Reproductive Potential and Population Growth of the Worm Enchytraeus buchholzi (Clitellata: Enchytraeidae) Under Laboratory Conditions as Well as Regression Models.

Abstract

The worm Enchytraeus buchholzi is a new pest injuring American ginseng Panax quinquefolium. To explore its reproductive potential and then estimate its population dynamics, the authors conducted two related experiments: (1) measuring individual fecundity in its lifetime by rearing each of the parent adults alone in a wet sandy dish at 18 and 21 °C indoors; (2) testing population growth by rearing each of the parent adults together with its offspring for a time longer than two generations at 21 °C. In Experiment I, five dependent variables, namely daily mean cocoons (DMC), cumulative cocoons (CC), eggs per cocoon (EPC), daily mean eggs (DME) and cumulative eggs (CE), were extracted, with each of them subject to a stepwise regression analysis on rearing time (T) and its power series as independent variables. Equaling to the net reproductive rate (R₀), the generational adult equivalent (GAE) was calculated via a conversion of F₁ generational eggs into adult equivalents (AE). In Experiment II, both an exponential and a logistic function were applied to construct regression equations. The results indicated that (1) a parent adult of E. buchholzi was able to live for a period as long as 10 and 13 full generations at the two temperatures tested and lay 84.8 and 110.6 cocoons containing 545 and 714 eggs, respectively; (2) DMC reached its maximum between 7 and 9 days of rearing and then declined slowly along a straight regression line; (3) CC rose steadily along a quadratic curve; (4) both EPC and DME varied following a cubic curve; (5) CE increased steadily along a cubic curve; (6) the new polynomial models suitably reflected the numerical growth trends of cocoons and eggs in the F₁ generation in a broad sense, while corresponding derivative equations quantified both the daily reproductive potential and resistance of the worm, thus revealed its daily reproductive capacity; (7) R₀ was 41.2 AE at 21 °C and 42.5 AE at 18 °C when a population of E. buchholzi lived in a niche with unlimited ambient resources; (8) this kind of temporal population generated by individual reproduction had fully demonstrated its significant, generational reproductive potential; and (9), through living in such a limited area as the wet sandy dish, bypassing an exponential growth process, the laboratory population grew up along a logistic curve from the F₁ to F₃ generations. The statistical relationships help to comprehend the individual reproduction of E. buchholzi, understand deeply the logical sequence and the difference between individual and population reproductions, predict population dynamics of the worm, and provide its integrated pest management with a solid basis. The experimental study has expanded theories on bionomics and population ecology, opening up a new area for research work in related fields.