Selection pressure by specialist and generalist insect herbivores leads to optimal constitutive plant defense. A mathematical model

• Published in: Ecology and evolution Vol. 13; no. 12; pp. e10763 - n/a
• Main Authors: Chakraborty, Suman, Gershenzon, Jonathan, Schuster, Stefan
• Format: Journal Article
• Language: English
• Published: England John Wiley & Sons, Inc 01.12.2023; John Wiley and Sons Inc; Wiley
• Subjects: Brassicaceae; Defense mechanisms; Differential equations; Evolution; generalist insects; Glucosinolates; Herbivores; Herbivory; Host plants; Host selection; Independent variables; Insects; mathematical model; Mathematical models; Natural enemies; optimal constitutive defense; Optimization; Ordinary differential equations; Predators; specialist insects; Theorectical Ecology; mathematical model; natural enemies; specialist insects; optimal constitutive defense; generalist insects; glucosinolates
• ISSN: 2045-7758; 2045-7758
• DOI: 10.1002/ece3.10763

Brassicaceae plants have the glucosinolate–myrosinase defense system, jointly active against herbivory. However, constitutive glucosinolate (GLS) defense is observed to occur at levels that do not deter all insects from feeding. That prompts the question of why Brassicaceae plants have not evolved a higher constitutive defense. The answer may lie in the contrasting relationship between plant defense and host plant preference of specialist and generalist herbivores. GLS content increases a plant's susceptibility to specialist insects. In contrast, generalists are deterred by the plant GLSs. Although GLSs can attract the natural enemies (predators and parasitoids) of these herbivores, enemies can reduce herbivore pressure to some extent only. So, plants can be overrun by specialists if GLS content is too high, whereas generalists can invade the plants if it is too low. Therefore, an optimal constitutive plant defense can minimize the overall herbivore pressure. To explain the optimal defense theoretically, we model the contrasting host selection behavior of insect herbivores and the emergence of their natural enemies by non‐autonomous ordinary differential equations, where the independent variable is the plant GLS concentration. From the model, we quantify the optimal amount of GLSs, which minimizes total herbivore (specialists and generalists) pressure. That quite successfully explains the evolution of constitutive defense in plants from the perspective of optimality theory. Specialist herbivores are attracted by constitutive plant defense while generalists are deterred by such a defense. Therefore, there is an optimal level of plant defense chemicals. By a mathematical model, we determine that optimum.