Attributes pertinent to overwintering potential do not explain why Bactrocera neohumeralis (Hardy) (Diptera: Tephritidae) does not spread further south within the geographical range of B. tryoni (Froggatt)

Abstract  The geographical range of Bactrocera neohumeralis does not extend as far south as that of its sibling species, B. tryoni . However, there was no evidence of any difference between the two species in terms of physiological limitation to southerly spread when comparisons were made of low temperature torpor thresholds of adults, survival time of adults at −4°C and development rates of all stages in either warm or cool regimes. The survival schedule of the two species was similar in the laboratory and also in the moderately cold conditions experienced by caged cohorts that were exposed to winter field temperatures between late April and early November at Richmond, New South Wales (500 km south of the usual southerly limit of B. neohumeralis ). Overwintered cohorts of both species laid similar numbers of eggs in September in terms of eggs per emerged female (an indicator of the reproductive potential). However, because the proportion of B. tryoni surviving to the period of 1–15 September was less than half that for B. neohumeralis , the production per surviving female was more than double in B. tryoni . The possibility of the southerly spread of B. neohumeralis being limited by an Allee effect is discussed.