206
Gene R. DeFoliart
have been identified that affect the survival of the virus in nature (DeFoliart,
1978), and undoubtedly others have not yet been detected. Suitable quantification
of those parameters could require several lifetimes of research.
In the meantime, Ae. triseriatus is an enticing target for control by source
reduction. The natural breeding sites are treeholes that contain water, and in
western Wisconsin, where LAC virus is endemic, large gallery forests are the
main centers of production and dispersion. Control or elimination of the vector
from these large forests is not feasible with current technology. However, Ae.
triseriatus also breeds readily in discarded tires and other man-made containers
that collect water. Quantitative documentation is lacking, but the available evidence suggests that a high proportion of La Crosse encephalitis cases in Wisconsin
could be prevented through simple sanitation-the removal of old tires and other
man-made containers from the vicinity of human habitations. Such containers
around farm and suburban hillside homes and in shaded residential neighborhoods
are readily colonized by female mosquitoes emigrating from their woodland
haunts, and with transovarial transmission of virus to progeny, they serve as
long-term foci of infection that are in intimate contact with humans.
Source reduction may also be warranted in some cases for reducing Ae.
triseriatus populations in small woodland areas within or adjacent to areas of
high human population density. Admittedly, treehole detection and closure is
a labor-intensive method, but treehole-filling materials are now known that last
3 years or longer (Scholl and DeFoliart, 1979), so that an area once worked
would not need to be reworked for several years. A previous attempt to reduce
Ae. triseriatus by basal treehole closure was considered only moderately successful (Garry and DeFoliart, 1975). Measured by oviposition, the mean reduction in four test plots ranging in size from 8 to 20 acres was 67%. Deterrents
to higher reductions in that study were considered to be open barrier zones of
insufficient width to prevent infiltration from adjacent wooded areas, failure to
remove old tires, and the possible presence of arboreal treeholes. In retrospect,
it also would probably have been better to tabulate results on the basis of biting
counts, as there is reason to suspect that there is a significant amount of mortality
between oviposition and the ensuing bloodmeal.
Two questions must be answered, however, in order to place source reduction efforts on a sound footing. Scholl et al. (1979) found a biting rate of
28 per hour at ground level in a hyperendemic locality, but the level of biting
activity at which virus transmission to humans is interrupted or significantly
reduced remains unknown. Secondly, the proportion of an Ae. triseriatus population produced by arboreal treeholes, i.e., those not readily detectable by
ground level inspection, is unknown. The ratio of arboreal to basal treeholes is
probably small, but undoubtedly varies somewhat depending on age, type, and
history of a given woodlot. A relevant factor is the presence of a competitor
species, Ae. hendersoni; although closely related to Ae. triseriatus, Ae. hen-
Gene R. DeFoliart
have been identified that affect the survival of the virus in nature (DeFoliart,
1978), and undoubtedly others have not yet been detected. Suitable quantification
of those parameters could require several lifetimes of research.
In the meantime, Ae. triseriatus is an enticing target for control by source
reduction. The natural breeding sites are treeholes that contain water, and in
western Wisconsin, where LAC virus is endemic, large gallery forests are the
main centers of production and dispersion. Control or elimination of the vector
from these large forests is not feasible with current technology. However, Ae.
triseriatus also breeds readily in discarded tires and other man-made containers
that collect water. Quantitative documentation is lacking, but the available evidence suggests that a high proportion of La Crosse encephalitis cases in Wisconsin
could be prevented through simple sanitation-the removal of old tires and other
man-made containers from the vicinity of human habitations. Such containers
around farm and suburban hillside homes and in shaded residential neighborhoods
are readily colonized by female mosquitoes emigrating from their woodland
haunts, and with transovarial transmission of virus to progeny, they serve as
long-term foci of infection that are in intimate contact with humans.
Source reduction may also be warranted in some cases for reducing Ae.
triseriatus populations in small woodland areas within or adjacent to areas of
high human population density. Admittedly, treehole detection and closure is
a labor-intensive method, but treehole-filling materials are now known that last
3 years or longer (Scholl and DeFoliart, 1979), so that an area once worked
would not need to be reworked for several years. A previous attempt to reduce
Ae. triseriatus by basal treehole closure was considered only moderately successful (Garry and DeFoliart, 1975). Measured by oviposition, the mean reduction in four test plots ranging in size from 8 to 20 acres was 67%. Deterrents
to higher reductions in that study were considered to be open barrier zones of
insufficient width to prevent infiltration from adjacent wooded areas, failure to
remove old tires, and the possible presence of arboreal treeholes. In retrospect,
it also would probably have been better to tabulate results on the basis of biting
counts, as there is reason to suspect that there is a significant amount of mortality
between oviposition and the ensuing bloodmeal.
Two questions must be answered, however, in order to place source reduction efforts on a sound footing. Scholl et al. (1979) found a biting rate of
28 per hour at ground level in a hyperendemic locality, but the level of biting
activity at which virus transmission to humans is interrupted or significantly
reduced remains unknown. Secondly, the proportion of an Ae. triseriatus population produced by arboreal treeholes, i.e., those not readily detectable by
ground level inspection, is unknown. The ratio of arboreal to basal treeholes is
probably small, but undoubtedly varies somewhat depending on age, type, and
history of a given woodlot. A relevant factor is the presence of a competitor
species, Ae. hendersoni; although closely related to Ae. triseriatus, Ae. hen-