Why does the citrus red mite not try to flee when preyed upon by caterpillars? -A tragedy caused by an inflexible predator avoidance measure-

Why does the citrus red mite not try to flee when preyed upon by caterpillars? -A tragedy caused by an inflexible predator avoidance measure-

December 3, 2013


Assist. Prof. Yano

Herbivorous and carnivorous arthropods on a leaf experience deadly predator–prey interactions, just like the herbivorous and carnivorous mammals of the savanna. The citrus red mite Panonychus citri, which feeds on citrus leaves, lies down on leaf surfaces most of the time (Fig. 1, left), although the mite lifts its body when it copulates and oviposits or when it moves to uninfested leaf parts (Fig. 1, right). Swallowtail caterpillars, the body length of which is up to 35 mm, feed on citrus leaves regardless of the tiny mite’s presence (<0.5 mm). However, the mite rarely attempts to flee even when it is dying due to a caterpillar's attack. Although it may be a rare disaster, why does the mite not run away from the attack as wild animals do from a rare bush fire? Dorsal setae (hairs) of the lying mite extend in all upper directions, while its hairless under body parts adhere to the leaf surface (Fig. 1, left).


Figure1: The citrus red mite P. citri is well defended when lying (left), but less defended when moving (right).

The major predators of the mite are not the caterpillars, but predatory mites whose sizes are similar to that of P. citri. Regardless of the direction from which a predatory mite approaches the prey mite, the predatory mite cannot avoid confronting a hair before it bites on the prey body surface. The lying P. citri does not move when its hairs make contact with something, whether animals or artificial substances. Most predatory mites give up approaching P. citri because of the interfering hair, while some predatory mites forcibly approach the prey body surface, despite the hair and are eventually repelled by it (Fig. 2, left).


Figure 2: An inflexible antipredator defense of P. citri, lying down with protective hairs, is effective against its major enemy predatory mites, while it results in tragedy when being preyed upon by a gigantic caterpillar.

Compared to normal lying P. citri, both manipulated non-lying mites and hair-removed mites suffered much higher predation by predatory mites, demonstrating the defensive function of the lying posture and the hairs. Thus, not moving in response to stimulation of the hairs functions as an antipredator defense of P. citri, while it results in consumption by the gigantic caterpillar, against which the protective hairs are totally ineffective (Fig. 2 right). This tragedy resulting from this inflexible antipredator defense of P. citri suggests that the benefits of avoiding frequent attacks by predatory mites are larger than the costs of the extreme but rare disaster caused by gigantic caterpillars that presumably happens less than once in a lifetime of the mite.


Figure 3: Appearance of P. citri before (above) and after (below) hair removal