Plant structural changes due to herbivory: Do changes in Aceria-infested coconut fruits allow predatory mites to move under the perianth?

Abstract

Being minute in size, eriophyoid mites can reach places that are small enough to be inaccessible to their predators. The coconut mite, Aceria guerreronis, is a typical example; it Wnds partial refuge under the perianth of the coconut fruit. However, some predators can move under the perianth of the coconut fruits and attack the coconut mite. In Sri Lanka, the phytoseiid mite Neoseiulus baraki, is the most common predatory mite found in association with the coconut mite. The cross-diameter of this predatory mite is c. 3 times larger than that of the coconut mite. Nevertheless, taking this predator's Xat body and elongated idiosoma into account, it is-relative to many other phytoseiid mites-better able to reach the narrow space under the perianth of infested coconut fruits. On uninfested coconut fruits, however, they are hardly ever observed under the perianth. Prompted by earlier work on the accessibility of tulip bulbs to another eriophyoid mite and its predators, we hypothesized that the structure of the coconut fruit perianth is changed in response to damage by eriophyoid mites and as a result predatory mites are better able to enter under the perianth of infested coconut fruits. This was tested in an experiment where we measured the gap between the rim of the perianth and the coconut fruit surface in three cultivars ('Sri Lanka Tall', 'Sri Lanka Dwarf Green' and 'Sri Lanka Dwarf Green £ Sri Lanka Tall' hybrid) that are cultivated extensively in Sri Lanka. It was found that the perianth-fruit gap in uninfested coconut fruits was signiWcantly diVerent between cultivars: the cultivar 'Sri Lanka Dwarf Green' with its smaller and more elongated coconut fruits had a larger perianth-fruit gap. In the uninfested coconut fruits this gap was large enough for the coconut mite to creep under the perianth, yet too small for its predator N. baraki. However, when the coconut fruits were infested by coconut mites, the perianth-rim-fruit gap was not diVerent among cultivars and had increased to such an extent that the space under the perianth became accessible to the predatory mites.

Size of predatory mites and refuge entrance determine success of biological control of the coconut mite

“…This provides experimental evidence that the size of the opening to the mite's refuge may hamper the action of its potential predators, as was hypothesized in earlier work (Aratchige et al 2007;LawsonBalagbo et al 2007b;Negloh et al 2010;Lima et al 2012). …”

“…In young coconut fruits (up to one month), the perianth is so tightly appressed that coconut mites have no access to the meristematic zone (Howard and AbreuRodriguez 1991). In the course of fruit development, the distance between the perianth rim and the fruit surface increases to the point where coconut mites can access the space beneath the perianth (Howard and Abreu-Rodriguez 1991;Aratchige et al 2007;Negloh et al 2010;Lima et al 2012). This secluded microhabitat provides good conditions for development and reproduction of the mites, which feed on the meristematic tissue of the fruit.…”

“…Furthermore, coconut mites are protected from adverse abiotic factors (i.e. rain, wind, sun radiation, temperature and humidity variation), and, as long as the perianth is tightly appressed to the fruit surface, they are also protected from their predators (Howard and Abreu-Rodriguez 1991;Sabelis and Bruin 1996;Ambily and Mathew 2003;Fernando et al 2003;Siriwardena et al 2005;Aratchige et al 2007;Lawson-Balagbo et al 2007b).…”

“…Thus, coconut mites are protected from predation as long as predatory mites cannot enter. However, as the distance between the perianth rim and the fruit increases in the course of fruit development and in response to feeding by the coconut mite (Howard and Abreu-Rodriguez 1991;Aratchige et al 2007;Lawson-Balagbo et al 2007b), the predatory mites will eventually have access to the prey colony under the perianth.…”

“…Here, we tested the hypothesis that the distance between the perianth rim and the fruit (referred to as the refuge entrance below) is critical to biological control of coconut mites by predatory mites (Aratchige et al 2007;Lawson-Balagbo et al 2007b) in a small-scale field experiment. As candidate biological control agent, we selected the phytoseiid mite Neoseiulus paspalivorus De Leon, because it was the smallest, dominant predator on coconut fruits in the geographic area where the experiments were carried out (Falcon State, Venezuela) (F. da Silva, pers.…”