Environmental control of univoltinism and its evolution in an insect species

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Abstract
Chrysopa downesi, a conifer-inhabiting unicyclic insect species, shares many features in its seasonal cycle with some well-studied single-brooded birds: a quantitative response to winter daylengths, reproduction in early spring, a summer 'photorefractory' phase, and a short-day requirement for termination of this phase. Under natural conditions, C. downesi has an aestival–autumnal–hibernal reproductive diapause that is primarily controlled by photoperiod. The short-day requirement for diapause termination is fulfilled in October. Subsequently, during winter, the rate of diapause development is directly related to the actual duration of daylength, and diapause ends between March 1 and March 21. Oviposition begins in mid-April. The rate of egg and larval development is primarily regulated by temperature; the lower thermal threshold for development (t) is 11 °C, and K value (heat-degree days) for the period from egg to adult emergence is 378 day °C.The late ending of C. downesi's diapause, its relatively high t value, and its aestival diapause separate temporally the reproductive periods of this species and its sympatric sibling species, Chrysopa carnea. The asynchronous seasonal cycles of C. downesi and C. carnea form a major barrier to hybridization between natural populations of the two species in the northeastern United States.

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