Diapause and Polymorphism in California Populations of Psylla pyricola (Homoptera: Psyllidae)1,2,3

Abstract

Pear psylla, Psylla pyricola Foerster, from Wynola, San Diego County, California (approximately lat 33°5'N), could be induced to enter diapause by exposure to photoperiod of 14 hr light and 10 hr dark; but pear psylla from Danville, Contra Costa County, California approximately lat 37°50'N), required a photoperiod of13.5 hr light and 10.5 hr dark when both were reared identical temperatures. Females that entered diapausehad long forewings; females that did not enter diapause were short to long winged, and no females with short forewings entered diapause. At 16 and 27°C, the critical photoperiod for induction of diapause in females from Wynola differed only slightly. Females did not enter diapause when they were not exposed to the critical photoperiod until they had reached the 5th instar. Females from populations collected from several locations in California, north of Wynola, had similar mean lengths of forewing when reared under a 14.5-hr daily photoperiod. For males, the mean forewing lengths also were similar, but the forewings of males were always shorter than those of females. However, under the same rearing conditions, females and males from the Wynola population always had longer forewings than those from northern populations. In the field, during summer, mean length of the forewings increased much sooner at Wynola than at other locations. The difference may relate to the origin of the Wynola population.