Pulsed odors from maize or spinach elicit orientation in European corn borer neonate larvae

Lepidoptera larvae are capable of orienting towards or away from plants by using odors as cues but whether this attraction is innate or secondarily acquired remains unknown. We tested the hypothesis that European corn borer (ECB) neonate larvae express an innate attraction towards odors released from maize, and avoidance towards odors from spinach. Neonate larvae were placed on a locomotion compensator within a constant stream of humidified air that was loaded intermittently with airborne odors drawn from potted plants. The odor stream was delivered continuously or pulsed (1 to 10 sec pulses) at 40 ml/min. ECB larvae oriented toward maize odors pulsed at 2 to 6 sec but walked away from maize odors delivered at lower frequencies (9 and 10 sec pulses or to continuous ones). They consistently walked away from spinach odors, irrespective of the pulsing regime except at 1 sec pulses that did not elicit orientation. We further explored odor intensity on orientation towards maize odors by adjusting the odor stream intensity. At higher intensity (60 ml/min), the direction reversal started at the 6 sec half period, while at lower intensity (20 ml/min), it showed up only for the continuous stimulus. ECB larvae exhibit a striking ability to lock on to a direction, which they maintained despite gaps of up to 10 sec in the odor stream. Our results demonstrate that ECB neonate larvae express innate orientation preferences towards natural odors from plants. These reactions correlate well with the biological value of these plants for ECB: maize generally is accepted by ECB larvae and adults, while spinach represents a poor host because it produces (non-volatile) phytoecdysteroids that are toxic and deterrent.