3-4.
FLY SURVEYS
Fly surveys are made to determine what kinds of flies and how many flies are
present in an area. By looking up the ecology of common species in reference
Iiterature, personnel determine what larval habitats should be located and eliminated.
By comparison of successive surveys, control effectiveness is evaluated. Since it is not
practical to determine the precise number of flies, surveys are designed to give an index
of the population. The weekly index is the average number of flies observed from the
total fly survey stations. For example, the fly counting stations are observed weekly at
10 or more locations where flies are prone to be present. The average number of flies
observed per station in a given time is the index for that period or week. A bar graph
can be prepared and maintained which indicates, at a glance, the status of the fly
population as compared with previous surveys.
a. Fly Trap Surveys. Trap surveys have the advantages of securing a
reasonable cross section of the population for careful identification, securing an
approximate count of the relative number of the various species, and trapping flies alive
for laboratory study. For qualitative surveys of the fly population, the fly trap is a useful
tool. Traps vary in size, but the outlines of their construction are similar. See figure 3-4
for detaiIs on the construction of a typical fly trap. The operating principle of the trap is
simple. FIies are attracted to the baited trap and fly upward toward the Iight. Once they
pass through the small opening at the top of the cone and into the cage, they are not
Iikely to escape. Since success of the fly trapping depends upon getting the flies to
enter the trap, the kind of bait used is of prime importance. Since different kinds of flies
have different preferences for food, it is important to use all-purpose bait that includes
some fish or chicken parts along with some waste vegetable or fruit. The bait should be
placed in containers about 2 inches in depth and slightly smaller in diameter than the
cage cylinder. The trapped flies are killed by placing the trap in a tight container and
exposing the flies to chloroform or carbon monoxide and dioxide, or by freezing. The
collections are measured by volume, weight, or actual count.
b. Fly Grills. Fly grills (see figure 3-5) are widely used in modern evaluation of
fly populations. They are faster than baited traps or fly cone surveys and give a highly
valid picture of the fly situation. The fly griII depends upon the tendency of flies to rest
on edges and so it presents many attractive resting sites. The griII is placed over
natural attractants and the number of flies landing on the griII during a 30-second
interval is tabulated. When the griII is put down, the flies are somewhat disturbed and
fly upward a short distance. When all is quiet again, they come back down, landing on
the griII instead of the attractant. The inspector then records the total number of flies
resting on the griII. If fly counts are so high that total counts are impractical, the griII
may be divided into halves, quarters, or sixths, with painted markings. At least one-
sixth of the griII must be counted. A minimum of 10 counts is made in each block
sampled, and the 5 highest counts are recorded.
MD0170
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