Octopamine mimics the effects of parasitism on the foregut of the tobacco hornworm Manduca sexta

The parasitic braconid wasp Cotesia congregata lays its eggs inside the body of the larval stage of its host, the moth Manduca sexta. The Cotesia congregata larvae develop within the hemocoel of their host until their third instar, when they emerge and spin cocoons and pupate on the outer surface of the caterpillar. From this time until their death approximately 2 weeks later, the Manduca sexta larvae show striking behavioral changes that include dramatic declines in spontaneous activity and in the time spent feeding. Coincident with these behavioral changes, it is known that octopamine titers in the hemolymph of the host become elevated by approximately 6.5-fold. Octopamine is an important modulator of neural function and behavior in insects, so we examined hosts for neural correlates to the behavioral changes that occur at parasite emergence. We found that, in addition to the changes reported earlier, after parasite emergence (post-emergence), Manduca sexta larvae also showed marked deficits in their ability to ingest food because of a disruption in the function of the frontal ganglion that results in a significant slowing or the absence of peristaltic activity in the foregut. This effect could be produced in unparasitized fifth-instar larvae by application of blood from post-emergence parasitized larvae or of 10(−6)mol l(−1)d,l-octopamine (approximately the level in the hemolymph of post-emergence larvae). In contrast, blood from parasitized larvae before their parasites emerge or from unparasitized fifth-instar larvae typically had no effect on foregut activity. The effects of either post-emergence parasitized blood or 10(−6)mol l(−1) octopamine could be blocked by the octopamine antagonists phentolamine (at 10(−5)mol l(−1)) or mianserin (at 10(−7)mol l(−1)).