The time has arrived to plan for controlling summer fly populations. Adequate fly control programs are multi-faceted, systematic, and, in addition to directly enhancing cattle performance, can aid in the prevention of acute heat stress in feedlots. We recommend a combination of at least two fly control strategies used in combination from the time of the last hard freeze of spring to the first killing frost in the fall in your local area. Before deciding which strategies are the best fit for your feedlot, you need to identify the problem species and the problem areas in the lot.
Stable flies and house flies are the most common fly species in feedyards. House flies peak in population in July and persist into the fall. While house fly populations can reach extremely high densities, they are mostly an annoyance. House flies do not suck blood, but rather feed on liquids or solids they can dissolve with saliva (Greene et al., 1998). While house flies may not be the principle enemy in terms of cattle performance, they can be a public relations concern. As fall approaches, house flies roost in buildings, vehicles, and other places that can be a concern to neighboring businesses or residences. Purely from a public relations standpoint, you need to control house flies.
Adult stable flies feed on blood by attacking the legs of cattle and piercing the skin (Greene et al., 1998). Stable flies are most abundant in spring and numbers generally decline with hot, dry weather. However, during cool summers, high populations of stable flies may persist for up to four months. The feeding actions of stable flies cause cattle to bunch, stomp, and can reduce performance. Research conducted over several years at the University of Nebraska (Campbell et. al., 1997; Cantagui et. al., 2001) evaluated cattle performance and economic losses as a result of stable flies using screens to prevent fly entry into protected pens while measuring fly populations in adjacent pens. Fly populations of 50 per calf resulted in ADG reduction of 13.2%, while gains were reduced up to 20% when 100 flies per animal were present (Campbell, 1977). Over seventeen years of research in this model (Cantagui et. al. 1997) reported average reduction in ADG of 8.46%. The economic loss associated with stable flies depends on the cost of cattle, ration cost, and duration of the fly season. With breakevens as high as we have today, this loss is undoubtedly substantial. In addition, high stable fly densities can contribute to heat-related cattle mortality by causing cattle to bunch during times of heat stress. Densities as low as five stable flies per leg can reduce performance and indicate control measures need to be in place.
Stable flies and house flies both breed in manure, so the first step in fly control is manure management. Without proper manure management, other fly control measures will produce limited results. Stable flies require a moist mixture of soil and organic matter between four and 12 inches deep in which to lay eggs. In a 4-year study (Skoda, et. al., 1996) of immature fly populations in feedlot pens, 62.5% of immature flies were found adjacent to feeding aprons, 24.6% around mounds, and 8.4% along pen side fences. Areas where hoof action constantly disturbs manure are poor breeding grounds for stable flies, but those areas where stagnant manure and soil mixtures collect are the problem. Another survey of 93 feedlots (Gilbertson and Campbell, 1986) found substantial immature fly populations also existed in and around feed storage areas, along bunk lines, and in drainage areas.
Since stable flies take about three weeks to develop, and house flies about two weeks, one may conclude that scraping aprons and around mounds every two weeks or less should greatly reduce fly populations, if manure is removed from pens. Early spring pen maintenance should include removing manure deposits under fences and around any other obstacles to provide less breeding habitat for flies. Spring is also a good time to clean out drains, settlement ponds, lagoon edges, and any other areas where wet, stagnant combinations of manure and soil exist. Take the time to clean up your feed grounds, as well, by removing spoiled feedstuffs, reshaping areas that hold moisture, and generally removing fly habitat.
Predatory wasps whose larva feed only on fly pupae are commonly used in feedlots as a stable fly control measure, and can be very effective. Muscidifurax zaraptor are commonly localized in house fly pupae, but are commonly sold because they have high survivability. While they should be included in your program for house fly control, they should not be the sole species released. Spalangia nigroaenea larva can be found in both house and stable fly pupae and appear to be adapted well to feedlot environments in the Midwest (Greene et. al., 1998), so they should be a part of your parasitic wasp program. Parasitic wasps should be placed early in the summer, usually early June, and it is important to place 20 to 50 parasites per head on feed on a weekly basis.
Stable flies feed most actively between 10 a.m. and 4 p.m. and after feeding they seek a shaded area to roost. Windbreaks, weed patches, shaded sides of buildings or bunks, and adjacent fields often provide a roosting area for stable flies. Some of these areas are necessary, but controlling weeds, in particular, will help reduce fly pressure on cattle. Chemical treatment of cattle produces transient results, and it is difficult to effectively treat the legs of cattle where stable flies congregate. So, chemical treatment should be used mainly when other measures have failed, or to achieve an initial knock down of fly populations prior to implementing other controls. Focus on spraying early morning or late evening in areas where stable flies roost. Treating roost areas with residual sprays can be very effective in reducing fly populations. Always apply insecticides in accordance with label regulations (make sure you or your applicator are licensed) and document use.
Feed additives that control fly populations via activity in the manure are another option. There are a couple options on the market today, with both targeting manure-breeding flies, each through a different pathway. One directly kills the larvae of all four fly species, while the other prevents development of flies by interrupting the life cycle. Both are effective if dosed correctly on a daily basis. In order for either to work well, feedlot operators should start feeding early in the spring, 30 days before flies appear and should be fed continuously until 30 days after a killing frost. A more recent study (Durunna et al., 2017) conducted in Canada demonstrated that garlic powder when added to mineral program could also help in fly control. The garlic treated group had 52-56% fewer flies per cow when compared to the untreated-control groups in the summer long study which suggests the use of other feed ingredients such as garlic have potential to be part of an effective fly control program for both feedlots and pasture cattle. These products should be used as part of a fly control program and are not a substitute for manure management.
We encourage feedlot operators to develop fly control programs that focus on at least two methods of control, and manure management to reduce fly habitat should always be one of those methods. We generally recommend parasitic wasps or feedthrough control measures with chemical application reserved for acute fly problems.
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