Diapause Induction And Post-diapause Life-history Traits In The Cabbage Beetle, Colaphellus Bowringi (Coleoptera: Chrysomelidae)

The cabbage beetle, Colaphellus bowringi Baly 1865 (Coleptera: Chrysomelidae) is a serious pest of crucifers in China. The beetle's population in Jiangxi Province enters summer and winter diapause as adults in the soil. There are two distinct infestation peaks in the field, one in spring and a second in autumn. The beetle showed a considerable intrapopulation variation in its life history due to the difference in duration of diapause. Some individuals took more than one year to produce one generation. Some were univoltine, i.e., only one generation in spring or autumn; Some were biovoltine, i.e., one generation in spring, another in autumn; Some were multivoltine, i.e., one generation in spring, two or three generations in autumn. Thus, there are four generations per year in the field. The cabbage beetle is a short-day species, but its photoperiodic response highly depended on temperature. All adults entered diapause regardless of photoperiods if the mean temperature was ≤ 20℃; Whereas the diapause-averting influence of short daylengths was expressed only at high temperatures (above 20℃).In this thesis, the effects of environmental factors on diapause induction and diapause duration on the post-diapause life-history traits and offspring performance were investigated in the cabbage beetle.1. Diapause induction and clock mechanismThe cabbage beetle, C. bowringi is a short day species. Using 24-h light-dark cycles, the photoperiodic responses at 25 and 28癈 revealed a critical nightlength between 10 and 12 h. Using non-24-h light-dark cycles, it showed that the duration of the night was much more important than the duration of the light phase in diapause determination. Night-interruption experiments with T = 24 h showed that diapause was effectively induced by a 2-h light pulse in most scotophases; whereas day-interruption experiments by a 2-h dark-break had a little effect on the incidence of diapause. Nanda-Hamner experiments displayed three declining peaks of diapause at 24 h circadian intervals. Bunsow experiments showed two very weak peaks for diapause induction, one being 8 h after lights-off, and another 8 h before lights-on, but it did not show peaks of diapause at a 24 h interval. These results suggest that the circadian oscillatory system constitutes a part of the photoperiodic clock of this beetle but plays a limited role in its photoperiodic time measurement. The experiments of alternating short-night cycles (LD 16:8) and long-night cycles (LD 12:12) during the sensitive larval period showed that the 'counter' in C. bowringi accumulated long nights as well as short nights, but in different ways.2. Effects of thermoperiod on the diapause inductionThe diapause response curves at the thermoperiod (C:T16:28℃) under continuously darkrearing conditions revealed that the critical cryophase duration was of about 10.5 h. The diapause response curves both at different constant temperatures and at the thermoperiod under continuously dark rearing conditions clearly showed that the incidence of diapause mainly depended on whether or not the mean temperature was ≤20℃ or >20℃. If the mean temperature was ≤20℃, all individuals entered diapause; if >20℃, the incidence of diapause gradually declined with increasing mean temperatures. The results demonstrated a cryophase threshold temperature of about 19℃ and a thermophase threshold temperature of about 31℃ in this insect. Thermoperiodic amplitude (temperature difference between cryophase and thermophase) was shown to have a significant influence on diapause induction at the mean temperatures of 22, 23 and 24℃, but not at ≤20℃ and at > 25℃. Thermoperiodic responses at LD12: 12 clearly showed that the incidence of diapause was strongly influenced by the photophase temperature. The thermoperiod at LD12:12 induced much lower incidence of diapause than the thermoperiod with the same temperature at continuous darkness.3. Effects of food-plants on the diapause inductionIn the cabbage beetle, C. bowringi, the incidence of diapause w...