Comparisons of Eastern and Western Strains of the Alfalfa Weevil in Nebraska12

Abstract

Both the western and eastern strains of Hypera postica (Gyllenhal) occur in Nebraska. The western strain has moved eastward at ca. 16.1 km/year, and it increases in numbers slowly after reaching new territory. In contrast, the eastern strain has moved westward in excess of 80.5 km/year and has had an impact on alfalfa production within 3 years of invading a county. Over an 8-year period, larval populations of the eastern strain consistently peaked 1 to 2 weeks earlier than those of the western strain. The most likely explanation for this earlier peaking (after eliminating temperature differences and harvest practices as possibilities) appears to be the earlier return in the spring of adults of the eastern strain to alfalfa. Results of the present study showed a much higher level of parasitism by Bathyplectes curculionis (Thomson) of the eastern than of the western strain in the field, even though the eastern strain has some inherent resistance to the parasite. Field observations failed to show any clear evidence of sex ratio changes (such as those resulting from laboratory studies) in areas where the strains occurred together (Dawson County). Evidence that hybrid weevils are more vigorous or damaging to alfalfa than either of the parent strains is lacking.