CASE NOTES

Blowfly strike waves: Biology and control

Peter James, Queensland Alliance for Agriculture and Food Innovation (QAAFI),
University of Queensland

INTRODUCTION

Efficient fly control consists of an integrated approach which may include:

• breeding for resistance
• mulesing by an accredited muleser
• alternative breech treatment
• docking tails to the correct length
• strategic crutching and shearing
• prevention of scouring
• fly monitoring and strategic use of flytraps
• paddock management to minimise strike risk
• strategic use of chemicals.

Many of these elements are about flystrike preparedness and must be implemented well before a fly wave begins. Three basic conditions are required for strikes to occur:

1. presence of flystrike flies
2. presence of susceptible sheep
3. occurrence of weather conditions favourable for flies to find the susceptible sheep and for strikes to establish.

This paper focuses on the conditions required for development of fly waves and the actions that can be taken once a fly wave threatens.

FLY FACTORS

L. cuprina is by far the most important fly strike species, responsible for more than 90% of strikes. Brown blowflies (Calliphora spp.) can cause strikes in cooler weather in some areas and are often the species involved in wound strikes (for example in poorly healed mulesing wounds). Hairy maggot flies (Chrysomya rufifacies and Chr. varipes) are secondary strike flies and generally only invade once a strike has been established by L. cuprina. Because of its overwhelming importance in strike, this paper focuses on L. cuprina.

L. cuprina is often thought of as an obligate parasite, because it breeds predominantly on live sheep (Waterhouse et al. 1947). As most L. cuprina do not disperse further than about 1-2 km (Vogt and Woodburn 1979) the majority of strikes will be the result of flies that have bred on the property. L. cuprina readily oviposits on carcasses of dead animals, but very few of these eggs develop to become adult flies (Waterhouse 1947, Anderson et al 1988). This is because L. cuprina larvae are out competed and sometimes predated upon by other fly species that occupy this ecological niche, the carcass often becomes too hot for L. cuprina, and many larvae that do complete development are killed by predators and parasites attracted to decomposing carcasses. The exception to this is sheep that actually die from strike. By the time the animal dies, many maggots will be second or third instars and be able to complete their development before large numbers of carrion colonising species invade. Therefore, carcasses of sheep that die from strike can be the source of large numbers of L. cuprina adults.

Population maintenance over winter

Once soil temperatures drop below about 15ºC maggots that burrow into the soil suspend their development as pre-pupae and do not continue their development until soil temperatures rise again in spring. Many larvae and pupae die in the soil over winter and this is a vulnerable time in their life cycle. Flies that survive the winter provide the seed population for the next seasons fly strike. As most L. cuprina breed on struck sheep, one strategy that has been examined with some success is the possibility of early treatment of sheep to remove early season breeding sites for the flies and retard build up in fly numbers (McKenzie and Anderson 1990). The use of fly traps, particularly LuciTraps® which have specificity for L. cuprina (Urech et al. 2009), is a good way to assess emergence from over wintering and to build a property profile of fly emergence times.

Population maintenance during hot dry weather

A number of studies have been conducted to examine how and where populations of L. cuprina are maintained in periods when strike incidence is low. In pastoral zones most flies survived these periods around the homestead or farm buildings (possibly in offal pits) and along water courses frequented by sheep (Anderson et al 1983). Other flies that contribute to population maintenance come from those few animals that become struck from time to time and particularly ‘covert’ strikes (Anderson et al 1988). Covert strikes are defined as strikes not seen during normal paddock inspection or yarding and found only by detailed inspection of individual sheep (Wardaugh and Dallwitz 1984). Some covert strikes develop to overt strikes, but some persist as small strikes for extended periods of time, presumably sustained by ongoing low level oviposition, contributing to the maintenance of fly populations. It is likely that L cuprina populations were severely impacted in many areas during the recent drought, which together with a reduction in sheep numbers, may help to explain the lag time between the development of wet conditions and the onset of fly strike problems.

Susceptible sheep

Breech strike is the most common type of strike in most years. Breech strike occurs following urine staining or fouling of the breech with faeces, usually when sheep are scouring. This provides odours that attract L cuprina and stimulate oviposition, moisture and possibly irritation and dermatitis that provide protein for young larvae and a focus for larval feeding. Scouring is due to a number of causes, but is very often associated with infestations of scour worms or challenge from ingested worm larvae (most commonly Trichostrongylus spp, Ostertagia or Nematodirus). Scouring can also be caused by nutritional upsets or other pathogens.

Body strike occurs most commonly over the shoulders or along the backline of sheep, usually after the development of fleece rot or dermatophilosis.  In wet years and during fly waves body strike often becomes the predominant type of strike. Body strike can be particularly damaging because it is harder to detect than breech strike. With breech strike the wool rapidly develops a visible dark patch and sheep bite at the breech wool or stamp their rear feet. In comparison, with body strike the struck area extends down over the sides of the sheep under the wool, making it difficult to see and there may be few behavioural signs. Often body strikes are not detected until they are well advanced.

Fleece rot occurs when susceptible sheep are wet to skin level for four or more days. This leads to bacterial proliferation, most commonly Psuedomonas aeruginosa, which causes dermatitis, protein exudation into the fleece and an odour which attracts L. cuprina and stimulates oviposition. The serous exudate associated with fleece rot provides protein for newly hatched larvae and a focus for primary skin invasion.

Dermatophilosis (‘lumpy wool’ or ‘dermo’) is a fungal disease causing large hard lumps in the wool in severe infections. The spread of Dermatophilus can be associated with prolonged wet conditions, but infections can also occur in young lambs before the skin wax layer is fully formed, in shearing cuts and following dipping. Dermatitis ‘lumps’ are very hygroscopic and rapidly become wet and attractive to flies following rainfall so sheep that have previously developed these lesions need to be closely watched for strike after rain.

Pizzle or belly strike is most commonly an issue with wethers or rams where the belly wool becomes stained and dampened by urine and attractive to flies. Susceptibility can be increased by development of ovine posthitis or urinary calculi, cuts to the pizzle during shearing or ringing or other morphological abnormalities that divert urine into the wool. During long wet periods, continually walking through long wet grass keeps the belly wool moist and significantly increases susceptibility.

Other strikes: These include poll strike, wound strike, strike in lambing ewes where afterbirth has adhered, strike in footrot lesions and strike following lice infestation. 

sUITABLE WEATHER CONDITIONS

Moisture is the critical parameter for strikes to develop. This can come from rainfall, faecal and urine staining, body fluids exuding from strikes or wounds and sometimes long wet pasture. L. cuprina females need to sense moisture with their tarsi before they will oviposit. Their eggs and early larvae are extremely susceptible to dehydration. A period of approximately 24 h of moist conditions in the fleece is necessary for eggs to hatch and for larvae to establish.

Temperatures need to be at least 17ºC for L. cuprina to fly and to actively seek sheep and there is also evidence that very high temperatures also suppress L. cuprina activity. Wind speed needs to be less than 30 kph (stiff breeze) for L. cuprina to fly and locate sheep.

Strike waves

During periods of prolonged drought, such as experienced through much of Australia in recent years, few sheep develop predisposing conditions for strike, there is little opportunity for flies to breed and fly numbers can drop to low levels. This is particularly so on properties with good strike preventative programs in place. When rainfall comes, although conditions are such that a fly wave might be expected, often few fly strikes occur. This is a reflection of the low fly numbers present and the low numbers of sheep with predisposing conditions for strike. However, depending on the nature of the rainfall event, some sheep may develop predisposing conditions and fly numbers start to build. If the rainfall is followed by a dry period there may be little opportunity for the next generation of flies to breed and fly numbers may again decrease.

With ongoing warm wet weather, or repeated rainfall cycles coinciding with the next generations of flies, classic fly wave conditions develop. Flies are able to go through repeated breeding cycles increasing in numbers with each cycle, the number of susceptible sheep increases and strike incidence rises. Once L. cuprina is abundant and there are many sheep with predisposing conditions in the mob, strikes can commence quickly following any rainfall event that is accompanied by suitable temperatures and wind speeds. Flystrike becomes an ongoing problem.

CHEMICAL PREVENTION- TIMING

In most cases once fly strikes start or threaten, protecting the sheep will depend mainly on the application of insecticides. A number of strategies for timing chemical applications can be used:

Treatment before risk periods

Treatment at a set time each year is a possibility in areas where flystrike is a problem in most years. For example, in some areas growers know that strikes will commence in October to November in most years and they may carry out preventative treatment to protect sheep through the spring risk period. This is especially common with autumn shorn flocks where sheep will be carrying 6-12 month fleece through spring and summer. Sometimes only the most susceptible sheep, for example weaners, will be treated.

The FlyBoss Decision Support Tools can help choose an optimal treatment time to suit different areas and different management systems. Sheep may also be treated to protect sheep through periods when labour for flystrike monitoring and emergency treatments is not available, for example while the manager or farm workers are on holidays or when they are occupied with other farm tasks such as harvest.

A variation of this approach, previously noted, is to treat before fly emergence from over-wintering to remove breeding sites for early season build up in fly numbers (Anderson and McKenzie et al 1990). This will be most effective when applied on an area-wide basis to minimise the effects of fly migration. LuciTraps® used in late winter- spring can be helpful with this timing by helping to build up a farm profile of dates of L. cuprina emergence from overwintering. This approach may be even more attractive as a strategy with the availability of dicyclanil (CLiK®) as a strike preventative. Dicyclanil can provide up to 24 weeks strike protection, which means that in addition to slowing early build up in L. cuprina numbers, protection is provided through much of the fly season. Further testing of this strategy is the subject of a research project currently underway in Victoria (Larsen et al. 2010)

Treatment when weather becomes suitable for strike

Sometimes in areas where strikes is frequently a problem or there are known to be susceptible sheep in the mob the option chosen is to treat once weather conditions become suitable for strikes. This approach can avoid early strikes, help to prevent unnecessary treatments and extends the period of protection by delaying treatment until a risk becomes apparent.

Treatment once strikes begin

Where only a few sheep are being struck, monitoring is possible and the level of strike is manageable sometimes the best option is to hand treat individual sheep when they become struck. However, particularly where labour is limiting, this may not be a viable strategy and a protective flock treatment may be needed. Common strategies are to treat when the number of strikes becomes too high to manage by hand treatment alone or once a pre-determined number or proportion of struck sheep in the mob has been exceeded.

CHEMICAL PREVENTION- CHEMICAL CHOICE AND APPROACH

Table 1 summarises the groups of chemicals currently available for fly strike protection.

Table 1. Chemical actives and flystrike preventative products

Some factors to consider when deciding the approach to be taken and the chemical to use include:

Method of treatment

Flock treatments for fly strike protection are most often applied by hand jetting, race jetting or in a spray on formulation from a hand held applicator. For a consideration of the relative advantages of each see the FlyBoss website. Fly chemicals are also sometimes applied by dipping or showering, although these are seldom efficient options. Some producers add a fly strike protectant to the dip when treating for lice after shearing to gain some blowfly protection. However, in most areas sheep are seldom struck soon after shearing and there will be little wool to ‘hold’ the blowfly chemical so the period of protection is limited. Where dipping is delayed to provide more wool to hold the chemical, often the effectiveness of lice control is compromised. Spray-on treatments are a better option if sheep need to be treated at shearing for management reasons. Dipping and showering are also occasionally used as emergency treatments, but this is generally undesirable because of potential wool damage, disease risk and management difficulties associated with using these methods with long woolled sheep

Protection versus treatment of struck sheep

Some products are very good blowfly preventatives, but because they act by interrupting moulting are not good at quickly resolving strikes. Products containing dicyclanil and cyromazine fall into this category and are not generally recommended for strike treatment. If there are significant numbers of already struck sheep in the mob and separate hand treatment of these animals is not anticipated, a chemical that has quick action against larvae, such as ivermectin, may be preferable

Period of protection required

Sometimes protection may be required for a short period when sheep cannot be closely monitored, for example while on holidays or during harvest. Often strike waves last only a few weeks. In these situations a cheaper chemical with shorter protection period may be sufficient. Label claims for protection period from different chemicals are shown in Table 1.

Level of monitoring possible and ease of emergency mustering

Where it is difficult to monitor sheep for strikes, or difficult or expensive to muster sheep for emergency treatment a product with long protection time may be most efficient. Treatment may be necessary when sheep are mustered for other reasons, for example crutching and shearing. This will require a product with a long treatment time.

Residues and withholding periods

Time to shearing and the wool withholding period may limit choice of product and method. Meat withholding and export slaughter intervals may also be a consideration if stock may be sold for slaughter. These are listed in the FlyBoss ‘Tools’ Products Module which contains a full listing of products registered for flystrike and lice control. In addition, some growers are targeting markets that stipulate particular residue limits (e.g. EU Ecolabel). The WoolRes module in FlyBoss tools can help choose chemicals for different situations to meet these limits.

NON CHEMICAL PREVENTION: Crutching, pizzle ringing, shearing

Strategic crutching, ringing and choice of shearing time should all be part of a well structured preventative program. The FlyBoss tools module can help time preventative shearings and crutchings for optimal effect. Crutching and shearing may also be useful as part of an emergency strike response. This can vary from a quick ‘dagging’ or ringing of the most susceptible sheep to a full mob crutching/ringing if shearers or other labour are available. During fly waves body strike is often the problem. Some sheep may need to be shorn and treated but even though shearing is effective, the difficulties in quickly getting shearers and other operational considerations mean that this is seldom a realistic option for the whole mob in strike waves.

NON CHEMICAL PREVENTION: Controlling scouring

Scouring, subsequent wool staining, dags and breech strike are often associated with gastrointestinal parasite infestation. This can be the direct result of high worm burdens, or due to ‘hypersensitivity’ scouring in sheep with low worm immunity, most commonly young sheep. Where breech strike is the problem and high worm burdens are the cause (this should be confirmed with a worm egg count) drenching can markedly reduce strike incidence (Morley et al. 1976). If scouring is thought to be nutritional, the cause should be identified and the animals moved to another paddock or other action taken. Occasionally scouring can be caused by bacterial or protozoan infections. Diagnosis and treatment in these instances will generally require specialist expertise.

The impact of scouring will depend on breech wrinkle, whether the sheep have been mulesed or had other breech treatment, whether the tails have been docked to the correct length (docking too short stops the sheep from lifting the wool out of the way when defaecating or urinating, increasing liability to staining), or whether they have been recently shorn or crutched.

NON CHEMICAL PREVENTION: Paddock choice

Most properties have ‘bad strike paddocks’ (often protected creek paddocks or wet flats) and others where there is much less strike (for example exposed hill paddocks where sheep dry out rapidly and fly activity is low). A possible option during periods of high strike incidence is to move high-risk flocks to these exposed paddocks

NON-CHEMICAL PREVENTION: Flock segmentation

Not all mobs and sheep on a property will be equally susceptible to strike and identifying those with high susceptibility can improve the efficiency of control. This can be at the mob level, for example, weaners will generally be the mob most susceptible to scouring and fleece rot and consequently generally most susceptible to breech and body strike. A preventative jetting or an extra crutch may be the best option with these sheep whereas it may not be necessary with older sheep. Sometimes it may even be advantageous to segment within a flock, for example crutching or dagging just those sheep with dags, or drafting off fleece rot or dermo affected sheep. The susceptible sheep can then be suitably treated or perhaps put into paddocks where they can be closely monitored.

INTEGRATED PREVENTION

In an ideal integrated and well structured preventative program the use of insecticides can be used together with crutching or shearing, drenching to control scouring or a move to less strike prone paddocks.

TREATING STRUCK SHEEP

The most important part of treating struck sheep is shearing or clipping away the wool from around and above the wound. Often properly clipped strikes will resolve without chemical treatment. Most recommendations suggest clipping at least 5 cm into the clean wool around the wound. This allows the wound to dry out and exposes maggots to environmental influences, removes attractive stained wool which attracts flies for further strikes, exposes maggot ‘runs’ and ensures the whole strike is treated. How closely the wool is clipped influences effectiveness and Levot and Sales (1990) found that only 8% of strikes shorn with a shearing handpiece still contained maggots the next day. This compared to a previous trial where clipping was conducted with hand shears and the failure rate was 35%. L. cuprina has developed resistance to some of the chemicals used in strike treatments, particularly the organophosphates, and many are not completely effective at killing maggots. The main groups of chemical actives used in flystrike dressings are shown in Table 2. Ivermectin-based dressings appear be the most effective in killing maggots (Levot et al. 1999).

Larvae that escape from strikes during treatment can pupate and contribute to the next generation of flies. For this reason it is important to collect and dispose of wool and maggots clipped from the strike so that flies that might cause future strikes cannot develop. A convenient method often suggested is to collect maggots and struck wool in a garbage bag and leave in the sun until all of the maggots are dead. This may be particularly important early in the season and during hot dry periods where these maggots can contribute to early season build after overwintering or population maintenance in low strike periods. In addition, maggots that have been exposed to treatment chemicals but not killed can add to selection for chemical resistance, so it is important that they not breed.

Table 2: Chemicals for flystrike treatment

References

Anderson JME, Shipp E and Anderson PJ (1983) Preferred habitats of blowflies in the arid zone of NSW. Proc. Second National Symposium on the Sheep Blowfly and Flystrike in Sheep, Sydney, December 1983. Dept of Agric NSW. Pp 133-5.

Anderson PJ, Shipp E, Anderson JME and Dobbie W (1988) Population maintenance of Lucilia cuprina in the arid zone. Aust. J. Zool. 36, 241-9.

FlyBoss website: http://www.flyboss.org.au/

Larsen JWA, Anderson N and Tyrell LD (2010) Comparison of Programs For the Control of Blowfly-Strike in Mulesed and Un-Mulesed Merino Sheep in South-Eastern Australia Proc. Aust. Soc. Anim. Prod. 28, 77.

Levot GW and Sales N (1998) Protection from restrike provided by flystrike dressings. Aust. J. Exp. Agric. 38, 551-4.

Levot GW, Sales N and Barchia I (1999) In–vitro larvicidal efficacy of flystrike dressings against the Australian sheep blowfly. Aust. J. Exp. Agric. 39, 541-7

Morley FHW, Donald AD, Donnelly JR, Axelson A and Waller PJ (1976) Blowfly strike in the breech region of sheep in relation to Helminth infection. Aust Vet J 52, 325-9.

Urech R, Green PE, Rice MJ, Brown GW, Webb P, Jordan D, Wingett M, Mayer DG, Butler L, Joshua E, Evans I, Toohey L, and Dadour IR (2009) Suppression of populations of Australian sheep blowfly, Lucilia cuprina (Wiedemann) (Diptera: Calliphoridae), with a novel blowfly trap. Aust. J. Ent. 48, 182-8.

Vogt WG and Woodburn TL (1979) In, National Symposium on the Sheep Blowfly and Flystrike in Sheep, Sydney, June 1979. (New South Wales Department of Agriculture) pp 23-32.

Wardhaugh KG and Dallwitz R (1984). Covert flystrike. Wool Technol Sheep Breed 32 (3), 15-19.

Waterhouse DF (1947). The relative importance of live sheep and carrion as breeding grounds for the Australian sheep blowfly, Lucilia cuprina. Coun.Sci. Ind Res Aust Bull. 217.