GEOGRAPHIC VARIATION IN THE PHOTOPERIODIC REACTION OF NEODIPRION SERTIFER (GEOFF.)

Abstract

Sullivan and Wallace (1965) showed that prepupal diapause in cocooned larvae of N. scrtifer from a population near Chatsworth, Ontario (lat. 44'30' N.; long. 81'00' W.; elevation 1000 ft), is eliminated in most individuals by rearing the larvae during the feeding stages a t 21 'C and a t photoperiods from 4 to 15 hours, with subsequent incubation of the cocoons in darkness a t 21 'C. The critical photoperiod is slightly less than 16 hours and the reaction curve in the threshold region is very steep. The length of time from cocoon spinning to adult emergence among diapause individuals is about 50 days and among non-diapause individuals, about 30 days. Rearings of N. sertifer from Chatsworth, Ontario; Blagoevgrad, Bulgaria (lat. 42'01' N.; long. 23'06' E.; elevation 1450 ft); and Kauhajoki, Finland (lat. 62'22' N.; long. 22'17' E.; elevation 525 f t ) , were conducted a t 20 f 1 'C and a 17-hour photoperiod in 1965. Eggs from all three populations hatched about May 13 and cocoon spinning took place between June 1 and June 15. Adult emergence from 90% of the cocoons of the Finnish population took place between June 26 and July 7, or less than 30 days after cocoon spinning. Emergence from the Chatsworth and Blagoevgrad cocoons had not taken place 45 days after spinning. Dissections showed healthy prepupal larvae present in the cocoons. These results indicate the presence of a diapause among Chatsworth and Blagoevgrad individuals, samples from populations existing a t about the same latitude, and the absence of diapause in the Finnish stock originating from a higher latitude. N. sertifer is a short-day species a t Chatsworth and Blagoevgrad, where the natural photoperiods, including civil twilight, are about 16 hours 45 minutes and 16 hours 10 minutes, respectively, a t the end of the normal larval feeding period. At Kauhajoki, the natural photoperiod is about 21 hours 15 minutes a t the end of the larval feeding period. As a short-day species in Finland, N. sertifer must have a critical photoperiod much longer than the 16 hours found for the Chatsworth population. Danilevskii (1965) documented the constancy of the photoperiodic reaction in geographical populations of insects and also has shown the adaptive significance of variation in the reaction among populations of a species from different latitudes. The findings presented here indicate the need for detailed studies of latitudinal variation in the photoperiodic reaction of N. sertifer to understand the regulation of seasonal history in this widely distributed pest of hard pines.