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 finds 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 flat 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 x Sri Lanka Tall' hybrid) that are cultivated extensively in Sri Lanka. It was found that the perianth-fruit gap in uninfested coconut fruits was significantly different 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 different among cultivars and had increased to such an extent that the space under the perianth became accessible to the predatory mites.

Fig. 1

Bract arrangement of the perianth on a coconut fruit. Longitudinal sections were taken along line A and B. OB-Outer bracts of the perianth, IB-Inner bracts of the perianth

Fig. 2

Longitudinal section of a coconut fruit showing position L1 at the edge of the bract touching the coconut fruit and position L2, 1 cm away from L1 along the surface of the coconut fruit

Fig. 3

Mean (±SE) of the widest gap at L1 and L2 in infested and uninfested coconut fruits of three cultivars: SLT = ‘Sri Lanka Tall’ (white rectangles), DG = ‘Sri Lanka Dwarf Green’ (black rectangles), DGT = ‘Sri Lanka Dwarf Green × Sri Lanka Tall’ hybrid (grey rectangles)

Fig. 4

Frequency distribution of coconut fruits over classes of widest L1 gaps in infested (black rectangles) and uninfested (white rectangles) coconut fruits (all three cultivars together)

Fig. 5

Mean (±SE) of the number of coconut mites (white rectangles) and predatory mites (black rectangles) per coconut fruit from three coconut cultivars (SLT, DG, DGT; see legend of Fig. 3)