Behavior And Ecology

The biology and behavior of tabanid adults are better understood than of the immatures, The sex ratio at emergence is approximately 1:1, and emergence of males precedes that of females by 1 day to a few days. An important activity for both sexes is carbohydrate feeding, which provides energy for general body maintenance, flight, and mating. Sugars are obtained at floral or extrafloral nectaries or other natural plant sugar deposits. T. nigrovitta-tus adults feed on aphid honeydew. Males, in particular, engage in "dipping" behavior, touching the surface of pools of water with the mouthparts while in flight. This may serve to fill the crop with enough water to allow flies to regurgitate on honeydew deposits. The ingested sugars replenish energy reserves expended during daily flight and mating activities.

Tabanid mating occurs in flight, especially in the morning, but has never been induced in the laboratory. This is the key barrier to colonization. Most observations are of individuals or small groups of males (Tabaninae) hovering within 3 m of the ground along forest roads or ecotone areas or above natural features (e.g., plant clumps) which serve as markers. Some species apparently hover above treetops or forest canopies as high as 90—100 m above ground level. Males of other species, such as C. futigi-nosus and H. illota, perch on vegetation and other objects. In both case, males detect and pursue passing females. They also chase conspecific males and other passing insects. Males of H. hinei have been observed to pursue 8-mm beads shot past them at speeds of 27—30 m/sec.

Males of some species are thought to exhibit territoriality. However, what may appear to be agonistic interactions between males may actually be normal pursuit behaviors directed toward any appropriate-sized object moving through their response zone. Individual males do not necessarily use a particular site continuously over time. The occurrence of male aggregations -at the tops of hills, known as "hilltopping" behavior, is common for some species. The larger eyes of males reflect a mating strategy dominated by visual cues. The larger dorsal om-matidia may be more sensitive to ultraviolet light, allowing the male to detect a fast-moving female against the sky, and the smaller ventral ommatidia may be used to resolve visual details. While pheromones are suspected for a few tabanid species, this has yet to be proved.

Adult feeding activity is typically diurnal but occasionally crepuscular or nocturnal. It is affected by changes in environmental conditions, particularly temperature and barometric pressure. Species that feed diurnally generally attack hosts throughout the daytime, with discernible periods of higher activity. Females of T. wilsoni and T. pallidescens tend to feed near midday, whereas T. abactor feeds more frequently in late afternoon or early evening. Species which feed during crepuscular periods, particularly dusk, include Chlorotabanus crepuscularis,

FIGURE 13.7 Tabanus sulcifrons feeding on a cow. Note the blood droplet from a prior bite wound. (Photo by B. A. Mullens.)

L. annulatus, T. moderator, and T. equalis. Some crepuscular species feed into the early night.

Tabanid females usually mate before they seek a vertebrate host. Males do not feed on blood. Most species, particularly the Tabaninae, feed on large mammals such as cattle, horses, and deer (Fig. 13.7). Deer flies often attack large mammals, including humans, but there also are records of Chrysops species feeding on ravens, crows, ducks, and robins. Reptiles such as turtles are occasionally attacked.

Tabanids are strong fliers and readily disperse several kilometers in short-term flights. Adult dispersal probably is influenced by host availability. Mark-re lease-recapture studies with tabanids may yield 3—6% recovery, which is very high for insects. This suggests that local populations tend to remain in a given area, with dispersal occurring in a series of short flights. Marked T. abactor females have been shown to return to a host at the same site where they had obtained a blood meal 3—4 days earlier.

The attack rates by Chrysops species, and to a lesser extent by Tabanus and Hybomitra, vary substantially in different habitats. Many Chrysops species, for example, tend to frequent forest edges or ecotones, attacking in large numbers a host entering these specific areas from adjacent open fields. Dark-colored hosts, or even dark areas on a black and white animal such as a Holstein cow, are often favored for attack.

Many tabanids are selective in attacking specific body regions of their hosts (Fig. 13.8), regardless of color.

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