Feeding and foraging behaviour of a generalist caterpillar: are third instars just bigger versions of firsts?

Similarities and differences in foraging behaviour between first and third instar Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) on Vigna radiata (L.) R. Wilczek (Fabaceae) (mung bean) were determined by frequent, repeated observation of insects on whole plants. The time apportioned to feeding, resting and searching by these two larval instars differed. Third instars were often found feeding on exposed areas of the plant and, if in the terminal parts, would leave these areas more frequently than first instars. Third instars fed at fewer sites and spent less time searching and resting, fed in longer bouts and spent 20% more time feeding than first instars. Although both instars tended to move upwards to the top of the plant, this applied to approximately half of the third instars observed compared to all of the first instars. First instars were found to have a higher relative growth rate than third instars. The results show that it cannot be assumed that first instars are just smaller versions of late instars.