Method
Moth Life Cycle
Lower and upper temperature (°C) thresholds and accumulated heat units (degree days) for one life cycle of codling moth are, respectively, 11.1°C, 34.4°C and 603°days. Life cycles per annum of codling moth were determined by computing degree days with a base temperature of 11.1°C and an upper limit of 34.4°C for historical climatic conditions, using 50 years of daily values of maximum and minimum temperatures (1950-1999). This was repeated for projected intermediate (mid-century) climate change conditions using multiple GCMs for the Quinaries making up the region. To calculate the number of life cycles per annum of codling moth, the degree days accumulated over one year were divided by the °days required for one life cycle to be completed by codling moths, i.e. by 603°days.
In the case of the oriental fruit moth, the lower and upper temperature (°C) thresholds and accumulated degree days for one life cycle of oriental fruit moths are 7.7°C, 32.2°C and 535°days. Life cycles per annum of oriental fruit moths were thus determined by computing degree days using a base temperature of 7.7°C and an upper limit of 32.2°C. For historical climatic conditions, 50 years of daily values of maximum and minimum temperatures (1950-1999) were used at representative grid points for each of the Quinary catchments covering the region. To calculate the number of life cycles per annum of oriental fruit moths, the degree days accumulated over one year were divided by the degree days required for one life cycle to be completed by oriental fruit moths, i.e. by 535°days. For climate change assessments, the daily temperature values from the present (1971-1990) and intermediate future (2046-2065) climate projections of the multiple CMIP3 GCMs used in this analysis were interrogated in the same manner as for the historical climate.
Maps
Map Information
The modelling assumed that no factors other than 603 accumulated degree days affect the life cycle of a codling moth, and that, in theory, codling moths can be found throughout the region and in all seasons. Under historical climatic conditions the number of life cycles per annum ranges is generally up to 5 in the fruit production regions, with lower numbers in the mountain belts (up to 3) and higher numbers in the warmer areas. The map of codling moth life cycles per annum into the intermediate future shows distinct increases. When mapped as changes, these are generally of the order of 0.5-1.5 additional life cycles per annum, with lower increases (< 1.0) in the core pome fruit regions. However, when expressed as percentage change, the northern and western Langkloof pome fruit regions show a higher relative increase of >40% increase.