Adult host-seeking black flies (Simulium spp.) can often be severe biting pests of humans. The irritation associated with these bites can be considerable and can often make life miserable in areas where black fly populations are in great abundance. Moreover, bites may become itchy and swollen for a number of days. In sensitized individuals reaction to black fly saliva injected at the feeding site may cause a syndrome known as “black fly fever” that consists of headaches, fever, nausea, and/or inflammation of nymph nodes (Harwood & James 1979).

Larval habitats for black flies primarily consist of swift running water, with shallow mountain torrents being favored places (Harwood & James 1979). In Florida, these habitats are not present. Stone & Snoddy (1969) reported that some species prefer slow flowing streams and swamp rivers. These habitats are ubiquitous throughout the State. In 1998, a severe outbreak of adult Simulium slossonae Dyar and Shannon occurred in west central Florida (particularly Pasco County). Several reports of chicken mortality caused by adult black fly feeding had been reported in the State during the first three months of that year (Butler & Hogsette 1998). Although this species is primarily a bird feeder, large swarms were often attracted to people causing considerable annoyance (Butler & Hogsette 1998). Because this was an unusual event, the authors wanted to document seasonal occurrence and abundance of this species in Pasco County. In addition, we discuss the climatic events that led up to that outbreak.

Materials and Methods

Black flies were collected in stationary suction traps, primarily used for mosquito population surveillance, by Pasco County Mosquito Control District (PCMCD) personnel from 1997 through 1999. This trap is similar to that described by Bidlingmayer (1971). Collection data were obtained from daily catches from 35 traps placed throughout the District (covering 855 km²). In 1998, larval samples from submerged vegetation were periodically obtained from rain-swollen streams by PCMCD staff to determine production sites for emerging adults. Adult and larval samples were sent to Peter Adler, Department of Entomology, Clemson University for identification.

U.S. National Oceanic and Atmospheric Administration monthly total precipitation databases (including monthly normal levels) for 1997-1999 were obtained from their data monitoring station at Tampa International Airport (NOAA 1998b; NOAA 1999; NOAA 2000).

Results and Discussion

Suction trap collections from 1997, revealed that adult S. slossonae were present in Pasco County from May through November at an average of ≤2 black flies per trap month (Fig. 1). In 1998, collections of this species started to increase greatly with a primary peak in March and a slight secondary peak in October. During March, some suction traps had collected nearly 2,000 black flies.

Suction traps located along the Anclote River, Pithlachascotee River, and stream systems in the Starkey Management Area, consistently collected the greatest number of adult S. slossonae. These watersheds were probably the primary source of black fly infestation in the County and fit the description by Stone & Snoddy (1969) as slow moving southern swamp rivers/creeks favorable for larval development of this species. Indeed, submerged leaves and branches examined from those watersheds revealed several hundred attached S. slossonae larvae.

Simulium slossonae has previously been reported to occur widely in Florida with immature and adult specimens collected throughout the year (Stone & Snoddy 1969; Pinkovsky & Butler 1978; Butler & Hogsette 1998). But black fly populations reported for the State of Florida had never before increased to the pestiferous levels experienced in 1998. The outbreak of S. slossonae during that year appeared to have resulted from above average rainfall during October through December, 1997, and again February, 1998 (Fig. 2). Rainfall was reported to be 8 to 10 times above normal levels in several counties (including Pasco) often swelling stream and river systems to overflow in early 1998 (Morris 1998). Indeed, the National Climatic Data Center (NCDC) reported that the extreme rainfall experienced in central Florida during the latter part of 1997 and beginning of 1998 was associated with El Niño. This event produced 125% to nearly 300% that of normal precipitation levels (NCDC 1998a). According to NCDC, November 1997 to March 1998 had been the wettest reported since records were started in 1895.

During April-June, 1998, adult S. slossonae populations had declined considerably (Fig. 1). This period was the driest interval on record (NCDC 1998a). Obviously the precipitation deficits had a limiting effect on larval production (and subsequent adult emergence) through decreased aquatic habitat. During August, rainfall in west central Florida returned to normal or slightly below normal levels (Fig. 2) where, in October, a small peak of adult S. slossonae was recorded in traps from Pasco County.

In 1999, adult black fly populations were almost nonexistent (≤3 black flies collected per trap month) (Fig. 1). Larval habitats were not as abundant as the previous year with precipitation levels 1.5 cm to nearly 7.0 cm below normal (Fig. 2).

From our observations, and the data from west central Florida, we found that when above average precipitation events occur in the form of an El Niño weather system, they can trigger a quick build up of adult pestiferous S. slossonae populations. Apparently this species can rapidly exploit rain-swollen watershed habitats as larval production areas thereby producing enormous populations of host-seeking adults. Indeed, observations during the first half of 2003, revealed that S. slossonae again had risen to pest population levels in Pasco County (J.F.S., unpubl.) by exploiting rain swollen streams produced from another El Niño system during the winter of 2002-2003 (NOAA 2003a; NOAA 2003b).