Australian Journal of Entomology (2003) 42, 352–356 Diapause termination in two species of tropical butterfly, Euploea core (Cramer) and Euploea sylvester (Fabricius) (Lepidoptera: Nymphalidae) Andrea A Canzano, Rhondda E Jones and Jamie E Seymour* Department of Zoology, James Cook University, McGregor Road, Smithfield, Qld 4878, Australia. Abstract This study examined the factors used as cues for diapause termination by two Euploea species. Euploea core and E. sylvester were collected from two overwintering sites at Chillagoe, Queensland. Under tropical wet-season conditions, more butterflies terminated diapause (approximately 60%) than those kept under dry-season conditions (approximately 40%). However, 100% diapause termination occurred only when butterflies were kept under wet-season conditions and were fed with honey. For reproductive success, Euploea species need to terminate diapause rapidly to synchronise oviposition with fresh, rapid growth of their larval host plants, which often occurs after rain. The presence of rainfall, increased temperature and long days alone is not a fail-safe set of cues for this event. Our results suggest that food (as nectar), in conjunction with these combined environmental factors, provide Euploea species with a reliable signal that larval host resources are available for oviposition. Key words photoperiod, rainfall, temperature, relative humidity, oviposition. INTRODUCTION In temperate regions, of all the physical factors that change seasonally, photoperiod (which is highly positively corre- lated with temperature) provides the most regular and reli- able long-term indicator of future conditions (Tauber et al. 1986). Consequently, temperature changes between the seasons are clearly defined. In most insects, given their ectothermic nature, development is driven by temperature. Often temperatures in temperate regions will drop below the developmental threshold of an insect, preventing growth or reproduction for the duration of the unfavourable period (Danks 1987). Dormancy of temperate plants is positively correlated with temperature and photoperiod, so that changes in these variables widely control patterns of vegetative growth in temperate regions (Danks 1987). Because of this, many temperate-zone insects have developed strategies (using temperature and photoperiod) to synchronise their development with favourable conditions, such as more hos- pitable temperatures and food resource availability (Tauber et al. 1986). It is not surprising, then, that temperate-zone insects so often use photoperiod and/or temperature as a signal to terminate diapause. Tropical regions, however, present a different set of environmental parameters and the cycling of seasons is less directly determined by the position of the sun. Changes in daylength and temperature close to the equator are less marked than those in temperate regions. Periods of twilight are less variable and shorter than in the temperate zones, whilst air temperatures are seldom low enough to drop below the lower developmental threshold of most tropical insects, theoretically allowing development to occur all year round. This is not to imply that the tropics are aseasonal, as they can be driven by distinct changes in wet and dry seasonal patterns. As a result, rainfall events may provide a cue for terminating tropical insect diapause (Seymour & Jones 2000; Pieloor & Seymour 2001). Termination of diapause by rainfall and high relative humidity is being shown in an ever-increasing array of tropical species. Species of Lepidoptera (Jones 1987; Jones & Rienks 1987; Scheermeyer 1987; Braby 1995; Kemp 2001; Pieloor & Seymour 2001), Hymenoptera (Seymour 1991; Seymour & Jones 2000), Orthoptera (Tanaka 1987; Ingrisch 1996) and Coleoptera (Tanaka et al. 1987a,b) have the end of their diapause correlated with rainfall. This may be more so for species that rely on fresh plant growth for food resources and/or oviposition sites. In Australia, several families of Lepidoptera undergo diapause and two conspicuous species in the tropics are Euploea core (Cramer) and E. sylvester (Fabricius). Like other tropical nymphalids, these two species display a dia- pause strategy at the adult stage (Scheermeyer 1999). They share similar diapause characteristics to the monarch butter- fly, Danaus plexippus (L), such as migration, formation of aggregations, increased fat bodies and reproductive arrest, as well as sporadic feeding of some individuals at their over- wintering sites (Kitching & Zalucki 1981; Troyer et al. 1996). Our study aimed to determine whether, as in many other tropical insects, the factors acting as cues for diapause termination are related to humidity and/or rainfall, or whether, as in many temperate species, diapause termination is controlled by temperature and photoperiod. *Author to whom correspondence should be addressed (email: Jamie.Seymour@jcu.edu.au).