Instar-specific sensitivity of specialist Manduca sexta larvae to induced defences in their host plant Nicotiana attenuata

1. The time delay associated with the activation of induced defences is tho ught to be a liability for this type of defence because it allows herbivore s to remove biomass before the defence is fully induced. When defences are costly and plants grow with competitors, however, it may be more advantageo us not to induce defences too fast and motivate the herbivore to move to th e neighbour when it is most voracious. 2. Such a strategy can only work when the costs for the herbivore of moving to a neighbouring plant are smaller than the costs of staying on a fully i nduced plant. For lepidopteran herbivores, both the sensitivity to induced defences and the costs of moving may vary considerably between instars and this variation may constrain the plant's defensive opportunities. 3. This study was designed to examine whether the cost of moving, mimicked by a starvation period of 8 h. was larger than the cost of staying on a ful ly induced plant for each larval instar of the specialist Manduca sexta fee ding on induced and control tissues of Nicotiana attenuata. 4. For first- and second-instar larvae, the costs of moving were larger tha n the costs of staying on a fully induced plant. In contrast, feeding on in duced plant material retarded development in third-instar larvae more than did starvation, indicating that in this instar the costs of leaving are sma ller than the costs of staying on an induced plant. More than 98% of the li fetime leaf mass consumed by a M. sexta larva is consumed during the fourth and fifth instars, and during these instars larval development was not aff ected by either induced defences or starvation. Thus the third instar, the stage just before larvae cause the majority of damage, represents a window of sensitivity to induced defences during which larvae can be motivated to change plants. 5. These results suggest that N. attenuata plants, which commonly compete w ith conspecifics in nature. have the opportunity to manipulate the behaviou r of the specialist herbivore M. sexta to minimise the fitness effects of i nducing defences when these defences are most costly, i.e, when plants grow under intraspecific competition.