Voltinism and Diapause In the Spruce Web-spinning Sawfly Cephalcia-arvensis

Cephalcia arvensis Panzer (Hym., Pamphiliidae) spends most of the life cycle as a prepupa inside an earth-walled chamber in the soil of spruce (Picea abies Karst.) forests. There are two types of prepupae in diapause in the soil, the eonymph and the pronymph. Prolonged diapause as an eonymph occurs in prepupae just after soil penetration at temperatures below 12 degrees C. Eonymphs change into pronymphs during the following summer and the life cycle lasts for two years. Mature Larvae that enter the soil at temperatures above 12 degrees C change immediately into pronymphs and these individuals will emerge during the next spring. Other factors such as photoperiod do not appear to be correlated with prolonged diapause. The depth reached by the mature larvae in the soil is proportional to their size and inversely proportional to the soil temperature. Consequently, voltinism depends on both the conditions affecting development of the larvae in the crowns and the temperature at different soil depths.