As a result, many free-choice mineral supplements that are floor-stocked throughout the late spring and summer contain one of these technologies.
Feed-through fly control can be a valuable component of a comprehensive fly-control program. Unfortunately, its misuse often results in lost opportunities, limited return on investment or the perception that the product isn’t working. Thus, it is important to understand the science behind these products and the steps that should be taken in order to maximize their efficacy.
How or, more appropriately, where do they work?
One thing all feed-through fly control products have in common is: They are pesticides that have no direct effect on the animal. Instead, they wage their war against flies in manure. Once consumed, they move through the animal’s digestive tract relatively unchanged before being deposited in its manure. Although the different types of feed-through fly control are delivered to manure in a similar manner, their differences lie in their respective modes of action. As such, these additives generally fall into one of two categories: either insect growth regulators or larvicides.
Insect growth regulators vs. larvicides
Insect growth regulators – more commonly referred to as IGRs – are compounds that interfere with the processes required for normal fly development. Simply stated, they inhibit or delay the progression of fly larvae from one stage of development to the next. Because of this specific mode of action, IGRs are generally species-specific and thus target only a single fly species. Most commonly available feed-through IGRs are only effective on horn flies. The most common of these include S-methoprene and ADM methoprene.
In contrast to IGRs, larvicides cause a structural change in the fly that leads to its death before it is able to reproduce. More simply stated, they prevent flies from breeding. Because of this specific mode of action, larvicides are generally not species-specific and thus target more than one species of fly. As a result, feed-through larvicides target not only horn flies but also face flies and stable flies. The most common of these include Diflubenzuron and Tetrachlorvinphos.
Because of their differences, it is important to make sure you are using the most appropriate product to achieve your goals for fly control. Using the correct product will also increase the likelihood its performance meets your expectations. In certain environments or geographies, horn flies are the fly of primary economic significance.
But this is not always the case. In other situations, horn flies may not be as much of a concern as are face or stable flies. Situations arise where an IGR is being used, but the producer does not believe the product is working because they still see a lot of flies on animals. Since that product is intended to control horn flies, do not expect it to reduce pressure from face flies or stable flies.
In this situation, the issue was driven by a lack of understanding in what the product was designed to do rather than a lack of its efficacy. In situations where horn flies are your major concern, an IGR may be the best option. In situations where face flies or stable flies are more of a concern, a larvicide may be more appropriate. However, it is important to recognize larvicides may affect reproduction of insects other than flies. If considering a larvicide, be sure to consult with a product specialist to determine if it is the best option for your operation and how to minimize its effects on other insects.
Take the necessary steps to maximize efficacy
In order to maximize the efficacy of an IGR or larvicide, it is important to keep in mind they both elicit their effects on flies in manure. Therefore, they must be present in the manure in order to work. This means you need to be ahead of the game and begin feeding these products at least one month prior to fly emergence.
If you don’t begin feeding the feed-through product until flies have already become a problem, you’re going to limit the efficacy of the product. This is because flies have already been given the opportunity to breed and develop in manure that has not been exposed to the pesticide. They’ve essentially been given a head start. This doesn’t mean the product won’t work but rather that its impact will be limited by the delay in exposure. Their seasonlong efficacy is dependent upon feeding these products throughout the duration of the fly season. Plan to feed them throughout the fly season until at least the second major frost.
Keep in mind that these products work in a dose-dependent manner. Therefore it is important cattle consume the necessary (labelled) amount of product. This requires cattle consume the amount of feed required to deliver the necessary level of IGR or larvicide on a consistent basis. Expected daily consumption should be taken into consideration when formulating feeds or supplements in order to ensure they deliver the appropriate level of IGR or larvicide.
Then, ensure the feed or supplement is actually being consumed in a manner that allows for consistent delivery of the necessary amount of IGR or larvicide to manure. If using a free-choice mineral supplement as the vehicle of delivery, track mineral consumption and relocate mineral feeders as necessary to achieve consistent intake of the appropriate amount.
Another factor that limits the efficacy of feed-through products is their use as the sole means of fly control. While they may still provide some benefit when used alone, feed-through pesticides are not a “silver bullet” and perform best when used as a component of a comprehensive fly control program.
To achieve maximum fly control, use them in combination with other methods of control, such as insecticide-impregnated eartags (fly tags) and topical insecticides (sprays, pour-ons, rubs, etc.), as well as proper manure and wasted or spoiled feed management. The latter of these – proper manure and wasted or spoiled feed management – is the area most commonly overlooked. When these factors are overlooked, the efficacy of these feed-through fly control products is substantially limited, as flies almost always have access to moist organic matter that was not exposed to the product. Take the necessary steps to ensure old manure or excess feed doesn’t limit their efficacy.
While IGR and larvicide products can be a valuable component of a comprehensive fly control program, their misuse, along with a lack of understanding of their modes of action and factors that drive their efficacy, creates lost opportunity and substantially limits their value. Use this information to ensure you are using the correct tool for the job and using that tool in a way that maximizes your return on investment. 
PHOTO: It is important to make sure you are using the most appropriate product to achieve your goals for fly control. Getty Images.
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Jason Smith
- Assistant Professor
- Assistant Professor - University of Tennessee
- Email Jason Smith
