Diapause in the Nearctic Species of the virgifera Group of Diabrotica1: Evidence for Tropical Origin and Temperate Adaptations

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Abstract
Voltinism and diapause were studied in the species of the virgifera group of Diabrotica (Coleoptera: Chrysomelidae) that occur in the United States. The well-known univoltinism and egg diapause of the western corn rootworm (WCR), Diabrotica virgifera virgifera LeConte, and the northern corn rootworm (NCR), D. longicornis barberi Smith and Lawrence, was the basis for comparisons. Examination of museum collection records support univoltinism for D. cristata (Harris) (CRI), D. longicornis longicornis (Say) (LON), D. lemniscata LeConte (LEM), and the Mexican corn rootworm (MCR), D. virgifera zeae Krysan and Smith. Laboratory studies demonstrated an egg diapause in CRI, LEM, and LON. Eggs of MCR and WCR from 15 sites from throughout the geographic distribution diapause in the same embryonic stage, i.e., an early, undifferentiated germ band. Eggs of CRI and LEM diapause in that same stage; by contrast, LON diapauses as a segmented embryo with clearly defined cephalic lobes. The enhancement of diapause termination by a chill period (7°C) was demonstrated in CRI and LON and confirmed for the NCR. A cline in diapause duration was detected in eggs of the WCR; the more northerly populations had shorter diapause. The relationship prevailed at three temperatures: 15, 18, and 25°C. The results favor a hypothesis that these taxa, which now occupy the temperate climate, share, via common evolutionary origin, a diapause mechanism which originally evolved in the tropics. Adaptations that have evolved in the temperate climate incude low thermal optima for diapause development in the NCR, LON, and CRI and brief diapause in the WCR.