Bionomics of Codling Moth

Effects of Rainfall and Humidity

 
Air humidity is not apparently of great significance in the life of the active stages of the codling moth since the entire development of the caterpillars occurs within fruits, and that of eggs and pupae in situations that are adequately protected from desiccation (Shel'Deshova, 1967). Shelford (1927) noted that considerable mortality of pupae could occur if relative humidity was less than forty percent, and under optimum temperature conditions more than half of the pupae developed even when the relative humidity was 20-25 percent. No adverse effect of drought has been noted on the active stages anywhere under natural conditions. Moisture conditions during overwintering have less effect on the viability of the caterpillars than does the temperature. However, prolonged frost may have a desiccating effect (Shel'Deshova, 1967). The increased humidity in spring promotes simultaneous pupation of the overwintering caterpillars (Shel'Deshova, 1967).
 

Hagley (1972) found that rainfall was the most important cause of mortality of the first instar larvae when they were just beneath the epidermis of the fruits. Gier (1963) suggested that mortality of first larval instar occurred because of increasing water contents of the fruits and consequent flooding of tunnels. The second and third instar larvae in the fruits are also affected by the rainfall. Average hourly temperatures and the amount and duration of rainfall between 5 to 7 p.m. significantly influence oviposition in the first generation, but oviposition by the second generation was affected by temperatures (Hagely, 1976).
 

To summarize the main points, the relative humidity is not of critical importance for codling moth larvae because they occur within the fruits. The increased humidity in spring promotes simultaneous pupation of the overwintering caterpillars. The rainfall is the most important cause of mortality of the first instar larvae when they are just beneath the epidermis. The mortality of first instar larvae occurred because of increased water content of the fruit and consequent flooding of tunnels.
 

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