Susceptibility of field populations of cocoa mirids, Sahlbergella singularis Haglund and Distantiella theobroma (Distant) to bifenthrin

Cocoa, Theobroma cacao L. is the main export crop in Ghana. Production increased from 740,000 metric tons in 2005/06 to more than one million metric tons in the 2010/2011 season. The crop is very susceptible to attack by myriad of insect pests and fungal diseases with mirids (Sahlbergella singularis Haglund and Distantiella theobroma (Distant)) being the key insect pests. Following their detection as serious pests in the early 1900s, the main method for their management has been with the use of conventional insecticides. Misuse, improper application and failure to adopt full research recommendations on insecticide use are the causes of poor control, insecticide resistance in target pests, and adverse effects on non-target organisms. The general objective of the study was to determine the extent to which S. singularis and D. theobroma are susceptible to bifenthrin, one of the currently approved insecticides in large-scale use under the nation-wide government cocoa diseases and pests control programme (CODAPEC). Data was collected in the Ashanti, Central, Eastern, Volta, and Western Regions of Ghana. The study was executed in four phases: (i) Distribution of mirids using the visual hand-height assessment method, (ii) susceptibility of mirids to the recommended field concentration of bifenthrin, (iii) determination of the median lethal concentration values and (iv) a farmers’ survey (using structured questionnaires) to determine pesticide use pattern by farmers. The EPA Probit Analysis Programme was used to estimate LC50 values, the Confidence Intervals and the slopes of the curves. Mirid collection was done in 38 farms from a total of 134 farms visited. Sahlbergella singularis was the dominant mirid species on cocoa in the study areas. Their population was five times as high as that of D. theobroma. The highest numbers of both S. singularis (700 adults and nymphs) and D. theobroma (362) occurred in December in the Eastern Region, dropping to 25 and 55, respectively in March. The distribution of the two species in the other Regions was more variable over the period, with D. theobroma being absent in most locations. The efficacy of bifenthrin on the two mirid species was confirmed in this study. Excellent control (100 % mortality) of both S. singularis and D. theobroma was achieved with the recommended field rate of 240 mg/l and even with concentrations as low as 18.75 mg/l. No significant variation in mirid susceptibility to bifenthrin was observed in most locations as established by the LC50 values compared to the reference populations of S. singularis and D. theobroma from Twifo Praso (Central Region) and Pankese (Eastern Region), respectively. The farmer-survey showed that cocoa farmers use all kinds of insecticides including both approved and unapproved chemicals. Many of them know the field concentrations, but lack the necessary skills in safe handling and use of insecticides. The absence of no characteristic trend in the baseline data established by this study suggests that no major resistance problems so far have developed in all Regions. However, there is the danger for mirids to become resistant to bifenthrin as the recommended field concentration of 240 mg/l is much higher than the minimum effective concentration of 18.75 mg/l.

Source: KNUST