Observations on the effect of photoperiod on the control of polymorphism in Psylla pyricola

ABSTRACT. Psylla pyricola Forster (Homoptera) occurs as two adult morphs, a summer form (f. typica) and a winter form (f. simulans). The latter is larger, darker and overwinters in ovarian diapause. Eggs laid by either morph under LD 12:12 (short days) or under LD 18:6 or 16:8 (long days) gave f. typica when reared under long days. When reared under short days, almost all were f. simulans. No interforms were recorded. When the photoperiod was switched during the nymphal stage (whether from long to short days, diapause induction; or from short to long days, diapause termination), nymphs were progressively less sensitive with increasing age, with no morph changes when the switch occurred in the fifth instar. Nymphs derived from f. simulans reared under long days tended to be more sensitive to a change to short days in the early instars than when the nymphs were first reared under short days and then switched to long days. Interforms were frequent when the switch occurred during the nymphal stage, particularly in the third instar. Wing lengths varied within morphs depending on the timing of the switch. The largest f. simulans were derived from f. typica when maintained under short days, and the smallest f. typica from either morph when maintained under long days, as would be expected under field conditions. No f. simulans were found under field conditions in the first f. typica generation, but a small percentage were present in the second generation. It is concluded that, even within a given generation, not all P. pyricola are equally sensitive to photoperiod, but that photoperiod is one of the major factors controlling the change to f. simulans in the autumn. Whilst under laboratory conditions photoperiod can also affect the switch to f. typica in the spring, in the field it may be less important than low temperatures.