The field confirmation of cyromazine (e.g. Vetrazin) resistance (Levot 2012) and evidence of the increased prevalence of dicyclanil (e.g. Clik) resistance in NSW (Sales 2020) necessitates greater planning across the wool growing cycle for the prevention and treatment of flystrike. This planning may not be straightforward as it needs to mesh with other activities conducted on farm. In an attempt to slow the rate of resistance development, and contain the level of resistance, the selection of products for flystrike treatment and control should be part of an integrated approach to their use with an overall goal to reduce reliance on insecticides.
Insecticide resistance testing of populations of Lucilia cuprina, collected from strikes and submitted by producers, has continued since completion of the 2018-2020 AWI/DPI joint funded project. As was found in that study, 100% of submission to date from NSW have been classified as resistant to both cyromazine and dicyclanil which belong to the same (IGR) chemical group. Dicyclanil resistance has not been found in the absence of cyromazine resistance but cyromazine only resistance has been found in other states. We define dicyclanil and cyromazine resistant strains as follows:
Has survivors at the Susceptible Discriminating Dose (SDC) = Low level Resistance.
Has survivors at 4-fold the SDC but not 8-fold the SDC = Medium Level Resistance.
Has survivors at 8-fold the SDC = Higher level Resistance.
Little or no reduction in protection period should be observed within populations with low level resistance unless it is at the very end of the protection period or other factors come into play such as weather conditions which are deleterious to the product applied and the presence of mycotic dermatitis or fleece rot which then coincide with fly wave conditions. However, as the level of resistance, and the frequency of resistant individuals in the fly population, increases a reduction in the protection period will be observed. This was demonstrated for both cyromazine and dicyclanil based products against a strain selected for its higher level cyromazine resistance (Levot et al., 2014). A dramatic reduction in the protection periods provided by the three dicyclanil based products of varying concentration and cyromazine jetting fluid when challenged with a dicyclanil resistant strain displaying higher level resistance (Sales et al., 2020). The protection periods provided by these two insecticides have been reported to have decreased by varying degrees by NSW producers.
There are long-, medium- and short-term strategies that should be adopted against flystrike with an overall view of reducing reliance on chemicals. Long term strategies include breeding a line of sheep which are less prone to breech strike, fleece rot, mycotic dermatitis and which lack negative conformational issues such as 'devils grip'. Many producers have this underway and, if not, should commence as a priority.
Medium term strategies include the scheduling of shearing and crutching to utilise the 6 weeks protection they provide. In addition, the timing of joining and lambing may be manipulated in an attempt to minimise the flystrike risk to ewes with stained breeches and marked lambs.
Short term strategies include the rotation of chemical groups to reduce additional selection for resistance to these and other chemical groups. This requires the exclusive use of a single chemical group for the treatment of existing strikes and as a wound dressing for the wool growing cycle. A second chemical group should be identified for the treatment of lice and used only if there is evidence of an infestation. The eradication of lice through effective treatment, good fencing and good quarantine should also occur as a priority. Prophylactic flystrike treatments require the selection of a third group of chemical and consider using a risk-based approach to the timing of this treatment rather than application according to the calendar. The overall aim of this rotation strategy is to achieve fewer insecticide applications and no second treatments from the same chemical group in the same wool growing cycle.
To aid with flystrike risk assessment the flyboss Tools section has a modelling tool which uses the closest weather data gathering point to the property of concern. Scenarios can be compared, for example, two short acting treatments versus one long-acting treatment. Crutching and shearing times can also be manipulated to determine the times of greatest flystrike risk and the degree of that risk. It is important to note that this program models flystrike risk not actual occurrence and assumes that the products selected will achieve the protection period listed on the label.
Recently Horton (2021) has modelled the effectiveness of a number of these strategies in reducing the development of resistance. This modelling has shown that spring shearing reduced the rate of resistance development, summer shearing increased it and autumn and winter shearing had almost identical effects. In addition, the model verified that rotating between chemical groups did reduce the rate of resistance development and that rotating three chemical groups was better than rotating two, but these must provide similar protection periods, resistance should not be present and lice treatments are considered a rotation. Interestingly, a cost benefit analysis of the level of flystrike monitoring of flocks that producers undertake found that a moderate level of monitoring (3 days per week) reduced both flystrike related costs and the development of resistance. In addition, if the level of monitoring was increased and the detected maggots removed and killed then the onset of resistance was delayed further.
The utilisation of non-insecticide management strategies for flystrike control will become of increasing importance as laboratory testing (Sales 2020) and modelling (Horton 2021) confirms that continuous use of the same active ingredient results in increasing levels and rates of resistance development. There are a number of excellent resources available on the AWI website www.wool.com with the most recent being a chemical group selection wheel www.wool.com. In addition, the flyboss strike risk tool has recently been updated by Dr Brian Horton www.flyboss.com.au and flyboss information of flystrike resistance is currently being updated in line with the development of resistance to this group of chemicals.
The 2018-2020 study was jointly funded by Australian woolgrowers and the Australian Government through Australian Wool Innovation Limited and NSW DPI.