Plasticity in Induced Resistance to Sequential Attack by Multiple Herbivores in Brassica Nigra

In nature, plants interact with multiple insect herbivores that may arrive simultaneously or sequentially. Because insect herbivores often have a negative effect on plant fitness, plants have evolved mechanisms to defend their tissues from being eaten. There is extensive knowledge on the regulation of induced plant responses to single or dual attack. However, we lack information on how plants defend against the attack of multiple herbivores that arrive sequentially. In this study, we investigated whether Brassica nigra plants are able to defend themselves against caterpillars of the late arriving herbivore Plutella xylostella, when plants had been previously exposed to sequential attack by four other herbivores. We manipulated the order of arrival and the history of attack by four herbivores to investigate which patterns in sequential herbivory determine resistance against the fifth attacker. We recorded that history of sequential herbivore attack differentially affected the capability of Brassica nigra plants to defend themselves against larvae of P. xylostella. A sequence of attack with four episodes of attack by P. xylostella induced resistance to larvae of P. xylostella. The number of times the plant was attacked by herbivores of the same feeding guild, the identity of the first attacker, the identity and the guild of the last attacker as well as the order of attackers within the sequence of multiple herbivores influenced plant resistance to subsequent herbivory. In conclusion, this study shows that history of sequential attack is an important factor determining plant resistance to herbivores.