Counting Fly Numbers in the Environment

Monitoring fly populations on feedlots

Fly population monitoring is an important part of an integrated pest management (IPM) program. Information on the identity of the problem species and population fluctuations can provide early warning of fly waves. Fly control can be more effective if monitoring is started before fly numbers increase.

To keep track of fly population fluctuations and to assess the effectiveness of actions, population monitoring must be regular and systematic. The monitoring system and the site, timing and duration of the monitoring have to remain constant. The results should be assessed and recorded immediately after each monitoring period. Graphing the numbers of flies helps to identify trends in fly populations (Figure 1).

Several monitoring systems for fly populations are available. Adult flies can be monitored by using sticky sheets or traps, or structured observations of fly resting sites or animal behaviour. The extent of fly breeding can be established by inspecting numbers of larvae in major fly breeding areas.

To improve the consistency of the results, the same person should be responsible for monitoring flies in a feedlot. Some of the monitoring systems are more subjective than others and only a single operator can deliver useful results.

Monitoring fly larvae populations will give an earlier indication of fly population increase than monitoring adult fly populations, but it is more difficult to conduct and requires a trained operator.

Adult fly monitoring

Sticky trapping sheets

Place sticky trapping sheets on vertical walls or posts where flies rest, and away from dusty areas (Figure 2). Record the numbers and species of flies caught over a fixed time. Choose the period of time the sticky sheets are left out to avoid saturating the sheet with flies (1–7 days may be appropriate). The major feedlot flies can be identified using a magnifying glass and the pictorial key in the fly identification section.

Sticky trapping sheets are available from Bugs for Bugs (Mundubbera, Queensland) and Starkeys Products (Wangara, Western Australia). Smaller sticky surfaces such as fly tapes or ribbons can be used although these are less accurate fly monitoring tools.

Figure 2. Adult fly monitoring. Image taken by Jerry Hogsette USDA, courtesy of the Queensland Department of Agriculture and Fisheries

Traps

Use Alsynite traps (cylindrical alsynite panel with a clear sticky covering sheet) (Figure 3) to attract and trap stable flies and house flies in open areas. This trap is available from Olson Products (Ohio, USA).

Figure 3. Alsynite fly trap. Image taken by Jerry Hogsette USDA, courtesy of the Queensland Department of Agriculture and Fisheries

Observations

Counting flies where they rest, such as on fence railings, feed bunks or walls, can be used as an indicator of fly populations. This method is less accurate than sticky sheets because counts will depend on the time of day, weather conditions and other variables. It may also be difficult or impossible to identify the fly species present.

The number of adult flies in feedlots is closely related to observed cattle behaviour. The number of tail swishes, ear flicks and head tosses observed over a specified time (e.g. 1 minute) on several animals can be used to gauge house fly and bushfly populations. Counting the number of leg stomps can be used to measure stable fly populations.

Fly larvae monitoring

Monitoring larval populations gives an earlier indication of increases in fly populations than adult fly monitoring. The extent of fly breeding can be established by closely examining manure at major fly breeding sites. Manure needs to be turned over and examined at several locations and a larval rating assigned to each site, e.g. 1 for very low to 5 for very high numbers of larvae (Figure 4).

Figure 4. Recording fly larvae. Image by Jerry Hogsette USDA, courtesy of the Queensland Department of Agriculture and Fisheries.

House fly and stable fly larvae can be distinguished by inspecting their posterior spiracles (Figure 5).

Figure 5. Fly larvae with the location of the spiracles circled and close up images of house fly and stable fly spiracles. Images courtesy of Peter Green, Queensland Department of Agriculture and Fisheries

Graphing the numbers of larvae helps to identify trends in fly populations (Figure 6).

Figure 6. Example graph of fly larvae monitoring. Image courtesy of Geoff Brown, Queensland Department of Agriculture and Fisheries