The influence of climate on the establishment and success of the biocontrol agent Gratiana spadicea, released on Solanum sisymbriifolium in South Africa M.J. Byrne, a, * S. Currin, a and M.P. Hill b a Ecophysiological Studies Research Programme, School of Animal, Plant and Environmental Sciences, University of the Witwatersrand, Wits, 2050, South Africa b ARC—Plant Protection and Research Institute, Private Bag X134, Pretoria 0001, South Africa Received 1 May 2001; accepted 31 December 2001 Abstract Thetortoisebeetle Gratiana spadicea (Klug)(Coleoptera:Chrysomelidae)wasreleasedasabiologicalcontrolagentfortheSouth American weed, Solanum sisymbriifolium Lamarck(Solanaceae),inSouthAfrica.Manyofthereleaseswerecarriedoutinthehigh altituderegions(Highveld).However,theinsectfailedtoestablishatsomesites,andwhereestablishmentwasconfirmed,itsimpact has been variable. Cold winters were blamed for the insect’s nonestablishment or for the variable success at some sites. This hy- pothesis was tested by comparing the climate in the native range of beetles with that of the release sites in South Africa and by exposing different life stages of the beetle to the extremes of temperature and humidity, typical of winters at these sites. Climate comparison indicated that moisture stress would be expected at the high elevation release sites. The lower lethal humidity for the eggs was calculated to be 56.6% RH. Humidity at the high elevation release sites dropped below this value every month, creating potentialmoisturestressfortheeggs.Humidityhadnosignificanteffectonthesurvivalof G. spadicea pupae.Thecriticalminimum temperature ðCT min Þ oftheadultbeetleswas4:9  1:3 °C ðn ¼ 20Þ and6:8  1:3 °C ðn ¼ 20Þ for the larvae. The calculated LT 50 of the adult beetles was 7:1 °C ðn ¼ 120; 9:1to  5:3 °CÞ, while eggs and larvae survived 2-h exposure to 10 °C. Although the beetle is tolerant of temperature extremes experienced at the Highveld sites, this area was shown to be a marginal habitat for the beetles because of humidity stress. Ó 2002 Elsevier Science (USA). All rights reserved. Keywords: Climate compatibility; Critical thermal limit; Gratiana spadicea; Humidity; Solanum sisymbriifolium; Classical biocontrol; Establishment 1. Introduction Solanum sisymbriifolium Lamarck (Solanaceae) (wild tomato) is a shrubby weed, native to warm temperate South America. It apparently entered South Africa as a contaminant of imported horse fodder around 1900 (Nel,1988)andwasfirstreportedasapotentialproblem in1941(DuToitandvanderMerwe,1941).Theweedis also established in Australia, India, the United States, and China (Hill et al., 1993). Once mainly restricted to disturbed areas, infestations of S. sisymbriifolium have in the last two decades become increasingly vigorous in agricultural areas and forestry plantations, reducing the carryingcapacityandvalueoftheland(HillandHulley, 1995a). Birds spread the seeds of the attractive edible fruits, and because of the high density of glandular trichomesontheleaves, S. sisymbriifolium sustains little damage from native herbivorous insects (Hill et al., 1993, 1997). Attempts at mechanical and chemical controls have been thwarted by the weed’s ability to coppiceafterbeingcut,andbyitshighfruitproduction, which ensures the rapid recruitment of seedlings from a resilient seed bank (Olckers et al., 1999). The weed persists in cold regions by dying-back in winter, then coppicing from rootstock, and producing seedlings in the spring. In 1994, the South American leaf-feeding tortoise beetle, Gratiana spadicea (Klug) (Coleoptera: Chryso- melidae), was released as a biological control agent for * Corresponding author. Fax: +27-11-403-1429. E-mail address: marcus@gecko.biol.wits.ac.za (M.J. Byrne). Biological Control 24 (2002) 128–134 www.academicpress.com 1049-9644/02/$ - see front matter Ó 2002 Elsevier Science (USA). All rights reserved. PII:S1049-9644(02)00021-X