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

Abstract

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.

Selection Forces Driving Herding of Herbivorous Insect Larvae

“…One suggested mechanism is that insects feeding in synchrony maximize food intake before induced defenses become expressed (Anderson et al, 2015). The study of plant metabolic pathways underlying induced defense is a field that is progressing rapidly, which provides opportunities for investigating benefits to collective feeding at the molecular level [for example de Bobadilla et al (2021)]. Improved understanding of these advantages could generate meaningful insights into the temporal and spatial feeding patterns of herbivores in general.…”

“…One possible mechanism is synchronous feeding to outpace and overwhelm production of induced chemical defenses (Denno and Benrey, 1997). Indeed, induced defenses are activated in plant tissues in response to herbivore biting, but this process takes time; hence herbivores can avoid these toxic compounds by feeding together on one leaf until induced defenses appear, then moving away on to an undamaged leaf (de Bobadilla et al, 2021). Mathematical modeling shows that this time-lag in induced defenses can lead mobile herbivores to aggregate, feed synchronously and move from induced to undamaged plant parts (Anderson et al, 2015).…”