Managing Chemical Resistance

Lucilia cuprina
Lucilia cuprina

Lucilia cuprina, the Australian sheep blowfly, initiates most cases of flystrike on Australian sheep. Like all insect pests, it has the potential to develop resistance to chemical treatments. 
Resistance is the decreased susceptibility of a sheep blowfly population to a chemical that was previously effective at controlling blowflies. When exposed to a chemical, the more resistant individuals may survive and pass on this resistance to their offspring. Over time the proportion of resistant flies in the population may increase.

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. Some of the chemical groups that we used to rely on for blowfly control, such as dicyclanil and cyromazine, are no longer as effective because blowflies have become resistant to them.

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 not 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: 
 

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.

The following Two Producer Case Studies demonstrate integrated flystrike management success – one in a mulesed environment and one in a non-mulesed environment.