Effect of lure age and blend on sex pheromone trap catches of the mirid Sahlbergella singularis on cacao in Ghana

Highlights

• Lures releasing two pheromone components are essential for trapping cacao mirids.

• The ratio of the blend of major and minor components is non-critical.

• Lures should be replaced after two months exposure to prevent loss of efficacy.

• Potentially pheromone traps could benefit mirid management on organic cacao farms.

Abstract

Mirids, Sahlbergella singularis and Distantiella theobroma (Heteroptera: Miridae), are the main cacao pests in West Africa. Females of both species produce sex pheromones composed of hexyl (R)-3-((E)-2-butenoyl)-butyrate and hexyl (R)-3-hydroxybutyrate, the major and minor components, respectively. Lures composed of 1000:500 μg blends of the two components pre-aged for 2, 4, 8 and 12 weeks in a gauze-walled insectary were compared with fresh lures in a field experiment in Ghana. Lures were replaced monthly. A total of 272 S. singularis, all male, was caught. Fresh lures and those pre-aged for 2 and 4 weeks caught similar numbers in a month while lures pre-aged for 8 and 12 weeks caught 34% and 26%, respectively, than fresh lures (83). The attractiveness of five different pheromone blends were compared in a 15-month field trapping experiment. A total of 701 S. singularis, all male, was caught. The highest numbers were caught in traps releasing both components with no significant difference among 1000:50, 1000:500 and 1000:1000 μg blends. Traps releasing hexyl (R)-3-((E)-2-butenoyl)-butyrate alone caught over 98% fewer individuals than two-component blends, and those releasing hexyl (R)-3-hydroxybutyrate alone caught similarly low numbers to unbaited controls. We recommend that 2:1 blend lures, renewed at least every two months are used for mass-trapping cacao mirids. The results are discussed in relation to previously published mirid pheromone blend optimisation and longevity studies.

Introduction

The most damaging pests of cacao (Theobroma cacao L.) in West Africa are the mirids Sahlbergella singularis Haglund and Distantiella theobroma (Distant) (Entwistle, 1972; Collingwood, 1977). Although peak population densities rarely exceed 2500 individuals ha⁻¹ (Williams, 1954), annual losses from mirid feeding on cacao have been widely estimated to average 25–30% per annum (Collingwood, 1977; Babin et al., 2004; Anikwe and Makanjuola, 2013) and up to 75% in poorly-managed Ghanaian farms (Stapley and Hammond, 1959; Johnson, 1962). Since 1954, mirids have been controlled by foliar applied insecticides (Johnson, 1962; Owusu-Manu, 2002; Adu-Acheampong et al., 2015). However, an increasing market demand for organically produced cacao (Mahrizal et al., 2012), problems with pesticide-induced secondary pest outbreaks (Entwistle, 1972), farmers’ illegal use of pesticides either banned (Mahob et al., 2014) or unapproved (Adu-Acheampong et al., 2015), and loss of diversity and environmental pollution (Mahob et al., 2011), have stimulated research for more ecologically benign methods of control (Babin et al., 2004; Anikwe and Makanjuola, 2013) including sex pheromones (Padi et al., 2002; Ayenor et al., 2007; Mahob et al., 2011; Sarfo et al., 2018a,b).

Female S. singularis and D. theobroma produce the same two pheromone components in essentially the same ratio (Downham et al., 2002; Padi et al., 2002), however, separation is maintained temporally as adult S. singularis reportedly fly at night and D. theobroma by day (Leston, 1973). Males of Bryocoropsis laticollis Schumacher, a minor mirid pest of cacao (Johnson, 1962), also respond to the same pheromone blend (Sarfo et al., 2018a). The sex pheromone, which is attractive only to males, consists of two components: (I) a diester, hexyl (R)-3-((E)-2-butenoyl)-butyrate and (II) the corresponding monoester, hexyl (R)-3-hydroxybutyrate with an estimated naturally-occurring ratio of 2:1 (Downham et al., 2002; Padi et al., 2002).

Sarfo et al. (2018a,b) identified opportunities for managing cacao mirids by mass-trapping using their sex pheromone. To maximise trap catches and minimise expenditure it is important to identify both the length of time lures remain attractive in the field, and the most effective sex-pheromone blend for lures. We investigated the efficacy of lures pre-aged for up to twelve weeks before deployment in order to determine the longevity of the lures under field conditions. We also investigated the attractiveness of five different pheromone blends (Mahob et al., 2011) and an untreated control in a 15-month field trapping experiment.