Life Cycle, Growth, Survival, and Production of Macronychus glabratus (Coleoptera: Elmidae) in Northwest Arkansas and Southeast Texas Streams

The goal of this study was to better understand the ecology of Macronychus glabratus, a xylophagous aquatic beetle that plays an important role in the conditioning and breakdown of submerged woody debris. In the Illinois and White Rivers of northwest Arkansas, and Harmon Creek in southeast Texas, M. glabratus produced six larval instars. Primary recruitment in Arkansas was during March and April, but first instar larvae were also collected during February and May. Primary recruitment in Texas was during February and March, but first instar larvae were also collected during January and April. Adults made up -10-20% of the population throughout the year in both Arkansas and Texas, indicating that adults are probably long lived. In the laboratory, sixth-instar larvae pupated above the water line in a hollowed oval space beneath bark or in rotted wood. Emergence began -18 days later. After emergence adults crawled back the water. In the laboratory, adults oviposited and first-instar larvae began to hatch after 22 days. Production of M. glabratus in Arkansas was 1.65 g/m2/yr, standing stock biomass was 0.48 g/m2, and the P/B ratio was 3.46. In Texas, production was 0.51 g/m2/yr, standing stock biomass was 0.19 g/m2, and the P/B ratio was 2.63. Addtional key words: beetle, xylophagous insects, secondary production Macronychus glabratus SAY 1825 is a xylophagous (LeSage & Harper 1976; Gage 1983) aquatic coleopteran, collected in greatest densities from submerged wood debris in streams (Davis & Finni 1974; Gage 1983; Benke et al. 1984; Smock et al. 1985; Phillips 1995). The range of M. glabratus in the United States extends from Maine to Florida in the east, and the western range extends from eastern Texas northward to North Dakota (Brown 1972, 1975). M. glabratus is a member of the family Elmidae and subfamily Elminae, which are the most completely aquatic of all beetles (Brown 1987). Both the larvae and adults are aquatic and occupy the same macroand microhabitats (Phillips 1995). The preferred habitat for both adults and larvae is well-conditioned wood with many interstitial spaces, or wood that is just beginning to decay and has loose bark. Wood with rough bark and wood with greater volumes of biofilm contained greater densities of M. glabratus (Phillips 1995). In a review of the life histories of members of the family Elmidae, Brown (1987) found that elmids had short incubation times of 5 to 15 days, followed by larval periods lasting 6 to 36 months, with 5 to 8 ina E-mail: bio-ecp@shsu.edu stars. Development is generally faster at higher temperatures, and smaller species develop faster and have fewer instars than larger species (Brown 1987). Another xylophagous elmid beetle, Lara avara, showed an extended larval development period, with the larvae completing instars 1-3 in one year and instars 4-7 taking 3 to 5 years to complete (Steedman & Anderson 1985). White (1978) found that Stenelmis sexlineata had 7 larval instars, completed in -150 days in the laboratory. Natural populations had a similar life cycle, but the larval portion lasted considerably longer. Moubayed (1983) found that Grouvellinus coyei had 8 larval instars, and a life cycle of more than one year. The only life-history data for M. glabratus are from studies done in Quebec by LeSage & Harper (1976) and in South Carolina by Smock (1988). LeSage & Harper (1976) found that M. glabratus was semivoltine in Quebec; the larval stage had 6 instars and lasted two years, and the adults overwintered another year before mating. Smock (1988) found that M. glabratus was univoltine, and that the main recruitment period was June-July; no information was provided about the number of larval instars or the secondary production. Most species of the subfamily Elminae pupate above water, and most recently emerged adults disperse by taking flight (Seagle 1980). This is not true This content downloaded from 207.46.13.164 on Sun, 26 Jun 2016 07:38:05 UTC All use subject to http://about.jstor.org/terms Life cycle of Macronychus glabratus of all Elminae, however; only a small percentage of M. glabratus have functional hindwings and can fly (Segal 1933). Other dryopoids examined by Segal (1933) showed varying hindwing development: Helichus fastigiatus and Limnius elegans produced fully developed hindwings, Helmis quadrinotatus had short hindwings, and Stenelmis crenata had vestigial hindwings. Woody debris adds considerable insect production to stream habitats (Benke et al. 1984; Smock et al. 1985). M. glabratus, abundant on submerged wood, can be important to secondary production in stream ecosystems and, along with other aquatic xylophagous insects, to the conditioning and breakdown of submerged wood, providing both food and habitat for other stream insects inhabiting wood debris (Phillips 1994a). Understanding the dynamics of stream wood, therefore, depends on information about the ecology of these xylophagous beetles. Secondary production estimates are especially useful because they combine two characteristics of ecological significance, individual growth and population survival, into a single measurement (Benke 1984). The objectives of this investigation on M. glabratus were to: (1) determine the life cycle; (2) estimate the standing stock biomass, secondary production, and the production/biomass (P/B) ratio; (3) determine the hindwing development; and (4) compare these factors between populations in northwest Arkansas and southeast Texas.