The hypothesis that a circadian clock comprised of circadian clock genes is causally involved in insect photoperiodism has been supported by several studies. However, there remains a possibility that the effects of the circadian clock genes on photoperiodism are exerted through pleiotropic (non-circadian) functions provided by each gene independently from its role in the circadian clock. In the present study, we investigated the involvement of the circadian clock gene mammalian-type cryptochrome (cry-m) in photoperiodic regulation of ovarian development in the bean bug Riptortus pedestris by using RNA interference (RNAi). Injection of cry-m double-stranded RNA (dsRNA) induced expression of period (per), whereas did not affect expression of cycle (cyc), showing that CRY-m functions as a negative element on CYC-mediated transcription in the circadian clock. If the circadian clock is indeed involved in photoperiodism, a phenotype produced by RNAi of cry-m will be the same as that produced by RNAi of per, another negative element. The intact insects and insects injected with control dsRNA were found to enter diapause when kept under short-day conditions after adult emergence, while they developed ovaries when kept under long-day conditions after adult emergence. However, cry-m RNAi significantly increased the incidence of reproductive individuals under diapause-inducing short-day conditions, as per RNAi did, in accordance with our expectation. (C) 2011 Elsevier Inc. All rights reserved.