NON-DIAPAUSE OVERWINTERING BY PIERIS RAPAE 
(LEPIDOPTERA: PIERIDAE) AND PAPILIO ZELICAON 
(LEPIDOPTERA: PAPILIONIDAE) IN CALIFORNIA: 
ADAPTIVENESS OF TYPE III DIAPAUSE-INDUCTION 
CURVES* 
By Arthur M. Shapiro 
Department of Zoology, 
University of California, 
Davis, California 95616 
Diapause is generally regarded as a physiological adaptation 
which increases the probability of surviving the adverse season, and 
thus of reproducing after it is over. Many insect species show 
geographic differences in the environmental regimes which induce 
or inhibit diapause (e.g., critical photoperiod) and in the strength of 
the diapause induced. Such interpopulational differences are com- 
monly viewed as “fine tuning” to local climates, accomplished by 
natural selection and reflecting a genetic basis (e.g., Istock, 1981). 
Intrapopulational differences in photoperiodic sensitivity and dia- 
pause strength (e.g., chilling requirement) also occur, and have been 
interpreted as polymorphisms which “spread the risk” of environ- 
mental uncertainty over the population (cf. Bradshaw 1973, Shapiro 
1979, 1980a). In multivoltine insects in seasonal climates, offspring 
produced by the last seasonal generation of adults are commonly 
induced to enter diapause by specific combinations of environ- 
mental factors; in mid-latitudes these are likely to be decreasing 
photophase/ increasing scotophase and decreasing or consistently 
low night temperatures. Warmer nights tend to shorten the critical 
photoperiod for a given population, or may effectively inhibit 
diapause altogether under field conditions. 
Beck (1980) characterized diapause induction as falling into four 
response types. Type I is the common long-day response, in which 
long days inhibit diapause and there is a single threshold beyond 
which diapause occurs. Type II, the short-day response, is the 
reciprocal of Type I. Type III has two well-defined critical day- 
lengths, such that diapause is induced between, say, 8 and 14 hours 
* Manuscript received by the editor April 15, 1984. 
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