Resistance to insecticides in blowflies

Jeff Cave, District Veterinary Officer Wodonga, Agriculture Victoria

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 insecticide treatments.

Resistance is the decreased susceptibility of a pest population to a pesticide that was previously effective at controlling the pest. Pests evolve resistance to pesticides by a process of natural selection. When exposed to a pesticide, the most resistant individuals survive and pass on resistance to their offspring.

With repeated exposure to the pesticide, particularly at inadequate levels, the resistant pests are favoured, and their proportion in the population may increase. Eventually, there can be enough resistant pests in a population that the pest is controlled for only short periods or, ultimately, not at all.

Some cases of apparent resistance are the result of improper application or heavy rain following insecticide application. Before concluding that flies are resistant, check that:

  • the sheep affected were actually treated
  • the chemical was applied following the manufacturer’s instructions
  • the appropriate amount of chemical was applied
  • the wool length was adequate to retain the treatment
  • wool or dags did not reduce penetration of the product
  • there was not unusually heavy rain following treatment, resulting in chemical wash-out.

Sheep producers can use several management strategies to minimise the development of resistance or at least delay its onset:

  • Use an integrated approach to reduce reliance on insecticides.
    • –  Breed for resistance to flystrike.
    • –  Shear or crutch at times that maximise protection against flystrike.
    • –  Dock tails to the correct length.
    • –  Manage scouring.
    • –  Use breech modification, if required, until sheep are genetically resistant to flystrike.
    • –  If treatment is needed, make sure it is applied effectively.
  • Know which insecticides belong to which insecticide class.
  • Be aware that resistance to one insecticide may cause a cross-resistance to another related insecticide.
  • If treatments for lice and flystrike are deemed necessary, use different insecticides and appropriate classes for each pest. Also use insecticides from different classes for treating and preventing flystrike.
  • Minimise the number of insecticide treatments applied in a season.
  • Apply insecticides carefully, and strictly as specified on the label.
  • Monitor for flystrike frequently and kill maggots on all flystruck sheep
  • Monitor for flystrike frequently, and kill maggots on all flystruck sheep.
  • Understand the biology of the sheep blowfly.

Chemical groups

All registered products used to treat blowflies in a mob scenario (flock treatment) belong to one of six groups: organophosphates, insect growth regulators (IGR), macrocyclic lactones (ML), spinosyns, synthetic pyrethroids and neonicotinoids (see Table 1).

Organophosphates are older products that have mostly been replaced by more modern insecticides. As well as having high levels of resistance, organophosphates are nonselective, and their use carries work health and safety risks.

A recent chemical resistance survey conducted by the NSW Department of Primary Industries showed that blowflies are continuing to develop resistance to other chemicals used for treating flystrike. Samples sent in from Victoria showed that there is resistance to both dicyclanil and cyromazine.

Resistance should be suspected if a shortening of the protection period (specified on product labels), or flystrike in multiple treated sheep rather than just a few, is seen.

For further information, visit the FlyBoss website.

Table 1. Chemical groups and actives used for flystrike prevention

Chemical groupChemical activeExample productMethod of application1Protection period (weeks)2

Insect growth regulator (IGR)

Cyromazine

Dicyclanil3
* 12.5 mg/mL
* 50 mg/mL
* 65 mg/mL

Vetrazin®
Vetrazin® Liquid

CLiKZiN®
CLiK®
CLiK® Extra

Spray-on
Jetting/Dipping

Spray-on
Spray-on
Spray-on

11
Up to 14

Up to 11
18–24
Up to 29

Neonicotinoid

Imidacloprid

Avenge & Fly®

Spray-on

Up to 14

Macrocyclic lactone (ML)

Ivermectin

Coopers® Blowfly & Lice

Jetting

Up to 12

Synthetic pyrethroid (SP)

Alphacypermethrin4

Vanquish®

Spray-on

Up to 10

Spinosyn

Spinosad

Extinosad® Eliminator

Jetting

4–6

Chemical groups and actives used for flystrike prevention.

  • Always follow label directions.
  • Check the label before use as some products may differ.
  • Dicyclanil product protection periods vary due to their different concentrations of the active chemical.
  • Registered for the prevention of body strike only.

Source: Flyboss website

Page last updated: 26 Mar 2021