It takes 7-10 days for Spodoptera litura to grow from the third instar to the sixth instar. The reason is mainly related to its growth and development speed and environmental conditions. Spodoptera litura is an insect whose growth and development rate is affected by factors such as temperature, humidity, and feed. Under suitable environmental conditions, Spodoptera litura grows rapidly and the time required from the third to sixth instars will be relatively short. When the temperature is low, the humidity is low, or the feed supply is insufficient, the growth rate of Spodoptera litura will slow down, and the time required from the third instar to the sixth instar will be longer. In addition, Spodoptera litura needs to molt twice from its third to sixth instars, and each molt requires a certain amount of time and energy. Therefore, during this period, the growth rate of Spodoptera litura will slow down and more time will be needed to complete growth and development. To sum up, the reason why it takes 7-10 days for Spodoptera litura to go from the third instar to the sixth instar is the result of the simultaneous action of multiple factors such as its growth and development speed and environmental conditions.