Occasionally, insects are detected in bottles of beer on the market. When insects are detected in pasteurized beer, we are able to determine whether they entered the bottle during the packaging process or after the bottle was unstoppered by measuring the catalase activity of the insect. However, this method is inaccurate and impractical for insects in draught beer. We have investigated and developed a new analytical method. This new method can determine the number of days an insect has been in the beer by measuring the cholinesterase activity remaining in the insect. The cholinesterase activity varies little with the individual. This method is very simple, requiring only a homogenizer, spectrophotometer, centrifuge, and reagents.