POTENTIAL WOOD EXCAVATION BY COMMON NET-SPINNING CADDISFLIES IN POOL 8, UPPER 
MISSISSIPPI RIVER

Roger J. Haro1, William B. Richardson2, and Reid M. Northwick1,2
1River Studies Center, University of Wisconsin-La Crosse, La Crosse, WI 54601
2U.S. Geological Survey, Upper Midwest Environmental Sciences Center, La Crosse, 
WI 54602

      Hydropsyche orris and Cheumatopsyche campyla (Trichoptera:  
Hydropsychidae) are ubiquitous and abundant net-spinning caddisflies in the 
upper Mississippi River (UMR).  These larvae are commonly found on hard natural 
(e.g., snags) and artificial (e.g., wing dams) substrates in the UMR where there 
is sufficient flow to support suspension feeding.  In the spring of 2006, 
Hester-Dendy artificial substrates (H-DAS) were placed in 3 main channel areas 
(9 per area) of Pool 8 to assess macroinvertebrate colonization patterns and to 
estimate macroinvertebrate instantaneous growth rates and production.  Each H-
DAS has 14 round Masonite™ plates (75 mm dia.) separated by smaller (25 mm dia.) 
spacers.  We removed a subset of H-DAS every month for analysis from each area.  
In the laboratory we discovered many of the plates covered with curved, oblong 
pits underneath the fix-retreats made by the caddisfly larvae.  Pit areas ranged 
between 3.84 – 44.69 mm2.  Larval body areas ranged between 0.75 – 25.12 mm2 and 
0.38 – 13.49 mm2 for H. orris and C. campyla, respectively.  Because larvae were 
removed from the plates prior to identification, it is not known whether one or 
both of the species were responsible for the excavation.  Larvae excavated a 
mean (±SD) total of 18.7±4.1% of exposed area per plate face.  The mean (±SD) 
volume of Masonite™ excavated per exposed face was 603.4±132.4 mm3; this 
processing represented a removal of 154 cm3/m2.  Why larvae excavated the plates 
remains to be determined.  Potential hypotheses include:  1) larvae excavate 
organic substrates to supplement suspension feeding; 2) larvae excavate organic 
substrates to minimize exposure to drag forces in the current; 3) excavation 
permits the larva to retain a fixed foraging location within a crevice while 
increasing in body size; 4) larvae use excavated material in the construction of 
their fixed retreats; and 5) pits are excavated through larval development in 
preparation for pupation.  Given the historical levels of natural wood 
substrates throughout the UMR, caddisfly excavation may have been and may still 
represent a significant amount of organic processing and seston generation.  
This is the first known example of wood excavating by hydropschid caddisfly 
larvae and the authors plan a series of laboratory and field-based experiments 
in the spring and summer of 2008 to address these hypotheses.

Keywords:  net-spinning caddisflies, Mississippi River, wood excavation, 
artificial substrates