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Common Cattle Grub
Hypoderma lineatum (Villers), Oestridae, DIPTERA
DESCRIPTION
Adult -- The heel or
warble fly is a nonbiting, bee-like insect about 13 mm
long. It has 2 bands of yellow and white hairs across
its body, and the end of its abdomen has reddish-orange
hairs. On the thorax, there are four tiny, longitudinal
lines, and the legs are covered with black and orange
hairs. The wings are dark brown to black.
Egg -- The smooth
egg is dull yellow in color. A narrow oval, it is about
0.8 mm long and 0.25 mm wide. At one end it has a short
stalk with a clasping device that holds the egg to the
host's hair.
Larva -- When
first hatched, the larva is creamy white, densely
covered with spines and about 0.65 mm long. After the
first molt, it is about 13.5 mm long and has a pair of
spiracles on the back end. The third stage larva may be
as long as 28 mm and nearly 13 mm wide, with the upper
side flat and the bottom side rounded. Tapering slightly
at both ends, the larva has spines on segments 2 through
10. It turns almost black just before emerging from the
back of the host.
Pupa -- The
puparium is the hardened, dark, wrinkled skin of the
third-stage larva which envelops the pupa.
BIOLOGY
Distribution --
Cattle grubs were first recognized in the U.S. in the
late 1800s. They now occur throughout this country and
Canada.
Hosts -- The
cattle grub normally infests cattle. It was once common
on American bison and has been found on sheep, goats,
and horses. Some cattle grubs have even been removed
from man.
Damage -- The
adult flies are nuisances, occasionally causing cattle
to run wildly about with their tails in the air
(gadding) or to stand for long periods of time in deep
shade or water. Any of these defensive activities result
in reduced milk production and/or subnormal weight
gains. After hatching, larvae irritate the host's skin
by burrowing into it. Larval migration to the esophagus
and other organs, involving abnormal contact with these
organs, is injurious, and the cysts on the host's back
are swollen, often pus-filled areas which adversely
affect the host's health. This is often reflected by
loss of weight and a decrease in milk production. In
addition, at slaughter some of the damaged meat must be
trimmed, often from expensive cuts, and discarded; the
hide's value is also greatly reduced by the holes and
scar tissue.
Life History -- At
least a year is required for the completion of a life
cycle. In spring, eggs are deposited on the lower leg
hair of hosts, usually cattle. Up to 12 eggs are
deposited on each hair, and each female lays a total of
about 500 eggs.
Newly hatched larvae
immediately burrow into the skin at the base of the hair
and migrate through connective tissues to the diaphragm,
small intestine, esophagus, or heart. After a few months
the larvae migrate via connective tissues to positions
just beneath the skin on the back. When they reach the
back, the larvae cut minute holes through the host's
skin, and cysts form around the grubs. A few days later,
the larvae molt for the first time; about 25 days later,
they molt again. In December to February, 3rd stage
larvae mature, work their way out of the cysts, fall to
the ground and seek pupation sites. From 15 to 75 days
afterwards, the flies emerge and are ready to fly and
mate. The adults do not feed and remain active for only
a few days.
Control--Control
of the larva is more practical than control of the fly.
Treatment for cattle grubs should be after all heel fly
activities cease (fly free date) and before grub larvae
reach the back. In New York, this would be after June 15
and prior to October 15, with the preferred treatment
dates being from August 15 to September 15.
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