Population changes in Heterodera glycines and its bacterial parasite Pasteuria sp in naturally infested soil

A two-year soil sampling study was conducted on four microplots naturally i nfested with Heterodera glycines and an undescribed species of Pasteuria. T he objectives of the study were to investigate the population dynamics of b oth organisms and to assess the potential of Pasteuria sp. as a biological control agent of H. glycines. Seasonal fluctuations were observed in number s of cysts, eggs per cyst, second-stage juveniles (J2) of H. glycines, numb er of Pasteuria endospores attached per J2, and percentages of endospore-en cumbered J2. Percentages of endospore-encumbered J2, Y, increased with the mean numbers of endospores per J2, X, according to the equation Y = 87.0(1 -e(-0.53x)). In contrast, numbers of J2 per 250 cm(3) soil, Y, decreased wi th the numbers of endospores per J2, X, according to the exponential decay model Y = 67.4 + 220.1e(-1.2x) The equilibrium J2 density (67.4 +/- 3.3) de rived from this function was consistent with the predictions of the Lotka-V olterra model of population dynamics based on the equation 0.0195ln(y) - 0. 000336y = 0.000049x - 0.00285ln(x) + 0.06589, where x and y represent the b iweekly means of J2 densities and the percentages of endospore-encumbered J 2, respectively. In all cases, predicted equilibrium densities of J2 were b elow the damage threshold reported from field studies. These results indica te that, given sufficient time following introduction into a field, Pasteur ia may increase to levels that would be effective as one component in an in tegrated pest management program to control X. glycines.