Effects of environmental factors on life cycle regulation in Lasius japonicus Santschi (Formicidae)

This study investigated environmental factors that regulate oviposition by queens and colony development in Lasius japonicus Santschi. Newly mated queens were collected from fields in Okayama, Japan. The insects were maintained outdoors or under a longor short-day photoperiod at 30 degrees C, 25 degrees C, 20 degrees C, or 15 +/- 1 degrees C. All queens died within 50 days at 30 degrees C, whereas all or most of the queens survived at lower temperatures. At 25 degrees C, many pupae were observed approximately 1 month after the onset of oviposition. Diapause in either queens or workers was not induced at 25 degrees C. At 20 degrees C, many larvae did not pupate, indicating that larval diapause was induced. At 15 degrees C, hatching was not recorded and eggs disappeared. Low temperatures may induce reproductive diapause in queens. There were no significant differences between long-day (LD 16:8 h) and short-day (LD 12:12 h) conditions at any temperature. Under outdoor conditions, when summer temperature was moderate in 2005, queens started their nuptial flights in June, and pupation was recorded three times. However, when summer temperature was high in 2006, pupation occurred 1-2 times before winter, with queens making their nuptial flights as late as mid-July. Eggs and pupae disappeared in most colonies before the arrival of winter. Only queens, adult workers, and larvae were observed during winter. The present study showed that queens of L. japonicus founded and developed their colonies for as long as environmental conditions remained favorable, by responding to changes in temperature.