Distribution and biology

Distribution

Buffalo flies are mainly a pest in northern cattle but have been steadily increasing their range southwards (Figure 1). This southerly range expansion is expected to accelerate as the climate warms. Buffalo flies have now been found as far south as Maitland in eastern NSW and were found as far west as Narromine, Dubbo and Bourke during the wet years of 2010-11.

Figure 1. Map of Australia showing the southward spread of buffalo fly over time, since it was first reported in Darwin in 1838. Image courtesy of Peter James.

In northern Australia, the main fly season extends from November to April with lower numbers present for the rest of the year. Buffalo flies are a consistent pest and in highest numbers in the high rainfall coastal areas and then spread and infest much larger areas of western Queensland, the Northern Territory and northern Western Australia following heavy summer rainfall (Figure 2). In winter buffalo fly numbers are limited by lower temperatures, dry conditions and frosts and flies disappear from most of these areas. In southern areas, the main season is shorter with flies also limited by cool temperatures and frosts. In these areas the flies persist through winter in low numbers on cattle running in sheltered, moist, hilly areas that are less likely to be affected by frost. During spring and summer fly numbers build up and spread out from these overwintering foci, sometimes by direct flight, but more commonly with cattle when they are moved.

Figure 2. Distribution of buffalo flies in Australia showing regions with permanent and seasonal populations. Image courtesy of Meat and Livestock Australia.

Biology

Both male and female buffalo flies feed on blood, with the adult flies spending all of their lives resting on or in close association with cattle. The eggs of buffalo flies are deposited in or under freshly deposited dung pats, and hatch in 15-24 hours to first stage maggots which are a few mm in length and difficult to see with the naked eye. The maggots feed on the dung and pass through three larval stages with the fully fed larvae growing to approximately 3 - 6 mm in length. Completion of the larval phase takes about 4 - 5 days and maximum larval survival occurs at about 25⁰C and 75-85% dung moisture content. After they have finished feeding the third stage maggots seek out a drier position, often in the soil, beneath or at the edges of the dung pat, and form a puparium which is brown and approximately 4 – 5 mm in length. The adult flies emerge from the puparium after 3 - 5 days and live for 2 to 3 weeks if they find suitable cattle hosts. During the main buffalo fly season completion of the life cycle takes 12-14 days from egg to egg (Figure 3).

As soon as wings of the adult fly have hardened it will seek a host animal. If cattle are close, the flies usually do not disperse far and will rapidly move onto a close-by animal and commence feeding. When cattle are not readily available buffalo flies can fly up to 10 km to find a host. The female fly requires a blood meal to mature her eggs and can commence egg laying within 4 days of hatching from the puparium. When she is ready to lay she moves to the lower flanks of the cow and waits for the cow to defecate. When this occurs she rapidly leaves the cow, flies to the fresh cattle dung and lays from 14 - 26 eggs.

Buffalo flies are extremely dependent on the availability of susceptible cattle to provide blood and most die within a couple of days once separated from cattle, or if they fail to find a host. Optimum conditions for the flies are annual rainfall more than 500 mm and mean daily temperatures of 27-30⁰C. Through most of their range fly numbers are highest in late summer and early autumn.

Survival of the eggs and larval stages is highly dependent on the availability of moisture and they die rapidly if dehydrated. During summer in northern Australia when conditions are moist as a result of summer rainfall and temperatures are warm, buffalo flies rapidly complete their life cycle and numbers build up quickly. However in winter, when conditions are drier and temperatures are lower, the buffalo fly larvae can’t complete their life cycle before the dung pat dries out, buffalo flies fall in numbers and disappear completely in marginal areas. In addition, at temperatures less than about 15⁰C the larvae and eggs don’t complete development, even if moisture is present, and it is often considered that the first hard frost marks the end of the buffalo fly season.
 

Figure 3. Buffalo fly life cycle. Image created by Madison Mayfield