Performance of Two Pea Aphid Biotypes on Glandular Haired Alfalfa

T.J. Davis, D.B. Hogg and A.M.A. Mohamed
Department of Entomology, University of Wisconsin, Madison, WI 53706

Pea aphid, Acyrthosiphon pisum (Harris), is an occasional pest of alfalfa in the midwestern U.S. Pea aphid populations in Wisconsin consist of what are believed to be two biotypes that differ in color, hereafter referred to as the green form and the pink form. The two biotypes occur together and appear to coexist in alfalfa fields, although the green form tends to maintain a higher density of the two. In 1997, glandular haired alfalfa varieties with professed resistance to potato leafhopper were released commercially. The purpose of this study was to investigate and compare the performance of the two pea aphid biotypes on glandular haired and non-hairy alfalfa.

In a no-choice study, life history attributes of the two biotypes were compared on the glandular haired cultivar 'Trailblazer' and the non-hairy cultivar 'Rushmore'. The study was conducted under fluctuating temperatures in a greenhouse with 16L:8D photoperiod and approximately 70% RH. Daily high and low temperatures during the study averaged about 22%C and 14%C, respectively. Aphids were reared individually in dialysis tube cages on alfalfa stems and were checked daily for development (molting), mortality and reproduction (nymphs).

Aphid developmental period (days to adulthood and first reproduction), reproductive period, and daily and total reproduction were analyzed using ANOVA with main factors aphid color form and alfalfa cultivar. No statistical differences were detected in developmental or reproductive periods. However, there were significant differences for color form in daily reproduction (green > pink) and for cultivar in total reproduction (Rushmore > Trailblazer). Also, survivorship values of the green aphids after 40 days on Trailblazer and Rushmore were 40% and 90%, respectively, whereas survivorship values for the pink aphids were 60% and 100%, respectively.

In addition, age-specific schedules of survivorship and reproduction were used to estimate life table parameters, most prominently the intrinsic rate of increase which provides an index of potential population growth, for the four combinations of color form and cultivar,. Despite the influence of cultivar on both total aphid reproduction and survivorship, the effect of cultivar on intrinsic rate of increase was not statistically significant. This unexpected outcome appears to have happened because 1) peak aphid reproduction occurred earlier on Trailblazer than on Rushmore, thus contributing more to potential population growth rate, and 2) aphid mortality on Trailblazer occurred after peak reproduction had already taken place. In contrast, the effect of aphid color form on intrinsic rate of increase was statistically significant, with the green form having a slightly higher rate than the pink form.

These results suggest that the glandular hair trait involved with alfalfa resistance to potato leafhopper has no measurable effect on pea aphid population growth. In addition, the difference in population growth rates between the green and pink color forms may contribute to the higher densities of green pea aphids observed in field populations and provides support for biotype designation.

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