CARNIVORY BY AN AQUATIC DETRITIVORE, CLISTORONIA MAGNIFICA ( TRICHOPTERA : LIMNEPHILIDAE) N. H. ANDERSON Reprinted from ECOLOGY Vol. 57, No. 5, Late Summer 1976 pp. 1081-1085 Made in the United States of America Ecology (1976)57: pp. 1081-1085 CARNIVORY BY AN AQUATIC DETRITIVORE, CLISTORONIA MAGNIFICA (TRICHOPTERA: LIMNEPHILIDAE)12 N. H. ANDERSON Department of Entomology, Oregon State University, Corvallis, Oregon 97331 USA Abstract. The limnephilid caddisfly, Clistoronia magnifica, was reared through three generations on a detritus-based diet enriched with wheat grains and green grass. Developmental time was reduced and weight of mature larvae and pupae was increased by addition of enchytraeid worms to the above diet. Supplemental feeding on animals is also likely in the field but its importance would be underestimated because, in comparison with detritus, animal tissue would be a small component of the gut contents. Key words: Caddisfly; carnivore; detritivore; feeding habits; growth rate; Oregon; rearing. INTRODUCTION and photoperiod, could also be contributing factors. Cummins (1973) review of trophic relations of The experiments reported herein were designed to aquatic insects indicates that a significant portion of determine whether the quality or vigor of laboratory the material cycling and energy flow in freshwater cultures of the caddisfly Clistoronia magnifica (Banks) systems involves the processing of organic matter could be improved by augmenting a plant-based diet by insects. Studies of ingestion, using visual analysis with some animal food. of gut contents, (e.g., Brown 1961, Thorup and The only field data for C. magnifica are from Iversen 1974, Hall and Pritchard 1975) have demon- Winterbourns (1971a) study from Marion Lake, strated that plant detritus forms the base of most B.C. He described it as an early-season univoltine food webs in these systems. Gut analysis is useful species inhabiting submerged marginal vegetation and in demonstrating the processing accomplished by open sediments throughout the lake. Based on gut aquatic insects. However, as Cummins (1973) sug- content analysis of 35 specimens, he classified C. gests, nutritional interpretation of the data is diffi- magnifica as a sediment feeder. The gut contained cult because items digested rapidly, and therefore a mixture of plant fragments, algal cells, amorphous least likely to be observed, may be the most nu- detritus and a very few arthropod remains. He tritious. Worms and other soft-bodied invertebrates could not determine whether the animals were are digested rapidly and their retention time in the `actively selected or ingested along with the sediment. gut will be short. Similarly, the microbial flora However, he implied that the prey ingestion was of associated with detritus is difficult to census but it is minor importance in C. magnifica and in the other more nutritious than the more obvious, but highly detritivores studied because there was rarely more refractory, lignin and cellulose components. than one to three animal fragments present in guts This study is part of a project on developing dominated by detritus. methods for continuous culture of aquatic insects. Studies of growth and trophic relations could then be METHODS AND MATERIALS based on the entire larval interval instead of restricted The C. magnifica culture was initiated from egg to short-term feeding trials or to gut analysis. In my masses collected from Fay Lake, Linn County, laboratory I have reared several limnephilid caddis- Oregon, in the Cascade Mountains. Larvae were flies, including species of Hesperophylax, Hydato- reared in aerated pans of tap water with sand sub- phylax, Limnephilus and Pseudostenophylax, through strate at 15.6°C and long days (16 h light : 8 h dark). a complete generation on a detritus diet. However, Pans were washed and the water replaced once or compared with field material the resulting adults twice a week to remove fecal material and excess were frequently undersized. Also, some had mal- decomposing food. formed wings and others failed to emerge from the Alder leaves (Alnus rubra), conditioned in water pupal exuviae. It seemed probable that these results for 1-2 wk to allow microbial colonization, were were due to the nutritional inadequacy of the food, the basic food for the first-generation larvae. Conifer though rearing conditions, especially temperature needles, conditioned for several months, were used primarily for case construction but may also have Manuscript received 29 August 1975; accepted 10 been ingested. Wheat grains and green grass were May 1976. also provided. These were readily consumed within Technical Paper No. 4083, Oregon Agricultural Ex- periment Station. Contribution No. 189 from the Conif- a few hours, demonstrating that microbial coloniza- erous Forest Biome. tion was not a prerequisite for feeding on these foods. 1082 N. H. ANDERSON Ecology, Vol. 57, No. 5 Care was taken not to have an excess of wheat as index of food consumption. Some larvae were dry the fermentation and/or deoxygenation from decom- weighed at this time to compare weights between position led to high mortality. In the later instars, the treatment and control larvae. the use of grass in the diet was discontinued largely The worm-supplement feeding was repeated with because it was not convenient to maintain a supply. larvae of the third laboratory generation in drippery Second-generation larvae were obtained from trays, starting with 30 larvae about 7 wk old (15 late laboratory-reared adults. The initial experiment was third- and 15 early fourth-instar larvae) in each of conducted with third-instar larvae (6 wk old) to the experimental and control groups. These larvae determine whether they would feed on live inverte- were progeny of adults that had been reared on the brates, and if so, the effects on growth and fecal standard vegetative-based diet. The larvae were production. White worms (Enchytraeidae) from a reared to newly-moulted pupae, at which time they laboratory culture were used as prey. Fifteen larvae could be sexed, and were then dry weighed. In were placed in a petri dish with sand, conifer needles, addition to comparing pupal weights between treat- and half of a conditioned alder leaf (150 mg dry wt) ments, the rate of larval development was compared and 12 white worms 8 mg dry wt). A control without sacrificing the larvae, using instar duration, dish with 15 larvae, paired by case length with the case size, and type of case constructed as criteria. worm-fed group, was set up with sand, conifer needles and the other half of the alder leaf. The RESULTS dishes were examined daily or every 2nd day for 10 Second-generation C. magnifica larvae with the days. Each time, the feces were removed from worm-supplement feeding initiated in the third instar the food and substrate by several rinsings and moulted about 1 wk earlier than the group fed on then collected by filtration on Millipore® mem- alder leaves and were 50% heavier after 30 days brane filters (5 pm pore size). Fecal collections (Table 1). Expressed as a growth rate, the worm- were dried at 60°C for 24 h and weighed. Twelve supplement group increased 0.14 mg/day compared live worms were added each time the dishes with .08 mg/day for control larvae. Fecal produc- were cleaned. The caddisfly larvae were observed tion rates were consistently higher for the control, feeding on the worms but no attempt was made to suggesting a lower assimilation efficiency of the quantify the amount consumed because of the diffi- detritus-based food (Table 1). culty of recognizing and collecting the fragments The data indicate that development rate of mid- of the dead worms. instar larvae was enhanced by the food supplement, Five larvae from both the experimental and con- but the apparent difference in growth rates could trol series were sacrified on day 10 for a compari- have been overestimated. The experiment was ter- son of weights. These were dried for 24 h at 60°C minated when the control series had all moulted to and then weighed. The remaining 10 larvae in each fourth instar. By this time most of the worm- series were reared until all had moulted to the fourth supplement group had been fourth instars for several instar (to day 30) and then dry-weighed individually. days and had completed the rapid spurt of growth An excess of alder leaves, and worm supplement in that characteristically occurs in the early part of an the experimental dish, was maintained. instar. Worm-supplement feeding was also conducted with Even with the addition of wheat to the diets, the second-generation larvae taken from the stock cul- trend for greater growth is also apparent for the ture at 12 wk when the larvae where late fourth or worm-supplement group of second-generation larvae early fifth instar. Differences in procedure from the reared from late fourth instar to the prepupal stage first trial were: (1) the control and experimental (Table 2). This interval represents all of the final diets included wheat grains, as this was standard for instar, which is the major feeding and growth period the stock cultures; (2) rearing was continued until when 80% of the total tissue is elaborated. The the larvae sealed off their cases for pupation, that results are consistent with those obtained for third- is, after achieving maximum growth; (3) experiments instar larvae, with more rapid development and were carried out in a "drippery," a series of trays heavier larvae being produced with the worm supple- with a variable rate of water exchange (Anderson ment. There was no difference in case length for 1973), to minimize 02 deficits due to decomposing the mature larvae of the two groups. The daily wheat or worms. growth rate for the final 10-12 wk of larval de- The cultures were examined at least twice a week velopment was 0.31 mg/day in the worm supple- and food added if needed.