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 535 accumulated degree days affect the life cycle of an oriental fruit moth, and that, in theory, OFMs can be found throughout the region and in all seasons. Under historical climatic conditions, the number of life cycles per annum is generally up to 8 in the fruit production regions, with lower numbers in the mountain belts (up to 6) and higher numbers in the warmer stone fruit areas.
There is a distinct projected increase in the number of life cycles of OFM into the intermediate future of the 2050s, with some of the warmer stone fruit regions showing 9 or more life cycles per annum. When expressed as changes in the number of life cycles from the historical to the projected intermediate future climates, there are 1.5 to 2.5 more life cycles per annum in the fruit regions. This translates into an increase of up to 60%, with a lower percentage increase in the south-west (up to 40%) and higher increases in the mountain belts of the northern production regions as well as in the western Langkloof.