Induced responses in Nicotiana attenuata affect behavior and growth of thespecialist herbivore Manduca sexta

Many plants employ induced responses against generalist herbivores. Special ist herbivores, however, may employ several mechanisms to overcome the nega tive effects of induced plant defenses. Here we test how the behavior and d evelopment of specialist Manduca sexta larvae are affected by induced respo nses in their natural host plant Nicotiana attenuata. On a spatial scale re levant to both the plant and the herbivore, we first determined how methyl jasmonate (MeJA)-induced responses, such as increased nicotine production, affect the tendency of larvae to leave induced plants. When larvae were all owed to move between two plants planted in one pot, they left an MeJA-treat ed plant faster than a control plant. When both plants in the pot were MeJA -treated, the larvae developed more slowly than when both plants were unind uced, or when the larvae had the opportunity to move to an uninduced neighb or. The sooner larvae moved from an MeJA-treated plant to an untreated neig hbor, the larger the body mass they attained. This demonstrates that M. sex ta larvae can compensate behaviorally for the deleterious effects of induce d plant responses. These effects were observed in plants grown under both l ow and high N supply rates, though the effects were more pronounced under h igh N. To examine the consequences of the timing and the direction of the h ost plant switching behavior for larval development, neonate larvae were fe d leaves excised from induced and uninduced plants. Larvae confined to MeJA -treated leaves had higher mortality rates and grew slower than larvae fed only control leaves. This demonstrates that MeJA-induced responses decrease growth and development of specialist herbivores that do not have the behav ioral option of moving to an uninduced plant. The sooner the larvae were sw itched to MeJA-treated leaves, the slower their development compared to lar vae fed only uninduced leaves. In contrast, the sooner larvae fed MeJA-trea ted leaves were switched to control leaves, the faster they developed. Agai n the effects of MeJA treatment were stronger in plants grown under high N supply. We propose that induced plants growing in close competition with an uninduced conspecific may offset the fitness costs of these induced respon ses and perhaps obtain a fitness benefit by motivating herbivores to move t o their neighboring competitors.