Traps are tools frequently used to monitor and control pests. Therefore, it is important to study the prerequisites of their use. The instant active area of a trap (the instantly available area of a trap that is able to capture a pest) is a determinant of trap efficacy. However, to measure the instant active trap area is difficult. Therefore, we developed a technique of computer image analysis of digital photography to evaluate the instant active trap area, occupancy (area of insect bodies captured on the trap) and contamination (area of filth left by pests on the trap, such as wing-scales or faeces) of it. This study illustrates its use with two types of sticky traps applied to monitor the German cockroach, Blattella germanica, and the Mediterranean flour moth, Ephestia kuehniella. We found that moths decreased the capacity of the sticky surface more than cockroaches because of its contamination by wing-scales. The sticky trap area covered by wing-scales was nearly as large as the area occupied by moth bodies. Cockroaches contaminated the sticky surface by their faeces but the contaminated area was less than 2% of the area occupied by cockroach bodies. The results indicate that moths are heavy contaminators while cockroaches are weak contaminators of sticky traps.