of the reasons why we found a vast number of adults outside the forests, such as at houses and buildings.
Outbreaks of H. breviantenna sometimes occur in urban areas (Fig. 8). Both females and males were collected, indicating that they may also be reproductive in this area. We performed this investigation in 2022, and there were no outbreaks during this year (Tagami personal observation).
Outbreaks of H. breviantenna have been reported in Honshu, Shikoku, and Kusyu, Japan10. The adults appeared only in the rainy season and died immediately after emerging. It appears difficult for the species to spread over distant areas in nature, and it remains unclear why it spread over a broad area of Japan. Hyperlasion wasmanni has been found in many regions of the world15,16 and occurs in various habitats17. Many agricultural pest insects enter Japan from other countries with crops18,19. Furthermore, H. breviantenna is found in Australia9, although the relationship between these two populations remains unclear.
Sexual organisms oviposit on both females and males. Two modes of sex determination were found in Sciaridae: monogenic and digenic20. A monogenic female oviposits either only females or males, and a digenic female oviposits both females and males20. Moreover, Sciara ocellaris changes the sex of progeny depending on temperature20.
In this study, outbreaks of H. breviantenna occurred in different habitats in the same season, and only females were found in these habitats at Shizuoka University, investigated in 2008, by yellow sticky traps (Fig. 3), and IES was instigated in 2020 by emergence traps (Fig. 4). In 2021 and 2022, however, we collected many males from forests using hyaline sticky traps (SMP, IES, and urban area of Tajimi city: Fig. 6–8). H. breviantenna males have intact wings9. We collected males using many hyaline sticky traps 30 cm above the ground (Figs. 6 and 7). Sciarid species need males to produce offspring20. Although some traps collected more males than females in our results (Fig. 7), the sex ratio biased females in total. Hyperlasion wasmanni was also observed to have a female-biased sex ratio4,16, 21. It remains unclear what kinds of environmental factors affect its reproduction. We did not observe whether the females collected were already copulated. Further investigations are needed to confirm monogenic or digenic changes in sex depending on temperature.
We need to review the reasons for outbreaks of H. breviantenna from various perspectives. Some factors stimulate outbreaks of several insects22, such as changes in food quality23, 24,25,26, suitable weather27, and decreases in natural enemies or diseases28,29. Some pest insects, such as Lymantria dispar, periodically experience outbreaks30. There are two rainy seasons in Japan, south of Honshu. The first is from the end of May to mid-July, and the second is from the end of August to October. In addition to the H. breviantenna outbreaks during the first rainy season in 2007, some individuals appeared in the second rainy season (Fig. 1). Our results suggested that humidity may affect the emergence of adults. To test this hypothesis, we need to research its annual occurrence successively in multiple outbreak areas in Japan.
The outbreaks of H. breviantenna occurred in the open area of Shizuoka University and the urban area of Tajimi City (Figs. 1, 3, and 8). It occurs and reproduces in urban areas; however, it may be difficult to obtain sufficient reasons why outbreaks occur in urban areas.
The outbreak of H. breviantenna induces nuisance and sanitation problems not only in humans but also in commercial buildings. From this study, outbreaks occur not only from forest edges but also from urban areas with small gardens, woodlands, and parks. It is possible for humans to spray themselves with a repellent, but it is difficult to spray chemical pesticides on all personal and commercial gardens and forest edges. Spraying insecticides in the field has a poor effect on protecting the natural environment. In Japan, many houses use mesh screens at the windows from the rainy season to summer for ventilation and mosquito avoidance, as shown in Fig. 1. We preliminarily tested mosquito screens (the smallest aperture was 0.76 mm) for preventing invasion. Most individuals could enter homes through mesh screens (Tagami, personal observation). Fortunately, the adults disappear after noon. In houses, closing doors until noon is the best way to avoid invasion, but large dead bodies of H. breviantenna must be disposed of. Wearing face masks and glasses is important to protect the eyes and noses from this species when outside. As mentioned above, this species is distributed widely in Japan. Thus, we need to develop effective control measures when this species is broadly distributed.