Altosid (methoprene) and Rabon (tetrachlorvinophos) are insecticides that are available as minerals or feed supplements for beef and dairy cattle fly control. These oral larvicides are consumed and pass through the animal’s manure at concentrations that kill fly maggots. With this approach, the only sources of horn flies and face flies in pastures are treated.
There are some important things to keep in mind when using this approach:
What pasture flies are targets?
Horn flies and face flies are key pests of cattle in Kentucky. They torment herds every year and often are present in damaging numbers: more than 200 horn flies per head and more than 10 face flies. Altosid products are labeled for horn fly control. Rabon larvicides are labeled for both horn flies and face flies. Supplemental fly control measures may be needed, depending upon fly pressure.
How do these products work and why is that important?
Altosid is an insect growth regulator (IGR). It interferes with normal development of horn fly maggots, preventing them from emerging as adults. Rabon, a Group 1 insecticide, attacks the maggot’s nervous system so that it dies before completing development.
Knowing that these products work differently is important when planning an insecticide resistance management program for horn flies. Altosid IGR only affects the larval stages of insects, so it is not included in insecticide ear tags, pour-ons, or sprays that control adults. However, the organophosphate insecticide Rabon is effective against both maggots and adults. It and other Group 1 insecticides are available in some brands of insecticide ear tags and fly sprays. This must be considered so a total herd fly control program is not based on products from a single insecticide group. Rotation among insecticides that attack pests in different ways is part of resistance management.
How important is dose?
All animals in the herd must eat the minimum daily amount of larvicide specified on the label. This ensures that a lethal concentration of insecticide is always present in all manure. It means keeping up with consumption and taking steps to increase or decrease it as needed. This may be done by relocating feeders to increase or decrease availability. Over-consumption increases control costs unnecessarily. Under-consumption can mean reduced fly control.
How can I tell if the product is working?
The best way is to keep a record of average fly numbers per head and compare the numbers with what is considered acceptable levels of flies. Write down a weekly count of horn flies and face flies per 10 randomly-picked animals in each herd. Face fly counts are straightforward but horn fly counts usually have to be estimates. Ten or fewer face flies per head is generally considered to be satisfactory control. Horn fly numbers should be less than 100 per side.
Counts on your cattle may be above the target values even if the oral larvicide is doing its job. Flies can move in from nearby herds (within 3 to 5 miles) where fly control is not practiced or is insufficient. In these cases, supplement fly control with dust bags, oilers, or some other means.
What conditions increase the effectiveness of oral larvicides?
The faster manure piles dry, the sooner they become less suitable for fly breeding. Extra rain can allow the development of greater fly numbers. The better the fly control is on nearby herds, the less pressure you will have from flies moving into your herd.
Start early. Consumption of oral larvicides should begin before the flies appear so that the only fresh manure available is treated.
Oral larvicides are a tool for pasture fly control. Success depends on careful monitoring of consumption and keeping track of fly numbers to assess the need for extra control measures.
By Lee Townsend, Extension Entomologist
Kentucky Pest News 