Chemical Ecology of the Coffee Berry Borer, Hypothenemus Hampei Ferrari (Coleoptera: Scolytidae): The Role of Two Spiroacetals in the Insect-Host Communication System

Abstract

The Coffee Berry Borer, Hypothenemus hampei Ferrari is the most important pest of coffee worldwide. The predominant method for the pest control is application of broad spectrum pesticides, but they are often toxic to humans and pollute the environment, hence the need to search for safer alternatives. Spiroacetals represent a group of semiochemicals often used to manage various pests through manipulation of their behaviour. This study which was carried out between January 2013 and December 2013 sought to investigate the effect of two synthetic spiroacetals viz. brocain (1,6-dioxaspirol[4.5] decane) and frontalin (1,5-dimethyl-6,8-dioxabicyclo[3.2.1] octane) obtained from volatiles of coffee berries on the behaviour of Coffee Berry Borer. To achieve the objectives, the behavioural response of H. hampei females to brocain and frontalin was evaluated in three experiments, comprising a Y- tube olfactometer and petridish arena, both in laboratory assays and a field experiment in a coffee plantation. In olfactometer assays, a Y- tube arena (1 cm i.d; stem 8.5 cm; arms 7.5 cm at a 60º angle to the stem) was used to study H. hampei response to various concentrations of authentic standards of brocain, frontalin, and blends of the two compounds. During the assays, Coffee Berry Borer females that walked towards the arm with the test compound were considered to have made a positive response (attraction) while those that chose the control arm (solvent) were considered to make a negative response (repellence). In a subsequent experiment, H. hampei infestation levels in coffee berries whose exocarp was treated with brocain and frontalin was compared with solvent treated berries (control). The choice tests were done for Coffee Arabica var. Ruiru 11 in the most attractive stage (150 days old, yellow exocarp stage) and conducted in a petri dish arena (14cm diameter). In addition, volatility of brocain and frontalin was analysed and quantified (ng/μl/min) by gas-chromatography-mass spectrometry (GC-MS). Field experiments were conducted between 18th November 2013 and 9th December 2013 xv in three coffee plots located in Kiambu County, Kenya to investigate responses of H. hampei to Brocap® traps baited with frontalin and brocain. Olfactory tests revealed that H. hampei are repelled by frontalin while brocain has a dual function, being significantly attractive at low concentration and repellent at high concentrations. In petridish assays, H. hampei infestation levels in coffee berries treated with brocain, was twice higher than the control (berries with solvent) while frontalin treated berries were significantly avoided. Tests investigating release rates of the spiroacetals showed that brocain amounts significantly reduced over time unlike frontalin. In the field trials, frontalin baited traps failed to catch any H. hampei and when mixed with the pest’s commercial attractant, (1:1 methanol : ethanol mixture) the captures were reduced to 23%. Traps captures with brocain were not statistically different from the solvent baited traps (95%water+5%DMSO). Moreover, when brocain was mixed with the commercial attractant, it neither improved nor inhibited the attractants’ performance. This study established that frontalin is a repellent of H. hampei and masks the pest’s attractants. The findings suggest that frontalin may be produced by coffee berries as a defensive compound (allomone) to avoid overcrowding of Coffee Berry Borers in an already attacked host. Brocain was observed to be highly attractive at a single concentration (40 ng μl-1) and repellent at high concentrations in olfactory assays. These results suggest that brocain is perhaps used by H. hampei as a host kairomone (attractant) at low concentration and as a repellent at high concentrations. Frontalin may therefore be incorporated in an integrated pest management program for H. hampei.