Abstract The tortoise beetle Gratiana spadicea (Klug) (Coleoptera: Chrysomelidae) was released as a biological control agent for the South American weed, Solanum sisymbriifolium Lamarck (Solanaceae), in South Africa. Many of the releases were carried out in the high altitude regions (Highveld). However, the insect failed to establish at some sites, and where establishment was confirmed, its impact has been variable. Cold winters were blamed for the insect's nonestablishment or for the variable success at some sites. This hypothesis 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 potential moisture stress for the eggs. Humidity had no significant effect on the survival of G. spadicea pupae. The critical minimum temperature (CT min) of the adult beetles was 4.9±1.3 ° C (n=20) and 6.8±1.3 ° C (n=20) for the larvae. The calculated LT 50 of the adult beetles was −7.1 ° C (n=120;−9.1 to−5.3 ° C) , 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.