Optimizing mirid control on cocoa farms through complementary monitoring systems

Abstract

Mirids (Sahlbergella singularis and Distantiella theobroma) are the most important insect pests affecting cocoa production across West Africa. Understanding the population dynamics of mirids is key to their management; however, the current recommended hand‐height assessment method is labour intensive. The objective of the study was to compare recently developed mirid sex pheromone trapping and visual hand‐height assessment methods as monitoring tools on cocoa farms and to consider implications for a decision support system. Ten farms from the Eastern and Ashanti regions of Ghana were used for the study. Mirid numbers and damage were assessed fortnightly on twenty trees per farm, using both methods, from January 2012 to April 2013. The mirid population increased rapidly in June, reached a peak in September and began to decline in October. There was a significant linear relationship between numbers of mirids sampled to hand‐height and mirid damage. High numbers of male mirids were recorded in pheromone traps between January and April 2012 after which there was a gradual decline. There was a significant inverse relationship between numbers of trapped adult mirids and mirids sampled to hand‐height (predominantly nymphs). Higher temperatures and lower relative humidities in the first half of the year were associated with fewer mirids at hand‐height, but larger numbers of adult males were caught in pheromone traps. The study showed that relying solely on one method is not sufficient to provide accurate information on mirid population dynamics and a combination of the two methods is necessary.

Optimal pheromone trap density for mass trapping cacao mirids

“…However, Sarfo et al (2018) demonstrated that traps at 1.8 m caught 8% of the numbers of male S. singularis caught in traps positioned just beneath the canopy. Similarly low-trap heights had been used in earlier pheromone trapping studies in cacao (Padi et al, 2002;Ayenor et al, 2007;Mahob et al, 2011), although, in response to the results of Sarfo (2013), Awudzi et al (2017) placed their traps at 2.7 m. Sarfo et al (2018) also demonstrated that only ca. 23% of mirid males that responded to the pheromone entered the trap, which suggests that traps at 1.8 m probably trapped out ca.…”

“…Similarly low‐trap heights had been used in earlier pheromone trapping studies in cacao (Padi et al., ; Ayenor et al., ; Mahob et al., ), although, in response to the results of Sarfo (), Awudzi et al. () placed their traps at 2.7 m. Sarfo et al. () also demonstrated that only ca.…”

“…Furthermore, cacao mirids may only slowly re‐invade plots previously treated with insecticide (Owusu‐Manu, ). In a 5‐year study, Owusu‐Manu () found that mirid populations were highest every year in June–August on Amazon hybrid cacao, but other studies show that populations may increase locally at almost any time of year on those genotypes (Gibbs et al., ; Adu‐Acheampong et al., ; Awudzi et al., ).…”

“…Furthermore, cacao mirids may only slowly re-invade plots previously treated with insecticide (Owusu-Manu, 2002). In a 5-year study, Owusu-Manu (2002) found that mirid populations were highest every year in June-August on Amazon hybrid cacao, but other studies show that populations may increase locally at almost any time of year on those genotypes (Gibbs et al, 1968;Adu-Acheampong et al, 2014;Awudzi et al, 2017). Collingwood (1971) reported that more than twice as many S. singularis than D. theobroma per unit area were recorded by pyrethrum knockdown than by visual inspections to hand height, which he attributed to a higher proportion of the former feeding in the canopy.…”

“…His conclusion that S. singularis was more numerous in the canopy was confirmed here with an average 119 more in knockdown samples than in searches to hand height, whereas for D. theobroma the ratio was about 3.3:1. Despite their bias in overestimating the abundance of D. theobroma, hand height inspections are used regularly for mirid population studies on cacao in Ghana (Owusu-Manu, 2002;Antwi-Agyakwa, 2013;Awudzi et al, 2017) including for establishing the relative abundance of the two species (Antwi-Agyakwa, 2013;Adu-Acheampong et al, 2014). Here, D. theobroma was the most abundant species in searches to hand height (96%) and by insecticide knockdown (71%) which suggests that its much lower representation than S. singularis in pheromone trap catches (23%) may reflect either a lower response to the sex pheromone, as numbers caught per trap showed no decline with increasing trap density, and/or a higher percentage failing to enter the traps, confirming Sarfo et al (2018).…”