Population Trends of the Alfalfa Weevil 1 Larvae and the Distribution of Its Parasites in Pennsylvania 2

Abstract

Larval population trends of the alfalfa weevil, Hypera postica (Gyllenhal), and the distribution of Tetrastichus incertus (Ratzeburg), Bathyplectes curculionis (Thomson), and B. anurus (Thomson) were investigated during 1969 and 1970 in the central and southern regions of Pennsylvania. Weevil larvae began to hatch in mid- to late April in the southern regions and about 2 weeks later in the central regions. The population increased steadily and reached a maximum in 3–4 weeks after the eggs hatched. The larval population declined sharply following the peak. A partial 2nd peak occurred in midsummer in several of the regions, but no more than 10 larvae per sweep were collected at any sample site throughout the remainder of the growing season. Peak weevil larval populations were somewhat lower in 1970 than in 1969 in 3 of the 5 sampling regions. The population densities were highest in the west and the lowest in the east. A significant correlation was observed throughout the State between the longitudinal minutes of the sample sites and number of weevils found during the population peak in 1969. Significant correlations existed for these parameters in the southern region in 1969 and 1970 when these sample sites were analyzed alone. T. incertus and B. curculionis were found in all the fields surveyed throughout the State. B. anurus, although released in the southeastern and south-central regions, was recovered only from the southeastern region. B. anurus appeared well synchronized with its host population. T. incertus was found in alfalfa fields in early May, but parasitization was exceedingly low while the larval populations were high, never being greater than 4.5% in any region prior to mid-June. Percent parasitism increased substantially following the peak larval populations, and although it remained relatively high it fluctuated for a major portion of the growing season. Dispersal of B. curculionis and its subsequent establishment throughout the State appeared to have occurred in a westerly direction and from release sites within the State. Percent parasitism at weevil larval population peaks was highest at or near release sites. Sample sites farthest from release sites, i.e. western regions, had the lowest percent parasitism. Effective parasitism by B. curculionis appears contingent on its ability to superparasitize the host and thus avoid encapsulation. In sample sites at or near release sites a more favorable density relationship appears to exist for superparasitization. The highest percent parasitism at the host population peak was observed at these sample sites.