Calculation of the Base Temperature and Investigation of Changes in Number of Codling Moth (Cydia pomonella) Generations, with Degree-Day Model in Chenaran

Codling moth (Cydia pomonella) is one of the most important agricultural pests which annually damages apples, pears, and walnuts orchards. Like other pests, coldling moth development depends to the air temperature. Climate changes have many impacts on the environment and pests are affected by these changes, too. Codling moth has two generations in cold regions and 4 to 5 generations in warm regions per year. The number of this pest generations and the damages which it caused in the orchards depends on the amount of received cumulative temperature during a growing season. Increasing the average daily temperature will increase the cumulative degree-day and subsequently, this can increase the number of pest generations annually. The increase in the number of pests as a result of global warming can damage the environment and food security significantly. One of the effects of climate changes on the pest can be identified with an overview of the codling moth generation and its changes over the past years.

Maximum and minimum temperature data of Golmakan synoptic station (Khorasan Razavi) were used to calculate the number of codling moth generations (1989-2018). Average daily temperature was calculated using maximum and minimum temperatures. To estimate the degree-day, the base temperature was obtained by the growth rate method at different temperatures. In this method, the growth rate of one stage of pest life (egg laying to egg hatching) was recorded at different temperatures and the temperature in which the growth rate becomes zero was considered as the base temperature. Then, degree-day was calculated by subtracting the base-day temperature from daily mean temperature and cumulative degree-day for each year was obtained by summing degree-days biofix to harvest. The biofix date in 2019 was obtained by the pheromone traps and the first male trapped in the pear orchard near Golmakan station. Pest activity initiates at temperatures higher than the base temperature, so the biofix date for each year (1989–2018) was selected based on the mean daily temperature that reached the base temperature and then increased. Biofix in the pear orchard also occurred under these circumstances. Trend of cumulative degree-day was analyzed by Mann-Kendall nonparametric test in the statistical period 1989-2018 to examine cumulative degree-day changes. Pheromone trap data in pear orchards in 2019 and previous studies show that codling moth need about 650˚C degrees-day temperature to complete each generation. Consequently, the number of pest generations for each year was calculated by dividing the cumulative degree-day by 650 DD. The number of male trapped in pheromone traps from biofix to harvest and flight peaks shows the number of the codling moth generations in the area of observation. Finally, the number of codling moth generations in 2019 were compared to the number of generations of this pest in the statistical period 1989–2018.

The average annual temperature in Golmakan station varied from 13.9˚C to 16.7˚C from 1989 to 2018. Considering the codling moth growth rate from laying to hatching at 8, 10, 12, 14 16, 18˚C, base temperature in which the growth rate was zero, was obtained 8.4° C. The cumulative degree-day for codling moth from biofix to harvest in the statistical period 1989–2018 was between 1780˚C in 1992 and 2456˚C in 2008. Recorded air temperature data by data logger in pear orchard showed that the pest received 2251˚C degree-day in 2019. The results of Mann-Kendall test showed an increasing and significant trend for cumulative degree-day. After calculating the cumulative degree-day, it was determined that the codling moth had three generations in the past 30 years except one year. The mean cumulative degree-day was 2067˚C in 1989-1998, 2215˚C in 1999-2008, and 2300˚C in 2009-2018. This shows that the average cumulative degree-day increased by 143˚C in the first ten years and 85˚C in the second ten years. According to the results of this study, at the beginning of the statistical period, pest's life cycle stops at the beginning of the third generation and it enters the pupal and overwintering stage. Toward the end of the statistical period, pest life cycle is going to complete and reach to the fourth generation. In 2019, pheromone traps registered 3 pike flights for codling moth in the pear orchard near the Golmakan station. Thus, in 2019, there were three generations of this pest in the area of observation. If these conditions continue, the fourth generation may be presented in the area of observation in next years.

The results of this study showed that increasing the mean of daily temperature increases the cumulative degree-day and if the temperature continues to rise, the occurrence of fourth generation will not be unexpected in the coming years. Damage to apple, pear, and walnut orchards increases by increasing number of codling moth. Therefore it will be more difficult to control this pest. Under these conditions, using pesticides will increase in agriculture and there will be environmental problems. According to the results of this study in order to manage codling moth, it is necessary to determine the exact number of generations and the appropriate time of spraying due to the time of egg hatching in each region.