Wide Ranging Insect Infestation of the Pioneer Mangrove Sonneratia alba by Two Insect Species along the Kenyan Coast

Abstract

Insect infestation of mangroves currently threatens mangrove forest health and management. In the Western Indian Ocean region, little is known about insect damage to mangroves despite the fact that numerous infestations have occurred. In Kenya, infestations of Sonneratia alba have persisted for almost two decades, yet the taxonomic identity of the infesting pest(s), the extent of infestation, the pests’ biology, the impacts of infestation on host and the ecosystem, the host’s defensive strategies to the infestation are poorly understood. S. alba is a ubiquitous, pioneer mangrove species of the Indo-Pacific, occurring along the waterfront in a variety of mangrove ecosystem settings. Our main objectives were to identify the pest(s) responsible for the current dieback of S. alba in Kenya, and to determine the extent of infestation. To identify the pests responsible for infestation, we trapped emergent insects and reared larvae in the laboratory. To determine the overall extent of infestation within the S. alba zone, we assessed nine sites along the entire Kenyan coastline for the presence or absence of infested mangroves. Insect infestation in two mangrove embayments (Gazi and Mida) was quantified in depth. Two wood-boring insects were identified: a metarbelid moth (Lepidoptera, Cossoidea) of undescribed genus and the beetle Bottegia rubra (Cerambycidae, Lamiinae).The metarbelid moth infests mangroves in both northern (from Ngomeni to Kiunga) and southern regions (from Vanga to Mtwapa) of the Kenyan coast. B. rubra appeared in low density in Gazi, and in high density in Mida, Kilifi, and Ngomeni, with densities gradually decreasing northward. Insect infestation levels reached 18% in Gazi and 25% of S. alba stands in Mida. Our results indicate that B. rubra has the ability to infest young mangrove trees and expand its range, posing a danger to rehabilitation efforts where plantations have been established. Thus, there is great need for forest managers to address the recent increased levels of infestation in Kenyan mangroves; apart from the ecological interest such plant-herbivore relations bring in this ecosystem.

Infestation mechanisms of two woodborer species in the mangrove Sonneratia alba J. Smith in Kenya and co-occurring endophytic fungi

“…rubra to cause serious damage to the host appears to be higher than that of the moth, especially since it was realized in a coastal survey (Jenoh et al ., 2016) that B . rubra is extending its range both northward and southward along the Kenyan coast [22,24]. However, the magnitude of the destruction that B .…”

“…Since the moth infests branches that are higher than the maximum tide level [24], many faunal species can colonize burrows abandoned by the moth without interference from the tide, possibly inoculating many different fungal species. Unlike the moth, the beetle infests branches below the high tide level, which become submerged twice daily for several hours.…”

“…Due to this association, it would be important to first understand the benefit the beetle derives from interacting mainly with Penicillium spp. since during laboratory rearing of beetle larvae, Jenoh et al ., [24] discovered that the beetles could not survive when fresh food (new branches of S . alba ) was introduced and they speculated that the beetle relied heavily on fungi.…”

“…However, the two insect woodborers had different spatial niche distributions along the Kenyan coastline. In areas where their ranges overlapped, such as Gazi, Tudor, Ngomeni, and Lamu, the beetle is infesting the lower branches of a tree and the moth the upper branches, above the high tide line[22,24].…”

The advantages of using drones over space-borne imagery in the mapping of mangrove forests

“…While having experience with visual interpretation of remotely sensed imagery [ 36 ], and despite not having applied this technique in the present paper, we believe that drone imagery offers an entirely new and promising suite of possibilities to unambiguously identify and distinguish mangrove genera and species (including congeneric species and maybe even subspecies and varieties). This is evidenced by the superior quality of drone imagery (Figs 3 and 5 ) and the possible application in studies that do not aim at mapping, but rather at pinpointing species or individuals of interest, for instance to recognise invasive species [ 24 ] or cryptic ecological degradation [ 22 ], to identify tree-top or branch die-off [ 76 ] or to quantify biomass loss resulting from tree fall or lightning strikes [ 77 ].…”