Heat Stress Inhibits the Completion of Pupal Diapause in <I>Rhagoletis mendax</I> (Diptera: Tephritidae)

Abstract

Blueberry maggot, Rhagoletis mendax Curran, pupae that are exposed to high temperature toward the end of diapause will not develop beyond a specific heat-sensitive stage. The temporal relationship between heat sensitivity and diapause was studied by exposing cohorts of pupae at different stages of diapause to 25:35°C (photoperiod of 12:12 [L:D] h) for 20 d and comparing the length of time to emergence. Results showed that the longest delay in emergence was induced in pupae that were exposed to high temperature <10 d before the completion of diapause. Also, pupae at the heat-sensitive stage exposed to high temperature did not show the increase in respiration rate characteristic of the completion of diapause. The duration of the heat-sensitive stage was studied by keeping pupae at 20°C for different periods of time between two 10-d exposures to 25:35°C. Results showed that a small proportion of the pupae kept at 20°C for 5 d, and all pupae kept at 20°C for 15 d, developed beyond the sensitive stage, indicating that the heat-sensitive stage lasted ≈10 d in individual pupae. The effect of frequency of exposure to high temperature during the sensitive stage was also studied. Data showed that the delay was similar or greater than the total duration of the exposure to high temperature. Exposure to high temperature caused ≈15% of the pupae used in these experiments to remain in a long-term quiescence, with a respiration rate similar to that of pupae in diapause. Potential endocrine causes for this quiescence were investigated by topically applying exogenous 20-hydroxyecdysone on these pupae. Application of the hormone caused 41% of the individuals to develop to the adult stage, suggesting that heat-induced damage to the endocrine system caused the failure to develop. Overall, this study indicates that in the blueberry maggot, during a ≈10-d sensitive stage immediately before the completion of diapause, heat-induced stress on the endocrine system can prolong diapause and delay adult emergence for a period of time similar to the duration of the exposure.