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Patterns of seasonal activity and density of host-seeking western blacklegged ticks, Ixodes pacificus, were investigated in central and southern California. Weekly to monthly drag sampling was undertaken at two sites in Santa Barbara County and one site in Los Angeles County over multiple years. Adult I. pacificusbecame active in the winter (late November) and were rare or absent by late April to early May. Nymphal ticks became active in early to late February, were absent by early May to early June, and were rarely encountered using the drag method throughout their period of peak seasonal activity. Larval ticks became active earlier in the season, or at the same time as nymphs (early to late February) and were absent by early May. These results suggest a highly truncated period of I. pacificusseasonal questing activity, particularly apparent in the juvenile tick stages, in central and southern California relative to observed patterns in Lyme-endemic northwestern California. Notably, the highly truncated period of questing activity of the juvenile stages has important implications for pathogen transmission dynamics in that there exists only a brief window for horizontally transmitted pathogens to be acquired by one tick cohort and subsequently transmitted, through hosts, to the next tick cohort in this system. The broader patterns observed also suggest low human risk of tick-borne disease in central and southern California, and have implications for reduced tick-borne disease risk in the western US more generally under projected climate change.

Autoparasitoids (“heteronomous hyperparasitoids”) are parasitoids that lay female eggs on homopteran hosts and male eggs on juvenile parasitoids of either the same species or another species. Males develop as hyperparasitoids and eventually kill the juvenile parasitoid. We present a series of stage-structured models that investigate the effects of autoparasitism on population dynamics. Autoparasitism causes density-dependent mortality on juvenile parasitoids and therefore has a stabilizing effect. This also leads to an increase in host population abundance. In most cases an autoparasitoid leads to higher host equilibrium densities than a comparable primary parasitoid (except when the primary parasitoid is arrhenotokous (sexual) and the autoparasitoid has a low preference for attacking parasitized hosts or can attack the parasitized host for only a small portion of its development). When male autoparasitoids are followed explicitly in the models, mate limitation reduces the stabilizing effect of autoparasitism and leads to a further increase in host abundance. Coexistence of an autoparasitoid with a nonprimary parasitoid or second autoparasitoid is possible when the level of conspecific autoparasitism is greater than the level of heterospecific autoparasitism. When an autoparasitoid coexists with a primary parasitoid, the resulting host density is always greater than that with only the primary parasitoid. Therefore, autoparasitoids have the potential to disrupt control achieved by primary parasitoids. When two autoparasitoids coexist, the resulting host density is always lower than that attained by either autoparasitoid alone. The effects of autoparasitism are compared with those of other forms of interference competition.

Abstract.:  Continuous-time, age structured, host–parasitoid models exhibit three types of cyclic dynamics: Lotka–Volterra-like consumer-resource cycles, discrete generation cycles, and “delayed feedback cycles” that occur if the gain to the parasitoid population (defined by the number of new female parasitoid offspring produced per host attacked) increases with the age of the host attacked. The delayed feedback comes about in the following way: an increase in the instantaneous density of searching female parasitoids increases the mortality rate on younger hosts, which reduces the density of future older and more productive hosts, and hence reduces the future per head recruitment rate of searching female parasitoids. Delayed feedback cycles have previously been found in studies that assume a step-function for the gain function. Here, we formulate a general host–parasitoid model with an arbitrary gain function, and show that stable, delayed feedback cycles are a general phenomenon, occurring with a wide range of gain functions, and strongest when the gain is an accelerating function of host age. We show by examples that locally stable, delayed feedback cycles commonly occur with parameter values that also yield a single, locally stable equilibrium, and hence their occurrence depends on initial conditions. A simplified model reveals that the mechanism responsible for the delayed feedback cycles in our host–parasitoid models is similar to that producing cycles and initial-condition-dependent dynamics in a single species model with age-dependent cannibalism.

Recent outbreaks of chytridiomycosis, the disease of amphibians caused by the fungal pathogen Batrachochytrium dendrobatidis(Bd), have contributed to population declines of numerous amphibian species worldwide. The devastating impacts of this disease have led researchers to attempt drastic conservation measures to prevent further extinctions and loss of biodiversity. The conservation measures can be labour-intensive or expensive, and in many cases have been unsuccessful. We developed a mathematical model of Bd outbreaks that includes the effects of demographic stochasticity and within-host fungal load dynamics. We investigated the impacts of one-time treatment conservation strategies during the disease outbreak that occurs following the initial arrival of Bd into a previously uninfected frog population. We found that for all versions of the model, for a large fraction of parameter space, none of the one-time treatment strategies are effective at preventing disease-induced extinction of the amphibian population. Of the strategies considered, treating frogs with antifungal agents to reduce their fungal load had the greatest likelihood of a beneficial outcome and the lowest risk of decreasing the persistence of the frog population, suggesting that this disease mitigation strategy should be prioritized over disinfecting the environment or reducing host density.

Documenting trophic links in a food web has traditionally required complex exclusion experiments coupled with extraordinarily labor-intensive direct observations of predator foraging. Newer techniques such as stable isotope analysis (SIA) may facilitate relatively quick and accurate assessments of consumer feeding behavior. Ratios of N and C isotopes are thought to be useful for determining species’ trophic position (e.g., 1° consumer, 2° consumer, or omnivore) and their original carbon source (e.g., C3 or C4 plants; terrestrial or marine nutrients). Thus far, however, applications of stable isotopes to terrestrial arthropod food webs have suggested that high taxon-specific variation may undermine the effectiveness of this method. We applied stable isotope analysis to a pear orchard food web, in which biological control of a dominant pest, pear psylla (Cacopsylla pyricola), involves primarily generalist arthropod predators with a high frequency of omnivory. We found multiple sources of isotopic variation in this food web, including differences among plant tissues; time, stage, and taxon-specific differences among herbivores (despite similar feeding modes); and high taxon-specific variation among predators (with no clear evidence of omnivory). Collectively, these multiple sources of isotopic variation blur our view of the structure of this food web. Idiosyncrasies in consumer trophic shifts make ad hoc application of SIA to even moderately complex food webs intractable. SIA may not be a generally applicable “quick and dirty” method for delineating terrestrial food web structure—not without calibration of specific consumer food trophic shifts.

The cecidomyiid midge, Rhopalomyia californica Felt, forms galls on the perennial evergreen shrub Baccharis pilularis De Candolle, which is found in coastal regions throughout central California. R. californica, however, is not found in the more southern part of its host's range and is essentially absent south of Santa Barabara. Reasons for this restricted distribution were examined by field experiments at 2 sites: Santa Barbara and Big Creek, which is located 290 km to the North, well within the area where galls are widespread and abundant. No differences in plant suitability for oviposition were detected between the 2 sites when the midges were protected from predators, but midges responded differently to the plants, each midge producing more galls at Santa Barbara but each gall containing, on average, fewer larval chambers. Mortality caused by predators (or other sources of egg or early-larval mortality) was significantly higher at Santa Barbara than at Big Creek, reducing both the number of galls produced per female and also the average number of chambers per gall that was produced. Mortality caused by parasitoids was also significantly higher at Santa Barbara than at Big Creek. There was no evidence for spatial density dependence at either site. When combined with realistic parasitism rates and likely fecundities, such mortality rates result in a negative growth rate over the period studied for galls at Santa Barbara but a positive growth rate at Big Creek, providing a possible reason for the restricted distribution of the midge.