Evaluation of organic pest management strategies to control the cocoa mirid (Monalonion dissimulatum Dist.), Alto Beni, Bolivia

Summary

The cocoa mirid (Monalonion dissimulatum) is one of the major pests in cocoa cultivation in Alto Beni, Bolivia, causing up to 50% yield losses. The most common control method is the manual removal of the nymphs sucking at the pods in the morning. This practice is very time-consuming, and farmers often do not dispose of enough time to perform these control measures. Therefore, there is an urgent need for more efficient practices to control this important pest.
The present study evaluated the pathogenicity of two strains of Beauveria bassiana for the control of Monalonion dissimulatum on cocoa: a non-native, commercialized strain (Probiobass MR, Probiotec S.R.L.), and a native strain of Alto Beni, which is not yet commercialized (isolated by the local association SIEMPRE). Moreover, a silicon-based product (TECSIL PM®) was tested. In addition, the effect of different degrees of infestation with Monalonion dissimulatum on different stages of cocoa pod development was examined. In order to investigate these questions, several different field trials were carried out at the experimental station of Sapecho between June and September 2013.
The foreign strain of Beauveria Bassiana was the most effective bio-pesticide, causing a mortality rate of 63.3% in adults and 49.1% in nymphs of Monalonion dissimulatum. Cocoa pods in their early stages of development were highly susceptible to attack by Monalonion dissimulatum. On the other hand, fully developed cocoa pods showed a rather strong resistance to attacks by Monalonion dissimulatum: up to about 70% of damaged tissue on the surface of the cocoa pods, no effects on cocoa wet bean yield were recorded. However, when the damage increased above 70%, it had a strong impact on yield, amounting up to 50.4% yield loss. Fully damaged pods started drying out, and if they were completely desiccated, the yield loss was total.
It is concluded that the foreign strain of Beauveria Bassiana may be the most efficient to control Monalonion dissimulatum in the field. More on-farm field trials need to be conducted over longer time periods in order to elucidate whether the observed effects will be reflected in higher cocoa yields in the farmer’s context. It is recommended to perform harvesting operations at regular intervals of two weeks in order to minimize losses caused by the desiccation of damaged cocoa pods.