Robert M. Nowierski

COMPLEX POPULATION DYNAMICS IN THE REAL WORLD: MODELING THE INFLUENCE OF TIME-VARYING PARAMETERS AND TIME LAGS

We propose a class of complex population dynamic models that combines new time-varying parameters and second-order time lags for describing univariate ecological time series data. The Kalman filter and likelihood function were used to estimate parameters of all models in the class for 31 data sets, and Schwarz’s information criterion (SIC) was used to select the best model for each data set. Using the SIC method, models containing density-dependent processes were selected for 23 of the 31 cases examined, while models containing complex density-dependent processes were selected in 19 of these 23 density dependence cases. The density-dependent models identified by SIC had various linear or nonlinear forms, suggesting variable patterns of population regulation in nature. Population dynamics may combine density-dependent, inversely density-dependent, and density-independent processes, which may operate at different times and under different density ranges. These results suggest that our approach offers an adv...

Infectivity of Pandora neoaphidis (Zygomycetes: Entomophthorales) to Acyrthosiphon pisum (Hom., Aphididae) in response to varying temperature and photoperiod regimes

The infectivity of the pea aphid, Acyrthosiphon pisum, by the aphid‐specific entomophthoralean fungus, Pandora neoaphidis, was studied in environmental chambers using computer simulated late‐season temperature and photoperiod patterns and an excised fava bean leaf system. A complementary log‐log (CLL) time–dose–mortality model was used to model time–dose trends in infectivity of A. pisum by P. neoaphidis as a function of time and dose, and calculate the LC50. The likelihood ratio test based on the CLL model was used to test for differences in levels of aphid infectivity by P. neoaphidis over different photoperiod regimes. Differences in the LC50 values from CLL modelling over different time periods were found among the two regimes of 11‐ and 16‐h photophases at a constant temperature of 20°C, and two regimes at daily fluctuating temperatures from 5.4 to 18.9°C (with half‐hourly changes of 0.56°C the mean temperature was 12.12°C). The LC50 for the same time period was lower under the higher temperature conditions than under lower fluctuating temperature conditions. The likelihood test ratio statistics showed no significant difference in the slope parameter for three out of four treatments (two from the constant temperature of 20°C, and one from 5.4 to 18.9°C, irrespective of photophase conditions). This suggests that different temperatures and photophases may change the temporal characteristics of P. neoaphidis, and result in higher or lower infectivity to the pea aphid at certain dosage levels. However, changes in the photoperiod appear to be less important for the infectivity of P. neoaphidis to pea aphid than changes in temperature.

Selected factors affecting seedling recruitment of dalmatian toadflax

Seedling recruitment of Dalmatian toadflax, (Linaria genistifolia ssp. dalmatica (L.) Maire and Petitmengin (Scrophulariaceae)), was examined in a 2-year field study in Montana using overseeding and plant/insect exclusion methods, to determine whether it was more limited by seed availability or interspecific plant competition. Overseeding test plots with toadflax seed had no effect on seedling recruitment. Exclusion of plant competition (via herbicide application and pruning) significantly increased total, and cumulative seedling recruitment of Dalmatian toadflax on the last sampling date in 3 of 4, and 2 of 4 cases examined, respectively. Insect exclusion (via insecticide application) significantly increased total seedling recruitment of Dalmatian toadflax on the last sampling date in only 1 of 4 cases examined, and had no effect on cumulative seedling recruitment of Dalmatian toadflax on the last sampling date. We conclude that seedling recruitment in Dalmatian toadflax was more strongly influenced by plant competition than herbivory in our study. Hence, microsite limitation (i.e., competition for “safe sites for germination”) rather than seed limitation appears to play a more important role in toadflax seedling recruitment. In light of this, current biological control agents that impact seed production will likely have minimal capabilities of influencing toadflax density. Thus, a premium should be placed on establishing biological control agents that are able to cause significant damage to the stem and root system of Dalmatian toadflax, and in maintaining a healthy plant community that, through interspecific competition, will negatively affect toadflax seedling recruitment.

Populations of Sitobion avenae and Aphidius ervi on spring wheat in the northwestern United States. Spatial distribution and sequential sampling plans based on numerical and binomial counts.

During a three‐year field survey of the English grain aphid, Sitobion avenae (F.) (Homoptera: Aphididae) and the parasitoid, Aphidius ervi Haliday (Hymenoptera: Aphidiidae; represented by aphid mummies) on spring wheat, data sets were generated which consisted of 47 estimates of mean density (m aphids per tiller), variance (s2), and the proportion of empty tillers (P0) for S. avenae and 22 estimates of the same statistics for A. ervi, respectively. Each estimate of the aphid and parasitoid populations was based on counts per individual tiller on each sampling occasion. Taylors power law was found to fit the data sets better than Iwaos m* ‐ m regression model. Taylors slope for S. avenae (1.3076) was significantly higher than that for A. ervi (1.1519), indicating that S. avenae was more aggregated than A. ervi. Based on temporal changes in spatial aggregation represented by an index, 1/k [where k was estimated as m2/(s2 ‐ m)], S. avenae was most aggregated at low densities during the early infestation period and tended to be less aggregated as density increased, whereas A. ervi was more likely to be randomly distributed. A common k was detected for neither S. avenae nor A. ervi because the slopes from the linear regression of k on m significantly exceeded 0. Sequential sampling plans, based on numerical (complete) and binomial (presence or absence) counts, for the aphid and parasitoid species were developed using Taylors parameters and those estimated from the linear regression of ln(m) on ln[‐ ln[P0)], respectively. Suggestions are made concerning the use of the sampling plans and the levels of precision that may be attained.

Overwintering Mortality of Urophora affinis and U. quadrifasciata (Diptera: Tephritidae) on Spotted Knapweed: Effects of Larval Competition Versus Exposure to Subzero Temperatures

Abstract The overwintering survival of Urophora affinis Frauenfeld and U. quadrifasciata (Meigen) was examined in a 5-yr (1989–1993), multiple-site field study in western Montana. These two tephritid fly species are introduced biological control agents of spotted knapweed, Centaurea maculosa Lamarck, and diffuse knapweed, Centaurea diffusa Lamarck. Significant differences in overwintering mortality of U. affinis and U. quadrifasciata larvae (whether occurring conspecifically or heterospecifically within the same capitulum) were found among sites. Overwintering mortality of U. affinis and U. quadrifasciata (whether occurring conspecifically or heterospecifically within the same capitulum) showed no relationship with site elevation. Overwintering mortality of U. affinis and U. quadrifasciata showed no relationship with the densities of conspecific or heterospecific galls found within the capitula. Extended periods of extremely cold temperature were found to explain most of the variation in overwintering mortality of both fly species. We conclude that periods of extremely cold temperature is the most important mortality factor affecting overwintering survival in U. affinis and U. quadrifasciata.

Supercooling capacity of Eurasian and North American populations of parasitoids of the Russian wheat aphid, >Diuraphis noxia

Supercooling points were estimated for seven populations of >Aphelinus albipodus, five populations of >Aphelinus asychis, and four populations of >Diaeretiella rapae to assess whether their supercooling points were sufficiently low to provide the potential for overwintering survival in colder temperate climatic areas. Test individuals from all 16 of the parasitoid populations were collected originally from mummies of the Russian wheat aphid, >Diuraphis noxia. Mummies containing parasitoid pupae were maintained for 1 wk under three different temperature conditions (treatments): at room temperature (24.8 ± 0.2 °C), 1 wk at 0 °C, and 1 wk −5 °C, and the supercooling points across treatments, and within and among species were compared. Statistical differences in supercooling points were found among populations of >A. albipodus for each treatment, and for >A. asychis when maintained for 1 wk at room temperature. No differences in supercooling points were found among populations of >D. rapae mummies maintained under the three temperature treatments. The lowest supercooling points obtained for the three parasitoid species maintained at room temperature were the >A. albipodus population from Montana (−31.68 °C), the >A. asychis population from Greece (−32.04 °C), and the >D. rapaepopulation from the Caucasus (−33.12 °C). Preconditioning the parasitoid mummies to cold had no effect on the supercooling points for >A. albipodus, and in some cases unexpectedly increased the supercooling points for >A. asychisand >D. rapae. In comparing the overall mean supercooling points of the three parasitoid species, no differences were found within species (among temperature treatments), nor among species (within temperature treatments). It was concluded that observed differences in supercooling points of only a few degrees Centigrade among parasitoid populations and species would not be expected to cause differences in their overwintering success, especially given the expected variability in temperatures within and among overwintering sites.

Supercooling capacity of Urophora affinis and U. quadrifasciata (Diptera: Tephritidae) on spotted knapweed: comparisons among plants, sites, time of season, and gall densities

Larval supercooling points of Urophora affinis Frauenfeld and U. quadrifasciata (Meigen) were compared among plants, six research sites in western Montana, four fall/winter time periods, and among gall densities. These two tephritid fly species are introduced biological control agents of spotted knapweed, Centaurea maculosa Lamarck, and diffuse knapweed, Centaurea diffusa Lamarck. Few differences in larval supercooling points for U. affinis and U. quadrifasciata were found among plants, and where differences were found, they were not consistent across fall/winter time periods. Significant differences in larval supercooling points were found among sites and across fall/winter time periods. No relationship was found between larval supercooling points and site elevation. Larval supercooling points of both U. affinis and U. quadrifasciata showed no relationship with the density of Urophora galls within spotted knapweed capitula. Mean larval supercooling points of U. affinis were consistently lower than those of U. quadrifasciata across sites and fall/winter time periods. In conclusion, temporal differences in temperature over the fall/winter time periods and microclimatic differences among sites appear to be the most important abiotic factors influencing the supercooling points in U. affinis and U. quadrifasciata.