Time of the season: The effect of photoperiodism on host-mediated cues for diapause induction in an insect herbivore, Leptinotarsa decemlineata

Diapause is the primary life history strategy used by insect species inhabiting temperate climates for seasonal synchronization. Biotic signals for diapause induction originating from host plants may provide dependable cues for insects expanding from subtropical regions. We investigated the effect of plant-mediated photoperiod cues on diapause initiation within the Colorado potato beetle, Leptinotarsa decemlineata. We tested the effect of photoperiod changes on potato leaf composition and its subsequent correlation with diapause initiation.  Analyses revealed a significant effect of short-day (8:16 L:D and 10:14 L:D) photoperiod growth regimens on both nitrogen content in potato leaves and diapause initiation within L. decemlineata. Potato plants grown under short day conditions displayed significantly higher levels of leaf nitrogen compared with long day treated plants. Over 65% of beetles feeding upon short-day treated plants initiated diapause compared with < 20% of beetles placed upon long-day treated plants. The observation of seasonality-induced diapause signals by host plants signifies an important addition to the current array of plant-insect interactions. Our results suggest that seasonal synchrony may be the result of a more integrated system whereby host plant photoperiodism acts as an intermediate or supplementary physiological cue for diapause initiation.