The most serious and economically damaging ectoparasites facing the NZ sheep farmer are fly and lice. The cost to our farming industry runs into millions of dollars annually. There are over 15 million dollars spent each year on chemicals alone. The farmer must consider a range of factors when designing an integrated ectoparasite control programme. These include animal welfare, economic benefits, environmental impact, human safety, chemical options, and parasite resistance. One golden rule is to treat fly and lice problems as separate issues.
Flystrike is a major animal welfare issue, and every effort must be made to prevent it. Native blowflies are seldom involved in flystrike – the problem flies were introduced from Australia and Europe mainly in the late 1900s. However, the Australian blowfly (Lucilia cuprina) is a relatively recent arrival and was first recorded in NZ in 1984. It has adapted well to the NZ environment and is now the most prevalent and significant strike fly here.
Flystrike is an opportunistic disease which occurs when there is a combination of specific environmental and animal factors. Blowflies fly during the day when it is warm, not too windy, and it is not raining. Flystrike occurs when there is moist, smelly, usually longer wool, often around the breech of the sheep.
Control strategies can reduce the animal risk factors. Through genetics is one pathway, for example the selection of breeds that grow less wool around the crutch (eg Wiltshires). Animal husbandry procedures like crutching and effective drenching can also help reduce flystrike. Finally, stock management, such as keeping stock out of sheltered gullies, reducing yarding of stock during risk periods, and not tailing too late in the season, is advisable.
Chemical application targeted prior to highrisk flystrike periods can be an important component of an integrated fly prevention programme. Organophosphates (OPs) should not be used for fly prevention due to their short protection period, recorded parasite resistance and human health risks. Spinosad gives short-term fly protection and treats active strike. There is a family of insect growth regulators (IGRs) called benzyl phenyl ureas or BPUs (triflumuron, diflubenzuron) that have recorded flystrike resistance in NZ. Many of these products are no longer in use in Australia for fly protection due to resistance issues.
There is a second group of completely different IGR chemicals which are comprised of either Cyromazine or Dicyclanil (eg Vetrazin™, CLiK™ ‘type’ products). This second group of IGRs is generally regarded as comprising the most effective chemicals to be employed for flystrike prevention, although they do not treat active strike. Despite widespread use, meaningful resistance has not been recorded in NZ or Australia. There are two IGR combination products, containing either Cyromazine and Spinosad or Cyromazine and ivermectin, which also treat active strike.
Lice infestation is usually due to the biting louse and can reduce liveweight, fleece weight, wool quality, and pelt grading. Farmers can often feel overly annoyed and embarrassed about lice in their sheep, due partly to redundant laws regarding compulsory dipping and saleyards. However, lice are a tenacious parasite, and very hard to eradicate or control (especially on extensive fine wool properties). It is important to keep this in perspective while working with your animal health advisor to draw up an effective control strategy.
Lice populations increase over the autumn and late winter. Sheep require continued close contact (basically penned together) for transmission to occur. However, lice spread within 24 hours from a lactating ewe to its lamb during feeding. Lice eradication should be readily achievable in theory as their entire life cycle is spent on the sheep. However, several factors, including the practical difficulties of getting a clean muster to dip all sheep at the same time and/or poor dipping practices, result in making this a challenging goal.
Shearing and exposure to sunlight naturally reduce lice populations significantly, but there is no residual protection effect. Surveillance of lice numbers in mobs by farmers would be a tool to assist decision-making on chemical use (much like faecal egg counts can assist with drenching decisions). This is not common farming practice, but it would allow for more targeted chemical use when evidence of lice is recorded.
Selection of the most effective chemical for your farm is a complex decision with options that include:
Reference: Ectoparasites of sheep in New Zealand and their control. Society of Sheep and Beef Cattle Veterinarians of the NZVA (2nd edition).