Black Fly Outbreak in Central Texas

Bastiaan “Bart” M. Drees, Professor and Extension Entomologist, Texas Agricultural Extension Service, May 22, 2012

Fig. 1. Black fly, <i>Simulium meridionale</i>.  Photo by Bart Drees.

Fig. 1. Black fly. Simulium meridionale. Photo by Bart Drees.

For about 2 weeks beginning in mid-April, reports of “gnats” have been received from the Navasota and Bryan/College Station area. Other people have reported mosquitoes that are biting people during the day – particularly around the neck and head, and leaving persistently itchy swollen areas at the site of the bites. The actual culprits have been a black fly species, Simulium meridionale, also commonly called buffalo gnats or turkey gnats (Fig. 1). Black fly species are small (5 mm or less) flies. The flies are day-active, fast and strong fliers. Female flies have mouth parts modified to bite and feed on blood. They attack people as well as domestic animals, poultry and wildlife. The number of species of black flies in the world is now well over 2,000.  See the world inventory of black flies.   (Pers. Comm. Peter Adler).

The developmental stages of black flies are larvae found just beneath the surface of rapidly flowing fresh water, and their presence may be an indicator of good water quality. However, S. meridionale doesn’t need waterfalls or rocky riffles for larval development.  They do well in sandy-bottomed piedmont streams and slow, muddy bayous in Louisiana. Evidently, the adult flies disperse great distances from their breeding habitats because they have been occurring far from streams and rivers in urbanized areas of Bryan and College Station.

Fig. 1. Black fly, <i>Simulium meridionale</i>.  Photo by Bart Drees.

Fig. 1. Black fly, Simulium meridionale. Photo by Bart Drees.

During periods of adult activity biting flies can become very annoying. Staying indoors is one option, but working outdoors may require dressing in long pants and long-sleeved shirt and wearing a cap and possibly a mesh veil. Insect repellents can be applied, particularly to the neck and head, but may not be very effective against these strong, fast flies. Attacking flies can be collected using a fine-mesh net swung around one’s head. Bites can be treated with over the counter insect bite remedies to help deaden the itching, but some people are very sensitive to the bites and may need to visit with their medical doctor.

For horses and livestock, individual animals may be stabled during the day when populations are abundant, but this practice is not practical for large herds. Fly repellents applied to the chest, belly and ears are somewhat effective but require daily application. Muslin or cotton bags fitted over horses’ ears may be used to prevent fly entry of those species attacking these areas.

Activity diminished and ended by early May. However, outbreaks are likely to reoccur in future years at this time whenever conditions for them to successfully breed in high numbers are present. The high amounts of rainfall received to date this year certainly could have contributed to aquatic habitats needed for their development.