Raymond M. Newman

Herbivory and detritivory on freshwater macrophytes by invertebrates: a review

Aquatic macrophytes have been viewed as important to invertebrates as substrates for periphyton and epiphytic food, refuges from predation, heterogeneous substrates for co-existence, and upon senescence and death as detrital food sources. Direct herbivory on living macrophytes has been considered unimportant and has been understudied until recently. A review of the literature shows that, when looked for, invertebrate herbivory of macrophytes is more common than generally thought. Most herbivory on macrophytes is usually by specialized oligophagous herbivores which are placed phylogenetically in families and orders of primarily terrestrial insect groups (secondary invaders) that include a number of pest herbivores in terrestrial systems (e.g., chrysomelid and curculionid beetles, aquatic lepidopterans, and specialized dipterans). Herbivores from primarily aquatic invertebrate groups (primary aquatic invertebrates) are often generalists and also detritivores. These patterns and the use of macrophyte detritus by primary aquatic invertebrates do not appear to be determined by a low food quality of aquatic macrophytes, but are more likely related to deterrent chemicals present in both terrestrial and aquatic plants. In order to more clearly ascertain the importance of aquatic macrophytes as food and their interrelations with aquatic invertebrates, more mechanistic studies are needed of aquatic herbivores, herbivore damage to plants, and the secondary chemistry of aquatic plants.

The watercress glucosinolate-myrosinase system: a feeding deterrent to caddisflies, snails and amphipods

SummaryWatercress (Nasturtium officinale) possesses the glucosinolate-myrosinase system. This system is regarded as a classic example of chemical defense for terrestrial crucifers. Damage of watercress initiates myrosinase-mediated hydrolysis of phenylethyl glucosinolate to a toxic endproduct, phenylethyl isothiocyanate. In multiple choice tests, the amphipod Gammarus pseudolimnaeus, the limnephilid caddisflies Hesperophylax designatus and Limnephilus sp., and the physid snail Physella sp. all strongly preferred (10X) yellowed senescent watercress (FY) over fresh green watercress (FG), despite the 2X higher nitrogen content of green watercress (6.9% for FG vs 3.8% for FY). Green watercress contained 10–40 X more glucosinolate than FY watercress (6.4–8.5 mg/g wet for FG vs 0.2–0.7 mg/g wet for FY). However, when the watercress was heated (ca 70°C), to deactivate the myrosinase enzyme, multiple choice tests showed that these species shift their preferences to heated green watercress (HG). Heating deactivated the deterrent effect and overall preference (consumption) was HG ≥ HY > FY ≫ FG for Gammarus. HG > HY ≥ FY ≫ FG for Hesperophylax, HG > FY ≥HY ≥ FG for Limnephilus, and HG ≥ FY > HY ≥ FG for Physella. Thus heating resulted in a shift in preference from the low glucosinolate, but low nitrogen, unheated yellowed tissue to the high nitrogen green tissue. These results suggest that deactivation of the myrosinase enzyme, and hence isothiocyanate production, results in a shift in preference. Preliminary results with Hesperophylax indicate that addition of myrosinase to the test water, which resulted in the formation of isothiocyanate, results in a significant decrease in HG consumption from control levels (p < 0.001) and no change in preference for HY watercress. With Gammarus, myrosinase resulted in reduced consumption of both green and yellowed watercress, but no significant differential effect. These results provide evidence that the glucosinolate-myrosinase system, recognized as the principle deterrent system of terrestrial crucifers, is the feeding deterrent in watercress and also suggest that in the absence of a functioning deterrent system, nitrogen content may influence consumption.

Conservation strategies for the biological control of weeds

Publisher Summary The chapter reviews the factors that limit the success of insects for weed biological control, including factors that regulate control agent populations and factors that influence weed response to insects. It addresses the use of conservation biological control strategies with native agents while drawing on experiences from studies on rangeland weeds and the aquatic weed Eurasian watermilfoil. The importance of establishing the factors responsible for the success or failure of weed biological control projects is becoming more and more apparent. Most conservation strategies include population protection or informed and appropriate use of pesticides to maintain native or exotic biological control agents. In addition, strategies such as habitat protection to preserve critical habitat or refugia and plant community management to maintain and enhance the effectiveness of existing biological control agents need to be exercised. The role of resistance has rarely been considered in weed biological control because most weed biological control agents are specialists and thought to be adapted to overcome their hosts defensive systems. Properly timed range management practices can protect populations of native biological control agents, increasing their effectiveness.

Biological control of Eurasian watermilfoil by aquatic insects: Basic insights from an applied problem

Eurasian watermilfoil (Myriophyllum spicatum) is one of North Americas most troublesome exotic weeds; it has infested 45 states and three Canadian provinces and millions are spent annually on control. Three insects have been considered for the biological control of Eurasian watermilfoil and all three taxa are present in the northern US and southern Canadian provinces. The naturalized pyralid caterpillar, Acentria ephemerella, has been associated with milfoil declines in some eastern states, but rarely reaches high densities in the midwest. The native midge, Cricotopus myriophylli, also does not generally reach high densities, but deserves further study. The native milfoil weevil, Euhrychiopsis lecontei, has been associated with milfoil declines across North America. The weevil, which is highly specific to watermilfoil species, has been shown to control Eurasian watermilfoil via stem mining in laboratory, tank and mesocosm studies, as well as in several field studies. During the summer, all life stages subsist on submersed watermilfoil, and after 3-5 generations are produced, adults move to shore to overwinter in shoreline leaf litter. The milfoil weevil has an induced preference for the exotic Eurasian watermilfoil and prefers and performs better on the exotic compared to native watermilfoils. Field declines of Eurasian watermilfoil due to weevil feeding will occur with sufficient weevil density and a positive native plant community response. At many sites, weevil populations are too low to effect control. Predation by sunfish (Lepomis) is likely an important factor limiting densities of adult weevils and other herbivores. Population modeling indicates that adult reproductive lifespan and fecundity are key factors for end-of summer population densities. The weevil and perhaps the other herbivores can be useful biocontrol agents if factors limiting their density can be ameliorated. This work demonstrates that herbivory by aquatic insects can be substantial, resulting in 50-95 % reductions in plant biomass and shifts in macrophyte community structure. Furthermore, important insights on the evolution of host range and host range expansion have been gained from study of this system and a better understanding of trophic cascades in littoral zones is being developed.

Diet and Feeding Periodicity of Ruffe in the St Louis River Estuary, Lake Superior

Abstract Ruffe Gymnocephalus cernuus, a percid native to Europe and Asia, is established in the Lake Superior drainage and could have negative impacts on native fish through competition for forage and predation on fish eggs. We investigated the diet of ruffes in the 4,654-ha St. Louis River estuary in May–October 1989–1990 and the feeding periodicity of ruffes in two adjacent habitats during five 24-h periods in summers 1990–1991. Ruffes were primarily benthophagous. Age-0 ruffes fed mostly on cladocerans and copepods in early summer and midge larvae (Chironomidae) in late summer and fall. Adult ruffes less than 12 cm fed mostly on midges and other macrobenthos but also consumed large numbers of microcrustaceans. Adult ruffes 12 cm and larger fed mostly on midges, burrowing mayflies Hexagenia spp., and caddisflies (Trichoptera). Ruffes consumed few fish eggs. Adult ruffes in deeper waters and all age-0 ruffes fed throughout the day as indicated by weight patterns of stomach contents. However, adult ruffes...

Effects of a potential biocontrol agent, Euhrychiopsis lecontei, on Eurasian watermilfoil in experimental tanks

Eurasian watermilfoil (Myriophyllum spicatum L.) is a submersed macrophyte, exotic to North America where it is a major nuisance. One potential biological control agent is the native weevil Euhrychiopsis lecontei (Dietz) (Curculionidae). To determine the effects of known densities of the weevil on Eurasian watermilfoil in controlled conditions, we stocked weevils into outdoor 0.38 m3 tanks containing watermilfoil. Watermilfoil stems (20 cm long; 150 m−2 were planted in each tank and given 3 weeks to root and grow. Plants developed extensive roots and grew 10–15 cm prior to stocking. Weevils were then applied at four stocking densities (0, 6, 12 or 24 adults) to 16 tanks arrayed in a Latin square design. Plant length was measured and weevils were counted weekly in the tanks. Sediment nutrients, plant mass and nutrient content, and weevil densities were determined from a sample of plants prior to stocking and 3 and 4 weeks after stocking. After 4 weeks, all plant material was removed, biomass was determined and weevils were counted. Weevils survived in the stocked tanks and eggs appeared soon after introduction. There was an average of over 200 weevils (25 adults) in each of the stocked tanks at 4 weeks. Weevil stocking density resulted in a significant decline in watermilfoil biomass (P < 0.005) with biomass in the tanks stocked with 24 weevils reaching only 40% of the control. Root biomass also declined with weevil density (P < 0.005) and biomass in the high density tanks reached only 55% of the control. However, detached sunken and floating watermilfoil biomass increased with density of weevils and no change in total above ground biomass (sum of standing, floating and sunken watermilfoil) was found, indicating that effects were caused by plant damage rather than direct consumption. Periodic estimates of plant height, mass and root mass showed that effects on plants resulted from reductions in stems and roots from peak levels at Weeks 2 or 3 and were thus not simply a suppression of growth. Percent sugars and total nonstructural carbohydrates declined with stocking density in both roots and shoots. The total stock (g per plant) of sugars, starch and total nonstructural carbohydrates was reduced in the roots. Weevil densities of ≈ 300 m−2 can have rapid and substantial effects on Eurasian watermilfoil both above and below ground. Herbivory by weevils may have long term effects via disruption of plant carbohydrate stores that are essential for overwinter survival and subsequent regrowth.

Developmental performance of the weevil Euhrychiopsis lecontei on native and exotic watermilfoil host plants

We investigated the developmental performance of the submersed watermilfoil specialist Euhrychiopsis lecontei (Dietz) (Curculionidae) on its native host, northern watermilfoil Myriophyllum sibiricum Komarov, and its newly acquired exotic host, Eurasian watermilfoil Myriophyllum spicatum L. The weevil is a potential biological control agent of Eurasian watermilfoil. To determine if performance is affected by rearing plant or by maternal plant, we examined development of eggs from Eurasian and northern watermilfoil-reared mothers on both species of watermilfoil. Hatching occurred in 3-5 d, and egg emergence rate ranged from 60 to 100%. Larval and pupal stages were completed in 8-11 d at 25°C and 16-19 d at 19.5°C with survival ranging from 50 to 100% for the larval stage and 60 to 88% for the pupal stage. Overall developmental time from egg to adult was 23-26 d at 25°C with survival from egg to adult ranging from 20 to 70%. Development times were significantly longer (1-3 d) on the native northern watermilfoil than on Eurasian watermilfoil. Survival did not differ significantly among rearing plant, but progeny of Eurasian watermilfoil-reared parents performed more poorly on northern watermilfoil than did progeny of northern watermilfoil-reared parents. Adult emergence weight did not vary by rearing plant, but was affected by maternal factors. Thus, there is no developmental performance penalty for shifting to the exotic host and there is evidence of better performance on the exotic host than on the native host.

Is Predation by Sunfish (Lepomis spp.) an Important Source of Mortality for the Eurasian Watermilfoil Biocontrol Agent Euhrychiopsis lecontei

ABSTRACT The aquatic weevil Euhrychiopsis lecontei is a potential control agent for Eurasian watermilfoil (Myriophyllum spicatum). Predation by fishes may influence populations of this beneficial insect. To determine if fish predation is an important source of mortality for E. lecontei, fish stomachs from two Minnesota lakes, Lake Auburn and Cedar Lake, were sampled monthly during the summer of 1994. None of the black crappie (Pomoxis nigromaculatus) or yellow perch (Perca flavescens) stomachs from either lake contained E. lecontei. The frequency of occurrence of E. lecontei larvae and adults in sunfish (bluegill (Lepomis macrochirus) and pumpkinseed (Lepomis gibbosus)) stomachs from Lake Auburn ranged from 10.3% in September to 28.6% in August. The mean number of E. lecontei adults per sunfish stomach from Lake Auburn ranged from 0.18 ± 0.13 (± 2 S.E.) in May and July to 2.14 ± 2.90 in August. Larvae of E. lecontei were found only in stomachs collected in August (mean = 0.21 ± 0.31) and September (mean =...

Weak top-down control of grazers and periphyton by slimy sculpins in a coldwater stream

Abstract We conducted an enclosure/exclosure experiment during summer 1998 and 1999 to test the strength of top-down control by slimy sculpins (Cottus cognatus) on benthic macroinvertebrates and periphyton. We used 1-m2 cages to manipulate slimy sculpin density within 6 riffles in Valley Creek, Minnesota. Each riffle contained an enclosure stocked with 3 slimy sculpins, a fishless exclosure, and an open control cage. In 1998, densities of total invertebrates and grazers on clay tiles were significantly higher in exclosures than enclosures and control cages; however, densities of individual grazer taxa (Glossosoma and Baetis) on tiles did not differ significantly among treatments. Fish had no significant effect on invertebrates inhabiting gravel substrate. In 1999, densities of total invertebrates and grazers on tiles and inhabiting gravel substrate did not differ significantly among treatments. Slimy sculpins did not have a detectable effect on the drift propensity of invertebrates during either year. Regardless of the effect of fish on grazers in either year, periphyton biomass did not differ significantly among the fish treatments even though there was a strong, negative correlation between Glossosoma density and periphyton biomass. Periphyton biomass was significantly higher on tiles with a barrier that excluded Glossosoma larvae from the surface, indicating that Glossosoma limited periphyton biomass. We conclude that the strength of top-down control by slimy sculpins on invertebrates was weak. We did not detect an algae-based trophic cascade during either year because Glossosoma larvae were relatively invulnerable to fish predation and did not alter their behavior in the presence of fish.

Effects of alum treatment on water quality and sediment in the Minneapolis Chain of Lakes, Minnesota, USA

Abstract The effects of aluminum sulfate (alum) treatment on water quality in 4 lakes of the Minneapolis Chain of Lakes (MN, USA) were examined. Lakes Harriet and Calhoun (treated in 2001) and Cedar Lake and Lake of the Isles (treated in 1996) all showed initial water quality improvement based on surface water total phosphorus (TP), chlorophyll a (Chl-a), and Secchi disk depth. Three lakes (Harriet, Calhoun, and Cedar) were at or below historical estimates of growing season average TP after treatment and showed continued improvement in surface water quality through 2005. Lake of the Isles, which received the lowest alum dose, returned to pretreatment conditions after 6 years. Estimates of sediment phosphorus (P) release rates, however, indicated that alum treatment still limited internal P release in all 4 lakes. Although the alum application to Lake Harriet was a littoral-only treatment, water quality improved in this lake as well. The aluminum hydroxide floc drifted to the deeper part of the lake, reducing internal P release from deeper sediments by 85% in the 2 years following treatment, leading to unexpected improvements in surface water Chl-a and TP concentrations.