Spiroacetals in the colonization behaviour of the coffee berry borer: a 'push-pull' system

Abstract

Coffee berries are known to release several volatile organic compounds, among which is the spiroacetal, conophthorin, an attractant for the coffee berry borer Hypothenemus hampei. Elucidating the effects of other spiroacetals released by coffee berries is critical to understanding their chemo-ecological roles in the host discrimination and colonization process of the coffee berry borer, and also for their potential use in the management of this pest. Here, we show that the coffee berry spiroacetals frontalin and 1,6-dioxaspiro [4.5] decane (referred thereafter as brocain), are also used as semiochemicals by the coffee berry borer for host colonization. Bioassays and chemical analyses showed that crowding coffee berry borers from 2 to 6 females per berry, reduced borer fecundity, which appeared to correlate with a decrease in the emission rates of conophthorin and frontalin over time. In contrast, the level of brocain did not vary significantly between borer- uninfested and infested berries. Brocain was attractive at lower doses, but repellent at higher doses while frontalin alone or in a blend was critical for avoidance. Field assays with a commercial attractant comprising a mixture of ethanol and methanol (1 ∶ 1), combined with frontalin, confirmed the repellent effect of this compound by disrupting capture rates of H. hampei females by 77% in a coffee plantation. Overall, our results suggest that the levels of frontalin and conophthorin released by coffee berries determine the host colonization behaviour of H. hampei, possibly through a 'push-pull' system, whereby frontalin acts as the 'push' (repellent) and conophthorin acting as the 'pull' (attractant). Furthermore, our results reveal the potential use of frontalin as a repellent for management of this coffee pest.

Figure 1. Number of Hypothenemus hampei (CBB) life stages found inside coffee berries (approx. 150 days of development), infested with 2, 4, and 6 H. hampei colonizing females per berry, after 2, 5, 15 and 30 days after infestation.

Figure 2. Release rate (ng/µl/hr) of (5S,7S)-conophthorin and frontalin in samples of coffee berries (approx. 150 days of development), infested with 2, 4 and 6 Hypothenemus hampei colonizing females per berry (CBB), after 2, 5, 15 and 30 days after infestation.

Figure 3. Responses of individual Hypothenemus hampei females (75 individuals per group/time after infestation) in olfactometer trials.

Red: coffee berries infested with 2 H. hampei colonizing females per berry (group 1); Green: coffee berries infested with 4 H. hampei colonizing females per berry (group 2); Blue: coffee berries infested with six H. hampei colonizing females per berry (group 3).

Figure 4. Olfactometer responses of Hypothenemus hampei females to frontalin (mean ± SE).

N =  total number of insects, and n =  total respondents (i.e. n = N less non-respondents)). Positive response to the various concentration levels is referred to as test while responses to DCM solvent is the control. The percent response for each arm was calculated relative to N.

Figure 5. Olfactometer responses of Hypothenemus hampei females to brocain (mean ± SE).

N =  total number of insects, and n =  total respondents. Positive response to various concentration levels is referred to as test while to DCM solvent as control. The percent response for each arm was calculated relative to N.

Figure 6. Olfactometer responses of Hypothenemus hampei females (CBB) to blends (mean ± SE).

Blend A: 40 ng/µl brocain +5 ng/µl frontalin, blend B: 160 ng/µl brocain +20 ng/µl frontalin, blend C: 640 ng/µl brocain +80 ng/µl frontalin. The asterisks indicate the significance levels (* =  significant at 0.05, ** =  significant at 0.01 and *** =  significant at 0.001).

Figure 7. Olfactometer responses of Hypothenemus hampei females (CBB) to blends (mean ± SE).

Blend A: 40 ng/µl brocain +5 ng/µl frontalin; blend D: 40 ng/µl brocain +10 ng/µl frontalin; blend E: 40 ng/µl brocain +20 ng/µl frontalin; and blend F: 40 ng/µl brocain +40 ng/µl frontalin. The asterisks indicate the significance levels (* =  significant at 0.05 and *** =  significant at 0.001).

Figure 8. Comparison of Hypothenemus hampei (CBB) responses to an optimal blend (A) against DCM solvents and individual components of the blend, 5 ng/µl frontalin and 40 ng/µl brocain respectively.

The asterisks indicate the significance levels (*** = <0.001).

Figure 9. Comparison of Hypothenemus hampei (CBB) infestation levels of ripe coffee berries treated with either 80 ng/µl frontalin or 40 ng/µl brocain.

The control is healthy berries treated with solvent (5% Dmso +95% water). The asterisks indicate the significance levels (* =  significant at 0.05 and ** =  significant at 0.01.

Figure 10. Comparison of Hypothenemus hampei (CBB) infestation levels of ripe coffee berries treated with solvent (5% Dmso+95% water), 80 ng/µl frontalin or 40 ng/µl brocain.

Figure 11. Number of coffee berry borer females (CBB) captured in traps baited with: brocain, frontalin, solvent, ethanol+ methanol mixture (EM) and blends of; ethanol: methanol + brocain (EMB); ethanol: methanol + frontalin (EMF) and ethanol: methanol + Solvent.