The effect of temperature and photoperiod on the time taken for a meal to pass through the gut, defecation and digestion in the last larval instar of Spodoptera littoralis (Lepidoptera: Noctuidae)

Abstract

Spodoptera littoralis (Boisduval) is a highly destructive and polyphagous insect pest of great economic importance. It develops throughout the year and the larvae are non-diapausing. Little work has been done on the time taken for food to pass through the gut of S. littoralis. Thus, this study on starving and well fed last instar larvae of S. littoralis aimed to determine the effect of temperature and photoperiod on the time taken for a meal to pass through the gut, defecation and digestion. The results indicate that it depended on temperature, photoperiod and hunger. The time that elapsed between a larva being fed and the production of the first faecal pellet, which is a measure of the time it takes for food pass to pass through the gut (SFP), differed significantly at different temperatures and photoperiods. The SFP was longer at 15°C than at 30°C. At 20°C and under a short photoperiod, SFP was very much shorter than under a long photoperiod (18L : 6D). In addition, at 20°C, the time to defecation was significantly longer under a photoperiod of 12L : 12D than under continuous light. In all the tests > 95% of the larvae produced four faecal pellets per meal. The digestibility values varied significantly in the different tests. This study offers new insights into the passage of food from the crop to the rectum at different temperatures and photoperiods. This might help in understanding the adaptability of S. littoralis and may also help in controlling this important pest.