The effect of fluctuating temperature regimes on the life history of the Asian citrus psyllid and its parasitoid Tamarixia radiata.

The effect of temperature on insect development has largely been examined using constant temperature regimes. Experiments conducted in this manner ignore the natural temperature cycles that insects are exposed to on a daily, seasonal, and annual basis. As a result, behaviors and biological processes associated with temperature variance may not occur, and caution must be exercised when extrapolating findings to insects in the field. Thus, to examine key life-history parameters of an economically important citrus pest, Diaphorina citri (Kuwayama) (Hemiptera: Liviidae) and its nymphal ectoparasitoid, Tamarixia radiata (Waterston) (Hymenoptera: Eulophidae) we used both constant and naturally fluctuating temperature regimes. Diaphorina citri is capable of vectoring the gram-negative bacterium Candidatus Liberibacter asiaticus, the causative agent of the lethal and incurable citrus disease huanglongbing (HLB). With the recent diagnosis of ten HLB-positive trees in San Gabriel, California, accurately estimating temperature-dependent biological traits of D. citri and T. radiata could provide valuable insight in the fight to protect California’s $2 billion-a-year citrus economy. For each temperature regime examined, 20 citrus seedlings inoculated with 10 fifth instar D. citri nymphs were either exposed to T. radiata or left unexposed and placed into growth chambers. Each growth chamber was maintained at one of five constant or fluctuating temperature regimes with an average of 15, 20, 25, 30, or 35°C, a photoperiod of 14L: 10D, and a relative humidity of 50 ± 20% RH. Cones were monitored daily for mortality and parasitism of D. citri nymphs, emergence of adult D. citri and T. radiata, and adult survivorship. From these data, several important biological parameters including developmental rate, longevity, and parasitism success were calculated and compared between constant and fluctuating temperature regimes for T. radiata. Using ANOVA, we found that both mean temperature and temperature fluctuation affected T. radiata developmental rate and longevity. Non-linear regressions including the Logan 6 and Lactin 2 models were used to fit a curve for the developmental rate of both D. citri and T. radiata and to calculate T_min, T_max, and T_o. Our data show that development of T. radiata is underestimated at low constant temperatures and overestimated at high constant temperatures when compared with fluctuating temperature regimes. A similar trend was observed for longevity. Mean temperature was found to affect parasitism success, which ranged from below 20% at 35°C to above 70% at 25°C. Exposure to T. radiata affected non-parasitism-related mortality of fifth instar D. citri, with this mortality up to three times higher in the parasitoid-added treatments. Thus, in addition to parasitism, T. radiata may be providing unseen but important biological control of D. citri in the field via host feeding or additional means of host disruption. The differences in developmental rate and survivorship of T. radiata observed when comparing constant and fluctuating temperature regimes are consistent with those of similar studies. These results indicate that natural temperature variation is a critical component of accurately measuring population growth parameters of T. radiata. The information obtained from these experiments could be used to further develop California’s IPM program for D. citri and to optimize biological control by T. radiata. This will be particularly important where biological control is the main source of D. citri management such as within urban sites and abandoned citrus groves.