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Disentangling the responses of insects to variations in their thermal environment is central to our understanding of the evolution of temperature-dependent performance in these species. Here, we report results of experiments examining the effects of high (upper lethal temperature = ULT) and low (lower lethal temperature = LLT) temperature and exposure time on the survival of larvae and adults of a multivoltine, nocturnal moth species, Pareuchaetes insulata, a biological control agent whose impact on an invasive weed, Chromolaena odorata has been variable in South Africa. The influence of temperature and acclimation on locomotion performance of the moth was also investigated. Temperature and duration of exposure significantly affected survival of both adults and larvae of P. insulata with more extreme temperatures and/or longer durations proving to be more lethal. Third instar larvae and adults are both freeze intolerant and had LT50 of −5.9 and −4.7°C, respectively, after a 2 h exposure. Although cold acclimation was beneficial to the nocturnal larvae, temperatures below 10°C significantly reduce their locomotion activities. The average daily minimum temperatures in the coldest months at three locations in South Africa are over 5°C lower than those of Fort Lauderdale, Florida, USA, where P. insulata was originally collected. Our results suggest that lethal high or low temperatures at short timescales are trivial in explaining the variable performance of P. insulata, but reduced locomotion at sub-lethal temperatures may be an important driver of the population dynamics of the biocontrol agent (especially in winter months) and may consequently explain the low population levels of the moth because of possible reduced feeding by larvae during night-time low temperatures.
Chromolaena odorata (L.) King and Robinson (Asteraceae), formerly known as Eupatorium odoratum L., is a weedy pioneering shrub native to the Americas from southern USA to northern Argentina (Gautier, 1992). Chromolaena odorata has become one of the worst terrestrial invasive plants in the humid tropics and subtropics of the Old World over the past century (Holm et al., 1977; Gautier, 1992). From its original point of introduction as an ornamental plant in northeastern India in the mid nineteenth century, it has spread throughout Southeast Asia, into parts of Oceania (Muniappan and Marutani, 1988; McFadyen, 1989;Waterhouse, 1994a), and into West and Central Africa (Gautier, 1992; Prasad et al., 1996). A different form of C. odorata (see below), first recorded as naturalized in the 1940s (Hilliard, 1977), has invaded a large part of the subtropics of southern Africa (Goodall and Erasmus, 1996).
Individual C. odorata plants are easily controlled by chemical and/or mechanical means. However, as it is a weed mainly of the tropics and subtropics, many of the countries in which it is a problem do not have the resources to implement comprehensive control programs using conventional methods. Consequently, biological control has become an important management tool (Goodall and Erasmus, 1996; McFadyen, 1996a).