uninjured rats are desired. Usually the catch in any type of trap will be best the first
night, so it is essential to set a sufficient number. A rat caught in each trap on the first
night indicates that an insufficient numbers of traps were used. If traps are kept set in
the same locations for a week, it is probable that no more rats will be caught, even
though rats are in the area. Rats quickly become "trap-wise," necessitating frequent
changes in settings and baits. If a rat proof building must be entirely freed of rats by
trapping, many snap and steel traps should be used in order to catch as many as
possible the first night. As the infestation is decreased, the number of traps may be
correspondingly reduced. More care then should be taken in the setting and
camouflaging of the traps to catch the more elusive rats. In a particularly difficult
situation, it usually pays to remove the traps for a week and to use poisoned baits
during the interim. Traps require servicing daily and sometimes twice daily. It is best to
set them late in the afternoon when there is less chance that they will be sprung by
careless people. Much time must be spent in this setting and daily servicing of traps if
good catches are expected. Rats often set off traps without being caught. Traps should
be cleaned whenever necessary to ensure efficient operation. A trap that rocks in its
setting will generally not catch rats, and warped traps are unserviceable. It is not
necessary to avoid handling traps, nor is it necessary to wash or sterilize them, because
human odors or odors from previously caught rats do not deter rats from approaching
them.
(2) Snap traps usually kill rats caught in them. All traps should be set on a
hairtrigger so that they will be set off in the slightest touch. A good test of sensitivity is
to touch the trigger with a flat piece of writing paper. If this is not sufficient to spring the
trap, the setting is improper or the trap needs adjustment. When set, the trigger should
be nearly parallel to the base of the trap and should not be up at an angle. Snap traps
may be baited or unbaited (see figure 3-2). A solid bait, about the size of the end of
Figure 3-2. Expanded trigger trap set in runway
MD0172
3-5