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Wet Weather Sparks Flywave Concerns

"The recent wet, humid weather has provided conditions that favour development of the sheep blowfly (Lucilia cuprina)"

By Department of Primary Industries - 7th December 2005 - Back to News

"The recent wet, humid weather has provided conditions that favour development of the sheep blowfly (Lucilia cuprina) and so the risk of flystrike in sheep may be significant," warned DPI (Bendigo) District Veterinary Officer George Downing recently.

Blowflies thrive in warm, humid conditions and don't like cool temperatures (less than 17 deg C) or excessively high temperatures (more than 38 deg C). They are particularly attracted to the breech and pizzle areas where the wool has been wetted and stained by urine or faeces, causing skin irritation and 'weeping' of protein-rich fluids from the inflamed skin.

Similarly, 'dermo' (or lumpy wool) and fleece rot (water stain) are also a source of protein and affected areas become particularly attractive to Lucilia cuprina when they are wet by rainfall or prolonged humid conditions.

"Any wounds from fighting amongst horned rams, lamb marking and mulesing, or possibly severe vaccination reaction sites, may also be potential areas for flystrike," said Dr Downing.

Under ideal conditions the lifecycle of the sheep blowfly takes only 14-16 days and, because females are prolific egg layers, populations of flies can build up very rapidly from fairly low numbers, causing severe effects in susceptible sheep.

According Dr Downing sheep need continual inspection for flystrike after heavy rain at this time of the year.

"Look for stragglers in the mob with darkened areas in the fleece. It only takes 2 or 3 days from when eggs are laid to the development of a significant area of flystrike.

"If the struck area is not detected immediately, it could be too late to save the sheep without regular daily inspections," said Dr Downing.

Most breech strike can be prevented by timely shearing and crutching, in conjunction with mulesing and marking. Breeding and selection against conformation and fleece faults that predispose sheep to flystrike (eg fleece rot, excessive wrinkle and devil's grip), and culling struck animals in years of low blowfly activity, helps to develop a mob of sheep that are fairly resistant to flystrike.

Occasionally, under adverse weather conditions, application of chemical may be necessary as a preventative measure although, in most years, it should only be necessary to treat struck sheep.

"Chemical preventive treatment will depend on various factors such as the perceived susceptibility of the sheep and the ability to respond to struck sheep in the event of a fly wave if preventative measures are not carried out," said Dr Downing.

Products available for fly control can be divided into 4 groups, ie Organophosphates (diazinon, propetamphos), Insect Growth Regulators (dicyclanil, cyromazine, diflubenzuron),

Macrocyclic Lactones (ivermectin) and spinosyn (spinosad).

Organophosphates will kill most maggots on existing strikes but will only provide very limited protection (2‑4 weeks) due to the high level of resistance to these products.

Insect growth regulators (IGRs) don't kill maggots immediately on existing strikes but they stop them feeding and they drop off in 2‑3 days. IGRs provide up to 14 weeks protection although there is evidence that resistance is developing to some of these products and the protection period may be significantly reduced.

Ivermectin kills maggots and protects against re‑strike for up to 12 weeks.

Spinosad provides 4‑6 weeks protection, although the period of protection in lambs and coarse-wool sheep is reduced. It does, however, have a nil withholding period and can be used on long wool.

Source: http://gippsland.com/

Published by: news@gippsland.com