NZPPS pesticide resistance management strategy

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Blowfly resistance management and prevention of flystrike

Lucilia cuprina is resistant to
some flystrike prevention chemicals.
(Photo: www.ento.csiro.au)

A.C.G. Heath
AgResearch Wallaceville, PO Box 40063, Upper Hutt

(Updated March 2003)

Reason for strategy

Both the Australian sheep blowfly, Lucilia cuprina, and its European counterpart, L. sericata, are resistant to some insecticide groups used in the prevention of flystrike in New Zealand sheep (Wilson 1999). Management strategies aimed at reducing reliance on insecticides will not only conserve the efficacy of existing products, but will help reduce residues in wool to levels that are acceptable to overseas markets.

Background

Individuals of the Australian sheep blowfly, Lucilia cuprina, that are thought to have entered New Zealand in the late 1970s, carried alleles that conferred resistance against diazinon, an organophosphate insecticide (Arnold & Whitten 1976; Wilson & Heath 1994), the basis of resistance being a microsomal esterase E3 (Hughes & Raftos 1985). Many years before, in Australia, L. cuprina had demonstrated resistance to cyclodienes and carbamates (Shanahan 1959; Hughes & McKenzie 1987). In New Zealand, L. sericata, a blowfly responsible for flystrike from the early days of sheep farming in this country had also shown cyclodiene resistance (Hart 1961). More recently, some populations of L. cuprina showed altered susceptibility to formulations containing benzoyl ureas, and L. sericata is demonstrating more widespread organophosphate resistance than previously (Kotze et al. 1997; Haack et al. 1999; Wilson 1999).

Flystrike (ovine myiasis) is a disease of livestock caused in New Zealand by four species of blowflies (Calliphoridae), either individually or in concert (Heath & Bishop 1995). Both L. cuprina and L. sericata are responsible for initiating the majority of flystrikes, but the primary lesion is often colonised a little later by either Calliphora stygia, the brown blowfly, or Chrysomya rufifacies, the hairy maggot fly. Both of these latter species alone can also initiate flystrike. Sheep affected with flystrike can suffer a variety of symptoms ranging from weight loss to death. Survivors lose wool and pelt quality as well (Heath 1994).

The principal means of preventing flystrike has always been dipping, which saturates the entire sheep's fleece, or body regions most prone to flystrike, in an insecticidal formulation that persists for long enough to provide the farmer with a worry-free period during times of high blowfly activity.

Because dipping sheep is not a straightforward or cheap procedure, farmers often use insecticides that combine flystrike prevention with louse control in one operation. This means that selection pressure for resistance can operate on both parasites simultaneously (Sales et al. 1996), which is not a desirable outcome.

Current status of blowfly insecticide resistance in New Zealand

Diazinon-resistant populations of L. cuprina and L. sericata occur throughout New Zealand (Wilson 1999) and have weak side resistance to other organophosphates. The result is that protection periods afforded against flystrike are substantially shorter (ca 75% less) than might otherwise be expected (Levot & Boreham 1995). There is also a degree of resistance to the benzoyl urea diflubenzuron in some populations of L. cuprina that seems to be directly correlated with diazinon resistance (Haack et al. 1999). There have been no reports of altered susceptibility to other chemical groups currently in use as flystrike preventives.

Resistance management and prevention strategy

There is no recorded resistance to triazines, pyrimidine carbonitriles or macrocyclic lactones among blowflies in New Zealand or overseas. These materials can be used in regions where diazinon and diflubenzuron resistance makes flystrike prevention difficult. Alternatively, although there is side resistance to organophosphates other than diazinon, it is not strong, and satisfactory flystrike protection periods can still be achieved using these chemicals.

For farmers wanting to reduce reliance on synthetic insecticides, a range of non-chemical methods available that, if used in an integrated fashion, can reduce flystrike prevalence (Cole & Heath 1999; Edwards et al. 2001). Blowfly trapping and behavioural disruption (Heath & Leathwick 2001), along with parasitoids (Bishop et al. 1996) and specialist forage crops that reduce breech soiling (Leathwick & Heath 2001) can, when used in conjunction with husbandry and grazing management provide flystrike protection, largely independent of chemical use.

Products with label claims for prevention of flystrike in New Zealand

¹For flystrike prevention only, no biting-louse claim.

Note: Control failure does not always imply resistance

The following guidelines are recommended:

• Dip using the minimum volume of insecticide necessary to achieve maximum protection, i.e. only treat body regions most at risk such as the backline and breech.
• Ensure that manufacturers' recommendations are scrupulously followed and that all apparatus is calibrated correctly and working effectively.
• Do not use diazinon or diflubenzuron formulations where they have given poor protection periods. Try another chemical group.
• Try and avoid using products that have combined efficacy against both blowflies and biting-louse.
• Follow good husbandry and integrated management procedures as described in Edwards et al. (2001).

References

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