ISSN 1392-3196 ŽEMDIRBYSTĖ=AGRICULTURE Vol. 99, No. 2 (2012) 203 ISSN 1392-3196 Žemdirbystė=Agriculture, vol. 99, No. 2 (2012), p. 203‒208 UDK 635.1:581.15:577.21(043.3) Cold hardiness of horseradish lea beetle (Phyllotreta armoraciae (Koch)) Külli HIIESAAR, Riina KAASIK, Ingrid H. WILLIAMS, Eha ŠVILPONIS, Katrin JÕGAR, Luule METSPALU, Marika MÄND, Angela PLOOMI, Anne LUIK Institute of Agricultural and Environmental Sciences, Estonian University of Life Sciences Kreutzwaldi 1, 51014, Tartu, Estonia E-mail: kylli.hiiesaar@emu.ee Abstract Supercooling ability and cold hardiness in the horseradish lea beetle Phyllotreta armoracia (Koch) (Coleoptera: Chrysomelidae: Alticinae) was investigated. Mean supercooling points (SCP) varied from −10.1 to −23.9°C depending on the season: SCP started to decrease in August, achieved their lowest value in mid-January after four months at 5°C, increased drastically in February and stayed at this high level until May. In contrast, some beetles had a distinctly high SCP during overwintering and survived freezing at their SCP. The beetles acquired suficient cold hardiness already in September–October with lethal time for 50% mortality of 448 h; the temperature necessary to kill 50% of beetles after 24 h exposure was −12.2°C. The beetles were able to survive very low temperatures only during diapause period. Their cold tolerance decreased on the ifth month of hibernation with Ltemp 50 above −8°C. At the same time the beetles retained a relatively good ability to withstand the moderately low temperature of −6°C for longer periods after termination of diapause as over 60% of them survived 312 h exposure in February; however, all the feeding beetles in May were dead already after 96 h exposure. Key words: Phyllotreta armoraciae, cold hardiness, supercooling, mortality, seasonality. Introduction The horseradish lea beetle Phyllotreta armora- cia (Koch) (Coleoptera: Chrysomelidae: Alticinae) is a univoltine monophagous species feeding in nature only on horseradish, Armoracia rusticana G.M. and Sch. (Brassicaceae) (Nielsen et al., 1979). In the laboratory, the beetles can be reared on several glucosinolate-con- taining cruciferous plants, such as Brassica napus L., Si- napis alba L., Brasssica nigra Koch, and they show no preference between B. nigra and A. rusticana (Vig, Ver- dyck, 2001). However, in choice tests, beetles have pre- ferred feeding on their host plant, horseradish (Hagerup et al., 1990). Horseradish is widely cultivated in Europe as a spice for pickles or as a side dish. It also tends to na- turalize outside the cultivated areas (on abandoned ields). The horseradish lea beetle is the most prevalent and most damaging insect pest of the horseradish plant. The beetles are irregularly distributed: in some habitats plants are densely colonized by them and the leaves entirely perforated, whereas the leaves remain intact throughout the growing season in other locations. According to our long-term observations, the irst beetles emerge from their overwintering sites on a sunny, warm day at the end of April and start eating immediately when the horsera- dish has barely sprouted. The major damage is caused by adults in springtime. Females lay their eggs on the peti- oles of young leaves in May, and the larvae burrow into the petioles (Nielsen et al., 1979). The new generation beetles emerge at the end of July and in the beginning of August and feed on the same plant for about one month. In mid-September, they start leaving the plants for their overwintering sites. The adults overwinter in debris and soil crevices not far from the host plants (Vig, 2002). In one of our observation stations, in herb collection garden where horseradish has been grown for many years, the plants were continuously heavily damaged, irrespective of the severity of winter. As a dispersed weed in open ields, plants were much less damaged. So far, there are no reports about hibernation and cold hardiness in the horseradish lea beetle. Gene- rally, insects survive low temperatures by using freeze tolerance or freeze avoidance strategies (Zachariassen, 1985), but some species are known to employ both (Du- man, 1984; Gehrken et al., 1991). It is not known which strategy enables P. armoraciae to survive long and un- stable winters; therefore this study focused on factors in- luencing supercooling ability and cold hardiness in this species. Our study aimed to assess: 1) seasonal changes in supercooling points (SCP) of adults, 2) seasonal chan- ges in cold tolerance of beetles after exposure to constant subzero temperature for different periods of time, 3) sea- sonal changes in cold tolerance of beetles after exposure of beetles to different subzero temperatures for a constant period of time. Material and methods Beetles. The experiments were performed from 2008 to 2009. The horseradish lea beetles were collected by a manual aspirator from horseradish in the medical