Abstract
Statement of Problem
Data regarding the life history of Edward's water penny beetle (Eubrianax edwardsil) are generally not known and are largely absent from the literature. The purpose of this study was to investigate basic life history aspects regarding Eubrianax edwardsii's four life cycle stages: the egg, larva, pupa and adult.
Sources of Data
Field investigations associated with this study were conducted at Weber Creek, located in western El Dorado County, California.
Investigations of the egg stage involved general descriptions of egg morphology and distribution. Hatching times were determined for eggs maintained in the laboratory, and the egg eclosion process described.
Investigations of the larval stage involved size group analyses of 100 larvae per month for one year to separate larval instars. Digestive tract contents of several larvae were analyzed to obtain preliminary data regarding diet. Various physical stream parameters were investigated to understand larval distributions and microhabitat choice. Investigations also assessed whether larval E. edwardsii undergo drift dispersal.
Investigations of the pupae characterized the general location of pupation and the distribution of pupae in the field. The duration of the pupation process (both in the field and the laboratory), and potential parasitism of the pupal stage were also investigated.
Analyses of the adult stage involved sex-ratio analyses, assessments of adult responses to light (phototaxis), pheromone production, and adult male and female life spans. Data regarding mating behavior and oviposition were documented as well.
Conclusions Reached
The eggs of E. edwardsii are yellow in color and measure between 250 and 300 microns in diameter. They are deposited in dense masses of approximately 500 eggs on the undersides of partially submerged rocks, usually during June and July. Hatching takes between 14 to 23 days following deposition (x= 17. 7 days). Newly hatched larvae are 0.75-0.85 mm long and lightly colored or translucent.
Annual summaries of larval sizes suggest the presence of at least five instars. Sexual dimorphism (females being larger than males) is evident during the third through fifth instar. Larvae overwinter at least once and possibly twice during their life span. Larvae feed primarily upon diatoms and detritus. The presence of larvae is positively correlated with the size (and possibly texture) of rocks with which they are associated. Larvae are generally found closer to stream edges where water velocities are usually slow to moderate. Larvae of E. edwardsii are highly thigmotactic and do not exhibit behavioral drift dispersal.
Larvae of E. edwardsii were found to pupate above water (typically during May and June), attached to various substrates within 1.0 meter of the stream edge. Female pupal cases (x=8.5mm) are generally larger than male pupal cases (x=6.2mm). Three pupae in the field completed pupation in between 12 to 15 days. Laboratory attempts to document the entire pupation process were unsuccessful due to fungal ( Cordyceps sp) infestations. Numerous pupal mortalities due to fungi were also evident in the field. A diapriid wasp (Trichopria sp.) was found assocciated with a dead pupa on one occasion.
Investigations of adult sex ratios revealed a prevalence of male E. edwardsii; observed male:female ratios ranged between 1.2: 1 and 3.8: I. Black-light sampling efforts failed to attract any adult E. edwardsii. Efforts to attract adults to pheromone traps yielded inconclusive results, though electron-micrographs of male antennae revealed the presence of potential chemosensory organs which suggest the use of pheromones in mate location. Adult males collected beneath pupal coverings lived in the laboratory for_between six and 17 days (x= I 0.1 days); females lived for between five and seven days (x=6.0 days) Field collected males lived for between two and seven days (x=5. l days); females also lived for two to seven days (x=4.3 days). Male and female E. edwardsii were found to mate readily
both in the field and under laboratory conditions. Females typically remain sedentary on top of partially submerged rocks until encountered by males which actively search for females. Copulation can continue for 1.5 hours. Following copulation females typically crawl into the water underneath the rock on which mating had taken place and begin to deposit an egg mass. Oviposition can continue for 125 minutes. Several females can oviposit on any given rock.