Abstract:
Antestia bug Antestiopsis thunbergii (Gmelin) is a key insect pest of Arabica coffee associated with high losses in terms of quality, yield and export market of produce from the African region. Although the pest prefer feeding on unripe (mature) green berries than other coffee berry maturity stages or coffee parts, the cues that influence their host choice are unknown. In addition, A. thunbergii is gregarious in the field and produces a strong distinct smell while disturbed, which is characteristic of defence and alarm pheromones. However, little is known about the identity of the volatile chemical compounds that this pest produces neither their biological functions. Hence, this study sought to identify and examine the role of bioactive coffee berry derived volatiles and A. thunbergii pheromones and, explore their potential for the pest management. Various laboratory experiments were conducted comprising: behavioral assays in a dual choice Y-tube olfactometer; electrophysiology tests using Gas chromatography-electroantennographic detection (GC/EAD) and chemical analysis in GC-mass spectrometry, and field tests in a coffee plantation located at Kiambu County, in Central Kenya. Behavioral tests showed that volatile emissions from unripe coffee berries (mature green stage) were highly attractive to A. thunbergii nymphs and adults, whereas those from ripe berries induced repellence. Antennal activity recording with A. thunbergii nymphs isolated five and ten electrophysiologically active components in unripe berries and ripe berries respectively. Through behavioral tests with synthetic standards of the EAD-active compounds, a three-component blend derived from unripe berries comprised of anisole, methyl 3-ethyl-4-methylpentanoate and (5S,7S)-conophthorin was found to be highly attractive to the pest. On the contrary, a five-component blend derived from ripe berries comprising 3-hydroxy-2-butanone, 2-heptanone, 2-isopropyl-3-methoxypyrazine, 2-isobutyl-3-methoxypyrazine and (E)-β-caryophyllene was repellent to the pest. These results suggest that the two distinct blends likely contribute to A. thunbergii host food choice amongst unripe and ripe coffee berries and are candidate semiochemical based components that could be used either as lures or repellents to control the pest. Intraspecific communication assays established that A. thunbergii males were attractive to males and females, but females were unattractive to either sex. Through chemical analysis, a candidate male specific aggregation pheromone was identified as methyl (2E,4E,6Z)-decatrinoate (MDT-EEZ) in Antestia bug males. In addition, chemical analysis of dorsal abdominal glands of nymphs, and metathoracic glands of adults and headspace volatiles of disturbed adults led to the identification of various candidate alarm pheromones that are produced by the pest and dominated by alkanes and aldehydes. In field trials that evaluated the performance of candidate lures which were dispensed from delta® traps, the highest tested dose of MDT-EEZ (5 mg) was significantly attractive but no significant attraction was recorded with the kairomone at the tested doses. MDT-EEZ attracted Antestiopsis spp adults and nymphs (A. thunbergii and A. facetoides) with aggregations appearing on pheromone baited plants (7 bugs/ tree) unlike in unbaited plants (control) (1 bug/tree). However, delta® traps did not capture the bugs, as bugs stayed near traps baited with pheromone without entering inside. Hence, future studies should investigate different trap designs that are compatible with the pheromone and as well as an effective formulation of kairomone. In conclusion, this study identified candidate chemical cues that govern host-food selection and avoidance; aggregation and alarm behaviors of A. thunbergii, and further explored the performance of attractive semiochemicals in the management of the pest in coffee plantations attaining promising findings. Optimization of attractant compounds identified in this study could lead to effective management of the pest through surveillance, mass trapping and ‘attract and kill’ strategies. Identified candidate repellent compounds may be used to expel the pest from coffee plantations or to mask susceptible host food (unripe berries). A push- pull system could also be developed comprised of attractive and repellent compounds to enhance suppression of the pest.