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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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