Manual for Diagnosis of Screw-worm Fly - xcs consulting

8.3 Sentinels (Fig. 35, Plate 8)
Agriculture, Fisheries and Forestry - Australia
A wounded sentinel steer will attract about 4-5 times more SWF than a swormlure trap.
Sentinel animals such as cattle and sheep are used primarily to attract gravid females
which lay egg masses on the edge of the wound. The egg mass and/or resulting larvae are
collected for diagnosis. During eradication programs employing sterile insect release, sterile
and fertile egg masses indicate the relative success of the releases and sentinels are crucial
for such monitoring.
A three-panel pen which can be used as a crush for cattle has been used extensively by
CSIRO for egg mass collections from sentinels throughout Papua New Guinea. Each panel
(1.5 x 2.7m) is made form 20mm OD galvanised steel pipe with rails and support struts at
25cm intervals. One or two panels are secured to a tree or steel post and the third panel
attached to form a triangle. All attachments are by 12 gauge fencing wire. The animal is
provided with water and a feed box containing lucerne chaff or hay. Fresh grass can also
be provided if available.
To restrain the sentinel, one panel is opened out a little to enable the second panel to be
pushed inwards, with the animal facing the point of anchorage.
Where smaller animals such as sheep and goats are available for use as sentinels,
transportable sheep pens, as developed by the Mexican-American Commission for the
Eradication of Screw-worms, can be deployed (Fig. 35).
Pens should be sited where SWF activity is most likely such as the edge of forested areas,
near sources of water, cattle and sheep camps or, in more open country, by using salt blocks
and molasses to attract livestock in the area.
To attract SWF and stimulate oviposition by gravid females, a wound with some
sero-sanguinous exudate is necessary. With a scalpel blade, a 30-50mm incision is made
through the dermis and into the underlying gluteal muscle of the rump. The upper rump
gives easy access for observation and removal of egg masses. Egg masses are laid around
the dry edges of the wound. Egg mass removal and servicing of sentinel pens is done daily,
preferably during late evening or dusk when SWF activity decreases. The sentinel is held still
(using the crush action of the cattle pen) and egg masses peeled off the host with scalpel
and forceps. Egg masses in labelled pill boxes are stored in a freezer unless required for sterility
check or rearing purposes.
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