Chemical-use strategies to slow the development of resistance to flystrike preventative chemicals by flies

Flies are increasingly becoming resistant to flystrike prevention and treatment chemicals, but there are ways to use products wisely to slow development of resistance in your local fly population.

However, these strategies can only be consistently successful if chemical insecticides are applied carefully and strictly as specified on the label, and in the correct position on the sheep.

Where possible, also use non-chemical strategies to manage flystrike.

Strategies to slow the development of resistance to flystrike preventative chemicals by flies
1. Rotate chemical groups within a flystrike season if more than one chemical application is required.

Use different chemical groups for successive preventative treatments and for treating struck sheep in case there has been a build-up of flies resistant to an earlier preventative treatment this season. Learn which actives are in which chemical group to make informed product choices for a rotation.

Where possible, rotation from one year to the next is also of value, but is harder to achieve when long-acting protection is required each year and only dicyclanil provides very long protection. Where there is heavy reliance on dicyclanil, it is particularly important to use as many of the strategies listed in points 2, 3 and 5, as possible, as well as non-chemical management of flystrike, because there is ample evidence of increasing resistance to dicyclanil, as well as other chemical actives.

2. Eliminate or reduce the effect of chemical treatment ‘tails’. 

The ‘tail’ is a period of time where chemical resistance is most likely to build. It occurs towards the end of the protection period of a chemical application. At this time, the chemical concentration of the product on the sheep has dropped to a level where maggots of resistant flies can survive, but maggots of susceptible flies will be killed. The concentration of the chemical at this time acts like a drafting gate “selecting” for resistance: resistant maggots live and susceptible maggots die. The time that this tail starts, relative to treatment time, and the length of this tail, will vary with the product and according to the current resistance status of the local fly population. Monitor for flystrike even when chemical protection has been applied, in case the protection period is shortening as a result of resistance developing. But note that no strike may occur during the tail period (and hence no selection will occur) if environmental conditions do not suit flystrike at the time (see point 3b, below).

The following practices can eliminate or reduce the tail:

a) Minimise the number of separate treatments applied in a season to decrease the number of chemical ‘tails’.

b) Choose a product that should give protection for at least a few weeks longer than the expected flystrike risk time so that the chemical ‘tail’ falls during cold or dry weather when flies are not active.

c) Remove the chemical tail by shearing or crutching treated wool just before the expected ‘tail’ period. The shearing/crutching will also provide a period of protection against flystrike.

d) If longer protection is required, apply another effective preventative chemical from a different chemical group just before the time of the expected ‘tail’ of the previous treatment (i.e., before any flystrike occurs within the protection period).

3. Use shearing and crutching to expand product choices.

Where the flystrike season is long, shorter acting products can still have a useful role. Crutch or shear sheep at the beginning or before the end of the risk period to shorten the time when sheep are susceptible, or shear or crutch part way through the season to break it into two shorter periods. This may allow the use of shorter acting products to cover the shorter risk period/s. Alternatively, two products can be used in a row, but they should be from different chemical groups and the second treatment should be applied before the tail of the first (see point 3d above).

4. Avoid using lice treatments to which flies will be exposed, and which are from the same chemical group/s as your planned flystrike preventative treatments. 

Eradicating lice provides a major advantage as it removes the need to apply lice treatments that can potentially result in chemical resistance in the local fly population. Flies exposed to lice products do not differentiate whether the treatment is for flies or lice. All exposure brings the chance that resistant flies live and susceptible flies die, so increasing the extent of chemical resistance.

5. Stop the life cycle of chemical-resistant maggots by monitoring treated sheep frequently for flystrike, and killing the maggots. 

Make the assumption that all maggots may be carrying genes for chemical resistance. Shear any flystrike wounds and the surrounding 5 cm of wool, and brush off as many maggots remaining on the sheep as possible; do this with the sheep on a surface from which all the struck wool and maggots can be collected. Do not apply chemical to this shorn wool and the loose maggots to kill them, instead, place the wool and maggots into a plastic bag, seal it, and leave it in the sun to kill all the maggots. A flystrike dressing should be applied to the shorn wound; a longer-term preventative may also be required because these sheep are more likely to become struck again.