Laboratory and Field Studies on the Seasonal Forms of Pear Psylla in Northern California

Abstract

The pear psylla, Psylla pyricola Foerster, has 2 forms. The summer form is generally of light color, and is observed in the fields from the beginning of spring until autumn; the winter form is darker and larger, and is present from autumn to spring. These 2 forms differ biologically as well as morphologically. The summer-form female is able to oviposit within a week after adult emergence, whereas the overwintering female has an ovarian diapause. The pear psylla was reared from egg to adult in the laboratory under 2 different conditions of photoperiod and temperature. The conditions used to produce summer and winter forms were a 16-hr photoperiod and 25°C, and an 11-hr photoperiod and 17°C, respectively. Statistical analysis of measurements showed that the laboratory-induced winter-form adults were significantly larger than the laboratory-induced summer-form adults. No significant differences in size were found between the laboratory winter-form females and the field-collected winter-form females, but in most cases the field-collected males were significantly larger than the laboratory winter-form males. In all cases the winter forms were significantly larger than the corresponding summer forms The wings of the 2 morphological forms exhibited distinct differences in the ornamentations. The veins of the forewings of the winter forms were strongly pigmented and almost black, whereas the summer forms were almost translucent. The cells of the forewings of the winter forms showed dark smoky zones, while the wings of the summer forms were usually dear with 2 dark smoky spots in the claval suture area. Laboratory results indicated that the factors which determined morphological form also determined biological form. The laboratory summer forms exhibited no ovarian diapause and started oviposition within 2 weeks of emergence, while the laboratory winter forms and the field-collected over-wintering forms exhibited ovarian diapause and required a considerably longer period of time before oviposition. Laboratory studies showed that the length of the diapause was influenced by the photoperiod to which the adults were subjected. The 16-hr photoperiod shortened diapause considerably as compared with the 11-hr photoperiod. Oviposition for the overwintering females in the field occurred in February in spite of considerable difference in mean temperatures for the different localities. The results emphasized the relative importance of the increasing daily maximum temperature and the day length during this period in inducing the overwintering females to oviposit.