Chemical Resistance and Flystrike – What You Need To Know This Season

Meet Lucilia cuprina, the Australian sheep blowfly, responsible for most cases of flystrike on Australian sheep. Flystrike control has relied heavily on chemical treatments- problem is, like all insect pests, Lucilia is able to develop resistance to these chemical treatments. 

What is resistance? – well, simply, it is when a chemical that was previously effective at killing a population of flies is no longer as effective at doing so. When this starts to happen, the problem spreads as more resistant individuals in the population survive when exposed to the chemical then go onto breed, passing their resistance to the next generations. Over time we end up with more chemically resistant flies in the population, making it harder to control the sheep blowfly.

Long-term use and over reliance on just one chemical group for any type of pest control almost always results in resistance if good resistance management plans aren’t in place. Resistance doesn’t mean that the chemicals have completely lost effectiveness; it just means that the period of protection may be less than what you previously expected or what is on the label. 

This is a timely reminder for sheep producers to implement resistance management strategies to maintain flystrike protection for their flocks and slow the development of resistance within their local fly populations.

Signs that indicate you may have resistance are:

•    a shortening of the protection period (that is specified on product labels); or
•    flystrike in multiple sheep that have been treated with the same chemical rather than just in a few sheep.

However, there are many factors that influence the protection period achieved on farm, so before you conclude your flies are resistant, check that:

•    the sheep affected were actually treated;
•    the chemical was applied following the label instructions and with the right equipment;
•    the appropriate amount of chemical was applied;
•    the wool length was adequate to retain the treatment;
•    wool or dags did not make penetration of the chemical difficult; and
•    there was no heavy or persistent rain following treatment, resulting in chemical wash out. 

You can slow the development of resistance by using chemicals strategically and following these steps:

1.    Use a range of chemical and non-chemical tools.
2.    Know the chemical groups and rotate them where practical.
3.    Optimise the number and timing of chemical and non-chemical treatments.
4.    Follow the label directions and keep treatment records.
5.    Regularly monitor for flystrike and kill any maggots from struck sheep.

For an explanation of all the factors that need to be considered in flystrike management and chemical resistance, check out the Managing Chemical Resistance document below: 

Managing Chemical Resistance (248 KB)

The following Flystrike Prevention and Treatment Chemical Guide will help you identify which flystrike chemicals are available for you to use for your specific circumstances, including the chemical groups, actives, application methods, label protection periods, withholding periods and intervals and other uses.

TIP- ROTATE THE CHEMICAL ACTIVE NOT THE BRAND NAME

 (This includes treatments for lice and dressings you may use)