When herbivores come back: effects of repeated damage on induced resistance

1. Plants are known to respond to damage with subsequent changes in resistance. The consequences of these changes for plant fitness and herbivore populations will depend on both the response of a plant to a particular attack at a given moment and on how plants respond through time to varying levels of damage and varying numbers of attacks. While a small number of studies document how induced resistance changes with time after attack (time course of induction) and different levels of damage, few studies have examined the cumulative level of resistance after multiple attacks, and little is known about how the plant’s response to damage changes with subsequent attacks. 2. Two experiments were conducted to address the consequences of repeated damage for resistance of tomato (Solanum lycopersicum, var Castlemart) to a common pest, the beet armyworm (Spodoptera exigua). The first experiment documented the time course of resistance following a single damage event using both growth and choice bioassays. The second experiment examined whether the plant responded differently to one versus two damage events. 3. Results show that plants were significantly induced by day 1 and remained induced until 15 or 20 days later, suggesting that repeated damage during the response to initial damage is possible. Plants receiving a second bout of damage were able to further increase their resistance level over the level reached in response to a first bout of damage, but the magnitude of response to the second damage event was initially smaller and slower than the response to a single damage event. 4. There was no evidence in this study for immune-like memory in induced resistance. Results of this study suggest that plants can respond to repeated damage, but that there is some limit on responses to repeated damage. Such limits on total plant resistance will affect the influence of induced resistance on herbivore populations and are consistent with assumptions of existing models of induced resistance and herbivore population dynamics, although models have not yet considered the consequences of slower rather than smaller responses to repeated damage.