Field Observations on the Cacao Mirids, Sahlbergella singularis Hagl. and Distantiella theobroma (Dist.), in the Gold Coast

Abstract

The two species of Mirids, Sahlbergella singularisHagl. and Distantiella theobroma (Dist.), are important pests of cacao (Theobroma cacao L.) in the Gold Coast. The damage sustained by the cacao is due, in part, to the direct effects of feeding by these insects, but more to the subsequent invasion of the resulting lesions by the weakly pathogenic fungus, Calonectria rigidiuscula (Berk. & Br.) Sacc.In the field, Mirid damage may be classified into three categories. “Blast” is the result of light diffuse attack and is so named because of its similarity to fire damage. “Stagheaded cacao” is more severe, the trees showing numeroussmall crown branches but forming a poor canopy. “Mirid pocket” describes severe damage, normally limited to a small area, the trees losing the crown completely and the polelike trunks bearing numerous lateral chupons. In general, stagheaded symptoms are more prevalent where cacao is grown without shade, and pockets where cultivation of cacao utilises shade trees.Consideration of the factors associated with Mirid damage shows that it is correlated with breaks in the cacao canopy. Evidence is brought forward to suggest that such breaks normally precede, rather than result from, Mirid attack. The canopy itself is unsuitable for the development of either species, the major part of the population being confined to the sub-canopy levels. The initial causes of the breaks are, most frequently, the die-backs associated with swollen shoot or with adverse water relations. The falling of shade trees causes a number of breaks, and, where cacao is grown without shade, the Mirids themselves may be a cause.The most important single factor influencing the form of the cacao canopy, and thus the course of Mirid attack, is the amount of overhead shade, particularly that provided by trees little taller than the cacao itself. Shade which is too dense causes etiolation of the cacao and thus renders it susceptible to attack, the resulting damage generally taking the form of a pocket. Shade which is too sparse does not shield the cacao from the adverse effects of exposure. The Mirid damage in such areas generally assumes the stagheaded form.It is suggested, from field experiments, that the invasion by Mirids of areas suitable for colonisation is not by random movements, but is determined by the change in some physical factor resulting from the broken canopy. Changes in light intensity are the most obvious results of such breaks, but further experiments are needed to determine which is the operating factor.Degraded cacao is prevented from recovery, not by Mirid attack alone, but by the interaction of it with the presence of C. rigidiuscula, and by increased exposure.The sampling methods employed are described, and the bearing of the results upon control measures is discussed.

True Bug (Heteroptera) Impact on Cocoa Fruit Mortality and Productivity

“…Feeding lesions on the peduncle, which usually are difÞcult to detect, may contribute to cherelle abortion as they can disturb the nutrient supply of fruit. Furthermore, mirid damage on the cocoa canopy may reduce photosynthetic activity and thus contribute to cherelle wilt (Williams 1953). After a mirid attack on the cocoa canopy, trees also have to offset the loss of leaves by increasing ßushing activity, and the incidence of cherelle wilt has been found to be positively correlated with vegetative growth (Valle et al 1990).…”

“…The cocoa mirid Sahlbergella singularis Haglund (Heteroptera: Miridae) is the most commonly found insect pest on Cameroonian cocoa farms (Babin et al 2010). This plant bug is particularly harmful because it damages fruits, twigs, and foliage, leading to a loss of productivity (25Ð 40% depending on the area), premature aging, and sometimes the rapid death of the most severely damaged trees (Williams 1953). Other Heteroptera (true bugs) that damage fruits are also found on Cameroonian cocoa farms, as are other plant bugs in the genus Helopeltis (Miridae), shield bugs in the genera Bathycoelia and Atelocera (Pentatomidae), and leaffooted bugs in the genera Pseudotheraptus and Callibaphus (Coreoidea) (Entwistle 1972).…”

“…True bug feeding lesions on cocoa pods usually appear as a black plug of dead tissue. On young pods, this damage may cause distortion during growth, sometimes leading to yellowing and the abortion of fruits (Williams 1953). In Ghana however, Bathycoelia thalassina (Herrich-Schaeffer) (Pentatomidae) has been found to feed on cocoa beans through the pod cortex, leading to brownish beans that are empty inside (Lodos 1967).…”

Rearing method and life‐table data for the cocoa mirid bug Sahlbergella singularis Haglund (Hemiptera: Miridae)

“…In the field, mirid densities are lower during the dry season as compared with the wet season (Williams 1954;Lavabre et al 1962;Gibbs et al 1968;Bruneau de Miré 1970). Mirid densities increase when trees bear fruits, which provide constant food and oviposition sites (Williams 1953a;Lavabre 1977;Piart 1978). However, some studies found that mirid populations develop similarly on unproductive cocoa (Williams 1954), or when pods are gradually removed from the trees (Bruneau de Miré 1977).…”

Effects of cocoa tree phenology and canopy microclimate on the performance of the mirid bug Sahlbergella singularis

“…Sahlbergella singularis is more *Correspondence: R. Babin, CIRAD, UPR Bioagresseurs Analyse et Maîtrise du Risque, Avenue Agropolis, TA A106 ⁄ 02, 34398 Montpellier Cedex 5, France. In addition, feeding lesions can become infected by parasitic fungi and usually develop into cankers, weakening and eventually even killing cocoa trees, when there is a serious infection (Crowdy, 1947;Williams, 1953a;Collingwood, 1977). In Cameroon, S. singularis is by far, the most common species on cocoa (Babin et al, 2010).…”