Flies that get stuck on the ribbons can then be counted and identified. It is important to use the same number of ribbons, and use the same locations each time, to accurately compare fly populations. Flies are more active during hot weather than when cool, so you need to factor in the temperature. Spot cards are another simple device for measuring fly activity. These are white file cards which can be stapled to pre-chosen locations in a facility so that both sides are exposed.
Flies resting on the cards leave brown regurgitation and fecal spots.
Up to 10 locations in a house should be used. After removal the number of spots per card can easily be counted. The number of flies per ribbon or spots on cards will vary according to the location. Doing it once will tell you nothing. You need to establish a base line to use as a reference, and when counts are above a certain level, controls need to be applied.