Foraging in nature by larvae of Manduca sexta-influenced by an endogenous oscillation.

Foraging by larvae of the moth Manduca sexta was studied in the field on the native host plant Datura wrightii. Continuous observation of individuals revealed that larvae moved very little, but fed for approximately one third of the time, independent of temperature or photoperiod, although the larger ones fed for the longest times. They were indiscriminate with respect to the part of plant fed upon, and analysis of feeding bouts and interbouts indicated that the pattern of feeding was not strongly influenced by direct nutritional factors. Attacks by parasitoids had a significant impact on feeding behavior. All individuals showed strong regularity in the times when feeding was initiated, and analyses demonstrated that feeding was initiated at times corresponding to fitted oscillation periods of approximately 4 min. Feeds were not initiated on every peak, but at times corresponding to the primary period, or harmonics of it. The period was not influenced by temperature, was different for all individuals, and was asynchronous among individuals. We conclude that an endogenous neural oscillation underlies the rhythm, and strongly influences the overall pattern of foraging. We discuss the oscillation in terms of its possible ecological significance, particularly the need for vigilance.