Selection pressure by specialist and generalist insect herbivores leads
to optimal constitutive plant defense. A mathematical model
Brassicaceae plants have the glucosinolate-myrosinase defense system,
jointly active against herbivory. Glucosinolates (GLS) are hydrolysed by
myrosinase to produce isothiocyanates as soon as herbivory begins.
Isothiocyanates exert detrimental effects on the feeding insects.
However, constitutive 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 high constitutive defense. The
answer may lie in the contrasting relationship between plant defense and
host plant preference of specialist and generalist herbivores. One of
the reasons plants are in this dilemma is that they do not know what
kind of herbivore will attack them in any given year, and thus have to
be prepared for different possibilities. GLS content increases the
susceptibility to specialist insects because these are attracted to
plants with a high GLS content and are capable of coping with the toxin.
In contrast, generalists are deterred by the plant GLS. Although GLS 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 optimal defense theoretically, we represent the
contrasting host selection behavior of insect herbivores and, in
addition, the emergence of their natural enemies by a non-autonomous
ordinary differential equation model, where the independent variable is
the plant GLS concentration. From the model, we quantify the optimal
amount of GLS, which minimizes the total herbivore (specialists and
generalists) pressure. That quite successfully explains the evolution of
constitutive defense in plants from the perspective of optimality