Allen W. Shostak

Parasitology Meets Ecology on Its Own Terms: Margolis et al. Revisited.

We consider 27 population and community terms used frequently by parasitologists when describing the ecology of parasites. We provide suggestions for various terms in an attempt to foster consistent use and to make terms used in parasite ecology easier to interpret for those who study free-living organisms. We suggest strongly that authors, whether they agree or disagree with us, provide complete and unambiguous definitions for all parameters of their studies.

MOISTURE AND TEMPERATURE EFFECTS ON SURVIVAL AND INFECTIVITY OF FIRST-STAGE LARVAE OF PARELAPHOSTRONGYLUS ODOCOILEI AND P. TENUIS (NEMATODA: METASTRONGYLOIDEA)

The survival of first-stage larvae (L1) of Parelaphostrongylus odocoilei and P. tenuis (Nematoda: Metastrongyloidea) and their infectivity to the snail Triodopsis multilineata were determined experimentally in a variety of temperature and moisture conditions. Survival of larvae of P. odocoilei increased with decreasing temperature. Survival of larvae in water was similar to survival in air at 17 and 45% RH; survival of larvae in air at 75, 85, and 95% RH was considerably lower at corresponding temperatures. The infectivity of larvae of P. odocoilei that survived desiccation was greatly reduced. Repeated freezing or repeated desiccation resulted in reduced survival of P. odocoilei and P. tenuis. Larvae of Parelaphostrongylus odocoilei from mule deer of Jasper National Park, Canada, were better able to resist the effects of freezing, but less able to resist the effects of desiccation, than were larvae of P. tenuis from white-tailed deer of Pennsylvania, USA.

Climbing Simulated Vegetation to Heights of Ungulate Hosts by Larvae of Dermacentor albipictus (Acari: Ixodidae)

Abstract Larvae of winter ticks, Dermacentor albipictus (Packard), ascend vegetation in autumn and form clumps that attach to passing ungulate hosts. We tested the hypothesis that vegetation height determines the height of clumps. During the vegetation-to-ungulate transmission period (early September to mid-November), larvae were released at the base of simulated vegetation (nylon rods 245 cm tall) in outdoor and laboratory trials and in the absence of host cues. Rod height exceeded the height of the tallest ungulate host, which is the moose, Alces alces (L.). Most larvae stopped climbing and formed clumps 50–190 cm above ground, which coincided with torso heights of moose; elk, Cervus elaphus L.; and deer, Odocoileus spp. Rafinesque. More clumps formed in outdoor trials than in laboratory trials and clump heights tended to increase over the course of the experiment, but clump number, size, and height did not correlate with weather conditions. Winter tick larvae appear to determine their height above ground in the absence of external cues, but this mechanism may be modified by external conditions.

Effect of age of the intermediate host Tribolium confusum (Coleoptera) on infection by Hymenolepis diminuta (Cestoda).

A cross-sectional study of 27 cohorts of Tribolium confusum aged 2–78 wk was done to examine effects of host age on exposure to eggs of Hymenolepis diminuta under standardized conditions. Pre-exposure, fasting, and postexposure mortality were low, sex ratio was equal, and fecundity of hosts was high during the first 30 wk, followed by increasing mortality and male bias of the sex ratio, and declining fecundity, in older beetles. These changes in the host were not associated with pronounced changes in infection results. Prevalence of infection was higher in females than males, but was unaffected by age in both sexes. Intensity of infection was similar between sexes in beetles up to 30 wk old, and thereafter declined in females, but not in males. Age-related changes in hosts were gradual, but unexpected levels of short-term variation in infection results suggest that some undetermined proximate factors may override general host age effects on the infection process.

Sublethal Exposure to Diatomaceous Earth Increases Net Fecundity of Flour Beetles ( Tribolium confusum ) by Inhibiting Egg Cannibalism

Population regulation results from an interplay of numerous intrinsic and external factors, and for many insects cannibalism is such a factor. This study confirms a previously-reported observation that sublethal exposure to the fossilized remains of diatoms (i.e. diatomaceous earth) increases net fecundity (eggs produced minus eggs destroyed/day) of flour beetles, Tribolium confusum. The aim was to experimentally test two non-mutually-exclusive ecological mechanisms potentially responsible for the increased net fecundity: higher egg production and lower egg cannibalism. Adult T. confusum were maintained at low or high density in medium containing sublethal (0–4%) diatomaceous earth. Net fecundity increased up to 2.1× control values during diatomaceous earth exposure, and returned to control levels following removal from diatomaceous earth. Cannibalism assays on adults showed that diatomaceous earth reduced the number of eggs produced to 0.7× control values at low density and to 0.8× controls at high density, and also reduced egg cannibalism rates of adults to as little as 0.4× control values, but at high density only. Diatomaceous earth also reduced cannibalism by larvae on eggs to 0.3× control values. So, while the presence of diatomaceous earth reduced egg production, net fecundity increased as a result of strong suppression of the normal egg cannibalism by adults and larvae that occurs at high beetle density. Undisturbed cultures containing sublethal diatomaceous earth concentrations reached higher population densities than diatomaceous earth-free controls. Cohort studies on survival from egg to adult indicated that this population increase was due largely to decreased egg cannibalism by adult females. This is the first report of inhibition of egg cannibalism by diatomaceous earth on larval or adult insects. The ability of diatomaceous earth to alter cannibalism behavior without causing mortality makes it an ideal investigative tool for cannibalism studies.

SHAPE VARIATION OF CYSTICERCOIDS OF HYMENOLEPIS DIMINUTA (CYCLOPHYLLIDEA) FROM FED, PARTIALLY FED, AND FASTED TRIBOLIUM CONFUSUM (COLEOPTERA)

Quantitative studies of a crowding effect on cysticercoids of Hymenolepis diminuta in the intermediate host are few and limited in scope. In this study, we developed a technique to rapidly collect morphological information on large numbers of parasites, and verified the utility of geometric models for simple and accurate estimation of cysticercoid size for quantitative studies. These models were tested using measurements from 4,899 H. diminuta obtained from 666 Tribolium confusum exposed 1–4 wk previously. Length, width, and depth of the body and cercomer (when present) can be used in conjunction with these models to provide the most accurate estimation of parasite size. However, parasite body length alone can be used, with adjustment for effects of host diet and infection intensity, to predict the remaining measurements in incomplete specimens. Parasites that developed in higher intensity infections, or in hosts with reduced food intake, were narrower and had a proportionately shorter cercomer. Host age, sex, and mating status, and parasite age also had statistically significant, but small-magnitude, effects on parasite shape.

Sequential and Concurrent Exposure of Flour Beetles (Tribolium confusum) to Tapeworms (Hymenolepis diminuta) and Pesticide (Diatomaceous Earth)

Abstract: The response of Tribolium confusum to sublethal levels of 2 environmental stressors was studied, i.e., parasitic infection represented by the cestode Hymenolepis diminuta, and a physical stressor represented by the natural pesticide diatomaceous earth (DE). These were applied sequentially (DE, then infection) to detect indirect or carryover effects of DE, and concurrently (DE applied immediately after exposure to parasites and DE presence maintained throughout the infection) to detect direct effects of DE. DE alone, but not parasitism alone, produced significant host mortality, and concurrent treatment with DE and parasitism did not increase mortality over DE alone. Parasite abundance was significantly higher following sequential, but not concurrent, DE exposure. Parasite abundance in mated hosts was significantly higher than in virgin hosts. Parasitic infection resulted in significantly fewer eggs retained in the oviduct of beetles, but there was no difference in the number of eggs that accumulated in the culture medium and no difference in the surface-seeking behavior of beetles. Mating status of beetles in all treatments, and DE exposure in concurrent treatments significantly increased their surface-seeking behavior. Concurrent exposure to DE also resulted in a 4- to 6-fold increase in host egg numbers that accumulated in the culture medium. Although DE exposure increased parasite numbers in the beetles, these 2 stressors otherwise appeared to act independently.

Response of Flour Beetles to Multiple Stressors of Parasitic (Hymenolepis diminuta), Environmental (Diatomaceous Earth), and Host (Reproduction) Origin.

Abstract:  Organisms face a multitude of potential stressors, and the way these stressors interact can provide insights into underlying biological processes. This study examined the flour beetle Tribolium confusum and its survival, net fecundity, and surface-seeking behavior in response to combinations of stressors from 3 categories. Infection by the cestode Hymenolepis diminuta provided a stress of parasitic origin. Exposure to diatomaceous earth (DE) provided a stress of environmental origin. Use of virgin and mated beetles evaluated reproduction as a stress of host origin. Single and multiple exposure of beetles to parasite eggs achieved a maximum mean abundance of 21 parasites/beetle and a maximum intensity of 90 parasites in an individual beetle. DE reduced initial parasite establishment, but did not directly affect survival of parasites after their establishment in the host. A rehydration technique was used to recover parasites from dead beetles, enabling this to be the first study to correlate H. diminuta intensity at time of death directly to mortality of T. confusum. A dichotomous intensity–mortality relationship was observed in 8% DE, whereby lightly infected (<20 parasites) hosts were killed by DE in an intensity-independent manner, but more heavily infected hosts were killed in an intensity-dependent manner. Host mating status did not affect host survival, but there were interactions among mating status, parasitism, and DE on net fecundity and surface-seeking behavior. However, these effects were minor compared to the host mortality that occurred when parasite abundance and DE concentration were both high. The aggregated distribution of T. confusum in beetles, the difficulty of achieving high mean abundances, and an apparent need for the stressors to have strong effects individually if they are to have enhanced effects when in combination, suggests that exposure to multiple stressors would seriously impact only a small proportion of the host population.