Relationship between host attack rates and CO 2 -baited insect flight trap catches of certain Symphoromyia species.

Abstract

Correlation coefficients of 0.612 and 0.884, respectively, for the numbers of females of Symphoromyia sackeni and members of the S. pachyceras complex attacking deer versus those caught in traps emitting ca 2.4 liters CO2/min. established that trap catches excellently reflected the day-to-day activity of the host-seeking portion of the fly populations as measured by their proportional activity at unrestrained hosts. The adjusted x¯ numbers of females attacking hosts per day were nearly identical to the x¯ numbers of females caught in traps per day. As traps also proportionately caught females in the same gametogenic states as those collected from deer, we conclude that the traps functioned as substitutes for deer for these host specific flies, particularly with respect to the numbers of the S. pachyceras complex caught each day versus the numbers attacking deer. This is the first report of a trap which quantitatively functions as a substitute for the natural host with respect to the daily attack rate of a hematophagous arthropod. There were highly significant differences among sites with regard to trap catches of both S. sackeni and members of the S. pachyceras complex. Catches of the latter were proportional to the percentage canopy cover surrounding trap sites, whereas wind direction had the greatest effect on the numbers of S. sackeni caught at each site. The relationships between canopy Cover, trap locations and catches, and wind directions showed that both S. sackeni and members of the S. pachyceras complex responded to the traps by orienting in an upwind direction and that the flies were being attracted from distances of up to 50 to 60 m. Comparison of the ratios of S. sackeni to the S. pachyceras complex at deer to their ratios in traps showed that the former species remained near its peak population density whereas the population of the latter declined rapidly during the study. Other data indicated that the traps were less attractive for S. sackeni than for members of the S. pachyceras complex. Although S. sackeni was considerably more abundant during the study than the members of the S. pachyceras complex, and the species which we nearly always caught in fewer numbers than expected in the traps, it was the species whose segments of the populations were strikingly trapped out from about a 5-ha area by a concentration of 5 traps. This seeming paradox is c1arified by postulated differences in the behavior of S. sackeni and members of the S. pachyceras complex. In addition to Symphoromyia species, CO2-baited traps caught large numbers of many obligate and facultative mammalophilic bloodfceding species. None or only few of any specieswere caught in unbaited traps.