Traditional methods of collecting, sorting, and identifying benthic macroinvertebrate samples are useful for stream biomonitoring and ecological studies, however, these methods are time consuming, expensive, and require taxonomic expertise. Estimating larval densities through collection of post-emergent exuvia can be a practical and time efficient alternative. We evaluated the use of multiple pass depletion techniques of the post-emergent exuvia of Pteronarcys californica to estimate larval densities at ten sites in three Colorado rivers. Exuvia density was highly correlated with both final-instar larval density (R2 = 0.90) and total larval density (R2 = 0.88) and the multiple pass removal technique performed well. Exuvia surveys found P. californica at three low density sites where benthic sampling failed to detect it. At moderate and high density sites the exuvia surveys always produced lower density estimates than benthic surveys. Multiple pass depletion estimates of exuvia proved to be an accurate and efficient technique at estimating larval densities and provided an effective alternative for traditional benthic sampling when objectives are detecting and monitoring P. californica, especially at low density sites.

Three benthic subsamples were taken at each site between 15-18 April 2010 from the Colorado and Fraser Rivers and 10 May 2010 from the Gunnison River, approximately one month prior to the typical adult emergence of P. californica. All sites were located in riffle areas dominated by cobble substrates interspersed with boulders and gravel. A modified Surber sampler with a 0.25 m² sampling frame (55.0 cm x 45.5 cm) and 150 μm mesh net was used. Cobbles larger than 10 cm in diameter were individually scrubbed with a brush, invertebrates washed into the net, and then the cobbles removed from the sampling frame. Remaining substrate within the frame was disturbed to a depth of 10 cm to dislodge invertebrates into the net. Contents were preserved with 80% ethanol in 2 L plastic jars. In the lab, all P. californica larvae were sorted, sexed [21, 38], and measured for total length (TL) from the anterior tip of head to the posterior tip of the epiproct to the nearest millimeter under a dissecting microscope with a calibrated ocular micrometer. Length frequency histograms for male and female larvae were separately constructed based on TL to separate annual year classes. Body size measurements can be used to separate cohorts of this merovoltine species due to the long lifespan, fast growth, and large size. We based our approach on separating annual cohorts on Townsend and Pritchard except we constructed length frequency histograms from total length measurements rather than head-capsule width and wing-pad length. The largest size cohort of both males and females was considered to be mature larvae, the final instar larvae expected to emerge in that year. Densities of mature larvae and densities of all larvae were calculated and used in separate analyses for correlation with exuvia densities. Exuvia sampling began with the onset of P. californica adult emergence on the Colorado River at site nine on 2 June 2010 and proceeded upstream to end at site one on the Fraser River on 21 June 2010; sampling at site 10 on the Gunnison River lasted from 16-23 June 2010. Each site was sampled beginning on the day when the first exuvia was found or winged adults were observed and continued daily until exuvia were no longer found. Data collection was performed by searching for exuvia within a maximum width of 10 m of the bank along two 30.5 m transects on one side of the river. Exuvia sampling sites were on the stream banks directly adjacent to benthic sampling sites. Searching at each individual site extended from the water’s edge laterally until no more exuvia were found, the search distance varying by site, generally depending on the complexity of the shoreline and thickness of riparian vegetation. The average width searched was measured at each site. Collections at a site were conducted by two to four people completing two or three removal passes with identical effort and personnel on each pass. Specimens were taken only when attached to dry riparian and emergent substrates; none were taken from the water to avoid counting ones that possibly drifted into the site. Exuvia were enumerated using hand held counters, stored in sealable bags, and removed from the search area.