The stable fly and house fly are the major insect pests
associated with cattle feedlots. The stable fly irritates cattle
by biting and feeding on blood, causing cattle to gain
less weight. The house fly is more of a nuisance, that
causes irritation to people at or near the facility. Reduced
animal weight gain attributable to house flies has not
been recorded. In the Midwest, stable flies are abundant
from June to mid-summer; temperatures over 100° F
greatly reduce their abundance. Reports from Iowa,
Kansas, Missouri, Nebraska, and Oklahoma show June
as the month with high stable fly populations, whereas
Illinois and New Jersey have high populations in August
and September. Further south in the US, peak populations
occur from March to June, with Texas reporting
spring and fall peaks. Population peaks vary and depend
on damp conditions required for fly development, in
such materials as animal waste mixed in straw, hay, or
other plant fibers. In the Midwest, house fly numbers
increase from July until cool weather in the fall. These
flies thrive during the hotter summer time.
Fly reduction measures should begin with sanitation
or cleaning and removal of manure, spilled feed (particularly
plant material mixed with animal waste), and
standing water. After the facility is cleaned, a few fly
breeding areas may remain. Those areas are where biological
control agents are most likely to be successful.
The only biological control agents currently available
for controlling flies in confined livestock facilities
are pteromalid wasps. Other types of biological control
organisms include pathogens and predators. Three
groups of beetles (staphylinids, histers, and scarabs)
have been studied for fly control, but none are currently available for commercial use. Staphylinid and hister
beetles are predators that are common in cattle feedlots,
and scarab beetles, or dung beetles, which bury cattle
dung as food for their young, are more prevalent and
effective in pastures.
Fungal pathogens have been observed attacking
flies. However, research has not demonstrated methods
to make them effective for fly reduction in livestock environments.
Pteromalid wasps are widely used and sold for fly
control. The tiny wasp parasite lays an egg on a fly
pupa. The wasp larva then feeds on the fly developing
inside the reddish-brown pupal case. Parasitism by the
wasp prevents fly development even when a parasite
does not emerge. Because wasps kill flies in the pupal
stage, they reduce the number of flies in the pestiferous
adult stage without disrupting the beneficial aspect of
fly maggots or larvae—helping to break down the animal
waste in the feedlot.
Fly parasites are about 1/16 inch long and can work
their way through animal waste searching for fly pupae
to sting. A parasite will live 10-30 days and produce 30-
60 eggs. Cooler temperatures extend their life period.
Tests show that 15-30 flies are killed by a single female
Spalangia nigroaenea. The parasites stay in the manure
searching for fly pupae in which to lay their eggs. When
a pupa is found, the female wasp taps its antennae on
the surface, then inserts her ovipositor through the pupa
case and deposits an egg on the surface of the fly developing
inside. The parasite egg hatches, and the larva
feeds on the fly. Female parasites may form a feeding
tube in the pupa from which they feed on the fly blood as it oozes out the tube. That is probably the only nutrition
for most species of parasites. Their feeding or
ovipositon activity kills the developing fly.
Several species of pteromalid wasps have been produced
and sold commercially for fly control. Early
research demonstrated success using Spalangia endius
and Nasonia vitripennis in poultry houses. These species
and others have been sold for fly control in Midwest cattle
feedlots. But in some studies, they were not recovered
from fly pupae collected from the feedlots where
they were distributed. This demonstrated that Spalangia
endius and Nasonia vitripennis were ineffective in controlling
flies at cattle feedlots. Another species,
Spalangia nigroaenea, was recovered commonly from
stable fly pupae at feedlots. Muscidifurax zaraptor was
common from house fly pupae. Thus, these species have
been considered to have a greater impact on fly numbers
in cattle feedlots. Mixtures of wasp species have
been marketed, but proven adapted species should be
used.
Based on samples from cattle feedlots that have
been cleaned, the release of Spalangia nigroaenea has
reduced stable fly numbers. Reductions ranging from 25
percent-50 percent, have greatly decreased the irritation
to cattle from biting flies. Delaying the early increase of
stable flies by 30 days decreases the total period of irritation
to cattle. That time delay, along with reduction of
the peak fly numbers, greatly benefits cattle gain and
requires only a small investment.
Fly parasite releases have been based on the number
of cattle present. This method is the best available
but must be improved. The fly breeding area is far more
indicative of the fly population than are cattle numbers.
Research has shown that fly population estimates made
by feedlot scouts are less than the number actually present.
During studies, samples of fly breeding material
examined weekly for three years produced four fly larvae
for every one estimated by experienced feedlot
scouts. The number of parasites (as parasitized pupae)
to release can be determined by using estimations of fly
pupae present then multiplying by four (1/4 being estimated)
and dividing by 15 (the average number of fly
pupae killed by one parasite). Other factors must be
considered when estimating the number of parasites to
release in the High Plains states, and they are probably
applicable in other areas:1. Are conditions dry and hot, or wet and cool?
2. Are fly breeding conditions changing toward larger
or smaller numbers of flies?
3. Is the feedlot clean or being cleaned, or drained, or
are cattle being removed?
4. How many fly pupae will each parasite kill; what
percentage will emerge as females; and how long
will they live?
5. Is there scientific proof that the parasite species
being released is effective?
6. Is there a dependable source of parasite species
adapted to your region?
7. Are insecticide procedures being used that will
reduce parasite populations, i.e., spraying directly
on feedlot manure where parasites live?
8. What is the degree of sanitation in the feedlot?
9. What percentage of the feedlot area supports fly
development?
10. Which fly species is abundant?
11. Can the feedlot afford parasite releases or should it
spend money on cleanup?
My results suggest that most parasitism occurs two
weeks following release, and weekly parasite releases
have had positive results. Less frequent releases result
in periods with few parasites present during hot weather
when parasites are active for only about one week.
Weekly releases during the fly season have been necessary,
because one period of releases has not maintained
parasite populations during the following weeks.
Several parasite release methods have been used.
Placing the emerging parasites directly in the location of
fly pupae and covering with an inch or more soil has
worked well. The fly pupae from which parasites are
emerging should be placed on damp soil or manure, so
they do not dry out quickly or reach high temperatures
with the summer heat. Placing the parasites out of the
sun and away from areas that will be trampled by cattle,
or removed and spread during manure cleanup, is
mandatory for successful fly parasite use.