Energy consumption (EC) is essential for the vital activities of living creatures. This energy is prepared in phosphorylation oxidative by burning high-energetic molecules same as carbohydrates, lipids and proteins. As insects, same as other organisms, have different activities throughout the daytime, knowledge of the amount of this activity can provide good insight into insect situation against pesticide application. So the amount of energy consumption of cotton aphid as an important and invasive crop pest, was assessed every four hours at different zeitgeber times (including 0 (24), 4, 8, 12, 16 and 20) in two photoperiod: 16 hour photophase and 8 hour scotophase and in constant light in a 24 hour circadian cycle. Also this amount was assessed with the application of LD50 of diazinon at those zeitgeber times under 16 hour photophase and 8 hour scotophase condition with selected doses and four replications. Assessment of EC was carried out by a little change according to Yuval and Coen-Janssen protocol. For doing EC experiments, frozen adult aphids of the same age were used in three replications. The graphs of EC changes under two photoperiod conditions in this experiment showed that the highest amount of that has occurred in ZT20 according to the night time under natural conditions. Moreover results showed that the amount of EC under two photoperiod conditions and in the presence and absence of diazinon, was at low level. On the other hand, it seems that circadian rhythm of EC is controlled by the endogenous circadian clock in this insect and it doesn’t control by environmental conditions. This is understandable by activity in same amount and same trend in both photoperiod conditions; but study the changes of EC trend at the presence of challenging condition, e.g. diazinon presence, has proved that the amount of EC is different in depends of existence of stress factors and of course organism reflection in front of pesticide at different times, so the amount and trend of EC is different at the presence of diazinon and the absence of it. According to the results of this investigation, it is completely obvious that after a 24 hours challenge with pesticide, the amount of EC highly fall down and this subject maybe cause by disruption of proteins in electron transport chain; because the parameters that are assessed as EC in cotton aphid are depended to the activity of electron transportation in electron transport chain that is considered as oxygen consumption.