Egg Mass Abundance Research Articles

Double-observer approach to estimating egg mass abundance of pool-breeding amphibians

Interest in seasonally flooded pools, and the status of associated amphibian populations, has initiated programs in the northeastern United States to document and monitor these habitats. Counting egg masses is an effective way to determine the population size of pool-breeding amphibians, such as wood frogs (Rana sylvatica) and spotted salamanders (Ambystoma maculatum). However, bias is associated with counts if egg masses are missed. Counts unadjusted for the proportion missed (i.e., without adjustment for detection probability) could lead to false assessments of population trends. We used a dependent double-observer method in 2002–2003 to estimate numbers of wood frog and spotted salamander egg masses at seasonal forest pools in 13 National Wildlife Refuges, 1 National Park, 1 National Seashore, and 1 State Park in the northeastern United States. We calculated detection probabilities for egg masses and examined whether detection probabilities varied by species, observers, pools, and in relation to pool characteristics (pool area, pool maximum depth, within-pool vegetation). For the 2 years, model selection indicated that no consistent set of variables explained the variation in data sets from individual Refuges and Parks. Because our results indicated that egg mass detection probabilities vary spatially and temporally, we conclude that it is essential to use estimation procedures, such as double-observer methods with egg mass surveys, to determine population sizes and trends of these species.

Patterns of composition and abundance in macroinvertebrate egg masses from temperate Australian streams

Quantitative samples of lotic invertebrate egg masses were collected on two occasions from three riffles along two rivers within the Acheron River catchment, south-eastern Australia. Sampling was stratified to compare the abundance and composition of egg masses on submerged rocks with rocks that protruded above the water surface within each of three substrate size classes. The egg mass fauna of both rivers was found to be both species rich and abundant. Egg masses belonging to Diptera and Trichoptera represented the majority of material collected, with approximately 70% of all taxa common to both rivers. In particular, egg masses of Chironomidae, Hydrobiosidae, Hydroptilidae and Hydropsychidae dominated most samples numerically. Descriptions of the egg masses belonging to 17 taxa are provided, as well as preliminary estimates of hatching times and observations of egg-laying and post-hatching behaviour for some species. Large rocks (> 30 cm maximum diameter) that protruded above the water surface consistently yielded the highest number of egg masses for all common taxa. For common taxa, between 74% and 100% of all egg masses were found on large emergent rocks. Of the common taxa, little variation in egg mass abundance was detected between sampling times or between sites within each river. Exceptions were hydroptilids and chironomids, which were completely absent from some time/site combinations, suggesting a degree of synchrony in egg-laying behaviour. For all common taxa, the greatest source of variation in egg mass numbers appeared at the level of individual rocks, where a high degree of spatial aggregation was apparent (as described by Lloyd's index of patchiness).

The distribution of aquatic invertebrate egg masses in relation to physical characteristics of oviposition sites at two Victorian upland streams

Summary1. Various physical variables were measured at rocks potentially used by lotic macroinvertebrates as oviposition sites at multiple locations on two occasions along the Acheron and Little Rivers, south‐eastern Australia. The associations between these parameters and the presence/absence and abundance of aquatic insect egg masses were explored as well as the small‐scale distribution of egg masses on individual rocks.2. Physical features that characterise oviposition sites of 17 different aquatic taxa are presented. No obvious differences in patterns of oviposition site selectivity were apparent between multiple sampling times or locations. For some common taxa, multivariate analyses revealed that measurements of rock size and local current speed were positively related to the likelihood of an egg mass being present. However there were no consistent relationships between the abundance of egg masses and either of these variables.3. The small‐scale distribution of egg masses on individual rocks revealed patterns in relation to surfaces that clearly differed as oviposition habitat, such as the underside, upstream and downstream surfaces. Presumably, these patterns are the result of small‐scale variation in flow conditions that characterise these particular rock surfaces.4. The results of this study indicate that the oviposition strategies of a number of aquatic taxa may be reasonably predictable based on measurable physical parameters. These findings have important implications for future studies wanting to incorporate the adult and egg life history stages of lotic insects into studies of population dynamics.

Experimental evidence for physical cues involved in oviposition site selection of lotic hydrobiosid caddis flies.

Cues involved in determining the distribution of invertebrate propagules within a stream landscape contribute greatly to our knowledge of the supply and arrangement of new recruits and thus an improved understanding of factors that might ultimately affect population parameters. Previous observations indicated that both current velocity and rock size were important determinants of the egg mass distribution of certain hydrobiosid caddis flies that lay their eggs in single masses beneath emergent rocks. These observations were tested experimentally in a temperate, upland Australian stream. Manipulations of current speed confirmed that females of Ulmerochorema sp. and the Taschorema complex deposited more eggs on rocks in elevated current speeds (>0.40 ms(-1)) whereas Apsilochorema sp. deposited more eggs on rocks in slow currents (<0.30 ms(-1)). This latter result did not coincide with previous observational data. The anomaly between observational and experimental data, however, was reconciled by the outcome of a further experiment that tested the influence of the emergent or 'landing pad' size of rocks as the abundance of Apsilochorema egg masses increased with landing pad size independent of the prevailing flow conditions. Landing pad size did not influence the abundance of egg masses of Taschorema or Ulmerochorema. Patterns of female visits to rocks indicated that taxa might distinguish between favoured egg-laying sites prior to landing on rocks. Large aggregations of adult male and female Ulmerochorema collected from rocks favoured for oviposition provide indirect evidence for mating induced swarming behaviour associated with oviposition site selection. This study provides a framework for more sophisticated questions relating to the influence of oviposition site selection on structuring populations of lotic macroinvertebrates.

Predation on Colorado Potato Beetle Eggs by Generalist Predators in Research and Commercial Potato Plantings

Abstract Field studies quantified predation on Colorado potato beetle [ Leptinotarsa decemlineata (Say)] eggs and determined the relationship between predation and egg mass abundance in research and commercial potato plantings in eastern North Carolina. Predator exclusion experiments were conducted weekly in research plantings. In addition, egg mass density and predation on egg masses were monitored throughout the season in research plots and commercial potato fields. Predation was an important source of mortality for Colorado potato beetle eggs. Survivorship of eggs exposed to predators was consistently, significantly lower than survivorship of eggs protected from predation. Averaged over 2 years, the mean survivorship of eggs protected from predation was 69%, compared with 26% survivorship of eggs exposed to predation. Regression analysis failed to detect any relationship between egg mortality due to predation and egg abundance. These results imply that efforts to reduce Colorado potato beetle populations selectively will not be offset by an according decline in abundance of natural enemies and therefore should be fully compatible with naturally occurring biological control.

Recruitment, maturation, and spawning of Loligo forbesi Steenstrup (Cephalopoda: Loliginidae) in Irish waters

A number of reproductive indices were compared with a subjective maturity scale for assessment of Loligo forbesi maturity. The ratio between nidamental gland length and mantle length corresponded well with female maturation, as did the ovary mass-soma mass and nidamental gland mass-soma mass ratios. For males, the ratio between spermatophoric complex mass and somatic mass was found to be the most suitable for maturity assessment. The timing of recruitment and maturation of L. forbesi in Irish waters was described from the size and maturity of squid in commercial landings in the south of Ireland during the years 1991–1993. Immature squid first appeared in commercial catches in July and August, and this represented the main period of recruitment. A second period of recruitment was apparent in December 1991, but was not identified in the 1992–1993 season. Mature females were present in the commercially exploited population between November and April, with a small number also found in the summer. The abundance of egg masses was used to indicate timing of spawning. Egg masses recovered from the Cork coast indicated that peak spawning occurred during the winter months, but continued on a small scale for much of the year.

Survival of spotted salamander eggs in temporary woodland ponds of coastal Maryland.

Temporary ponds on the Atlantic Coastal Plain in maryland were characterized according to water chemistry, rain input, phytoplankton, zooplankton and use by the spotted salamander Ambystoma maculatum during March-October 1983-1984. Neither the number of egg masses per unit of pond surface (abundance) nor the survival of spotted salamander embryos was significantly correlated (P>0.05) with pond pH. Rainfall during May-July significantly increased the hydrogen ion concentration of 5 of 11 ponds evaluated for the impact of rainfall during the previous 48h and the previous week. Survival of egg masses transferred among eight ponds with pH3.66-4.45 and one pond with pH5.18 was significantly reduced (P<or=0.05) only at pH 3.66. Embryonic survival was negatively correlated (P<or=0.05) with the concentration of aluminium in the pond water. The abundance of egg masses was positively correlated (P<or=0.05) with water temperature and magnesium concentration, and total chlorophyll during the larval period. Yearly variability of pond characteristics (e.g. water chemistry, pond longevity) and amphibian reproduction make it difficult to determine the effects of acidic deposition on the spotted salamander. At the present time, pond longevity, water temperature and possibly, oxygen content, seem more important to spotted salamander reproduction than chemical changes caused by annual acidic deposition.

Spring pond water chemistry and the reproduction of the wood frog, Rana sylvatica

To determine the impact of snowmelt water quality on egg mass abundance and embryo survival of the wood frog (Rana sylvatica) in Québec, 15 breeding ponds, with an early spring pH range between 3.4 and 6.7, were surveyed. Acidity and total organic carbon were correlated with the density of egg masses. Hatching success was reduced (and mould increased) in low pH ponds.

Grazing by the gastropod, Lacuna vincta, in the lower intertidal area at Musquash Head, New Brunswick, Canada

Lacuna vincta appeared at Musquash Head, New Brunswick, in large numbers from June to August 1981 principally on lower midlittoral and infralittoral fringe Fucus edentatus. The population reached a mean maximum of 280/m2 in June; by mid September the animals had disappeared. The L. vincta grazed heavily and deposited egg masses on Fucus edentatus but little on other species. All fine and filamentous and encrusting algal species and rock were avoided for egg deposition. Egg mass abundance peaked with the population in June with a mean of 180/m2. Mean egg production in June was 83000/m2.Both snail and egg mass distributions were correlated with the percentage cover of F. edentatus, reaching a maximum at 40–75 cm above mean low tide level.The snails ranged in length from 2–7 mm, consisting of three cohorts from different larval settlements. Growth was rapid in July averaging 0·6 mm/month slowing later.The food plant Fucus edentatus showed a heavy impact of the herbivore. Weight reductions attributable to grazing rose from 56 % in June to 79 % in August and grazed plants tended to thicken rather than elongate with growth. Grazing removed about 79 % of the net production of F. edentatus which averaged 61 g dry wt/m2/day. The impact of the L. vincta population on F. edentatus seems extraordinarily severe.

SPATIAL DISTRIBUTION OF ADULTS OFOOENCYRTUS KUVANAE(HYMENOPTERA: ENCYRTIDAE), AN EGG PARASITE OFLYMANTRIA DISPAR(LEPIDOPTERA: LYMANTRIIDAE)

AbstractThe spatial distribution of adultOoencyrtus kuvanae(Howard) (Hymenoptera: Encyrtidae) was examined in central Pennsylvania during 1978–1980. Data were gathered over a wide range of gypsy moth (Lymantria dispar(L.) (Lepidoptera: Lymantriidae)) densities and from all phases of population growth. There was a linear relationship between the log variance and log mean, fitting Taylor's Power Law with a power of 1.27. The degree of aggregation inO. kuvanaepopulations, as measured by the variance-to-mean ratio, was correlated positively with the abundance of the parasite, and was independent of host density. Aggregation was believed to result from the habit of the female parasite ovipositing repeatedly on the same egg mass, and the resultant progeny emerging synchronously. Behavior and density ofO. kuvanaepopulations were found to depend upon the gypsy moth population condition, i.e., low, rising, high stable, outbreak, or collapsed. The frequency distribution ofO. kuvanaepopulations fit the Poisson at densities less than 0.2 parasite adult per egg mass, a condition which occurred early and late (July, November) in the season, and the negative binomial above this density; a few distributions fit the log normal. The data were also analyzed using analyses of variance (multivariate and univariate), correlation, and regression techniques. All main effects, i.e., study area, host egg mass volume, egg mass height from the ground (within the 0–2 m sampling universe), aspect of the egg mass on the tree, and tree species, were important in explaining the variance in parasite abundance; study area differences were the most important. The effects of aspect and tree species were explained on the basis of small sample sizes, and not studied further. There was a positive correlation between host egg mass volume (= size) and parasite abundance. There was also a positive correlation between the height of the egg mass and parasite abundance during the summer; however, this relationship became negative by late fall. This seasonal change in vertical distribution was attributed to the tendency ofO. kuvanaeto be closer to the ground where they overwinter. Variation in adult abundance was generally greater among gypsy moth egg masses on different trees than among egg masses on the same tree, except at low parasite densities. This was attributed to the parasites searching for egg masses on one tree before dispersing to another.

Branch “Tip” Sampling for Determining Abundance of Spruce Bud worm Egg Masses

Counting of the spruce bud worm ( Choristoneura fumiferana (Clem.)) on balsam fir branches has always been a tedious task. A technique is presented which modifies previously developed methods by reducing the size of the sample without greatly decreasing its accuracy. The technique consists essentially of separating the tips of the shoots from the rest of the branch. The number of egg masses on the tips must be increased by only 16% to obtain the total number on the branch. Time saved by this technique ranges from 25 to 40%, depending upon the size of the branch, over previous techniques. Branches of any size and from any of the crown are within the scope of this technique. The method is not feasible on branches with severe or complete defoliation.

Relation between Egg Mass Abundance and Fall Populations of First-Generation Corn Borer and Justification for Insecticidal Control in Field Corn1

Relation between Egg Mass Abundance and Fall Populations of First-Generation Corn Borer and Justification for Insecticidal Control in Field Corn1