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Does sunfish predation foil augmentation of the ( leconti) for the biocontrol of Eurasisan watermilfoil ( spicatum)?

Mark Cornwell!

ABSTRACT

The aquatic weevil (Euhrychiopsis lecontei) is a potential biological control agent of Eurasian water milfoil () (Sheldon and Creed, 1995). The extent of predation on introduced used for the biological control for water milfoil is unknown. To determine if predation by sunfish is important in post-stock weevil survival, four thirty-gallon aquaria were each planted with fifty (50) M spicatum tips l70mm in length. Twenty-five aquatic weevils were then introduced to the tips in each aquarium. A mix of 10 (Lepomis machrochirus) and pumpkinseed (Lepomis machrochirus) sunfish were added to the three weevil tanks. Weevil numbers in the three tanks declined an average of77.3% (n=25 to n=5.6) in 48-hr and 85.3% (n=25 to n=3.6) in 72-hr. No weevils were lost in the tank without sunfish. In addition, fifteen pumpkinseed (Lepomis gibbosus) were collected from ponds known to contain milfoil weevils and were dissected to determine if sunfish feed on the weevils in a natural setting. No milfoil weevils were found in these stomachs. The lack of an alternate food in the experimental aquaria and an absence of weevils in natural diets suggest the opportunistic nature of the sunfish. These results indicate sunfish impact weevil populations under controlled conditions and may impact stocking success if natural alternate prey is scarce.

INTRODUCTION

Eurasian watermilfoil (Myriophyllum spicatum), hereafter called milfoil, is an exotic, invasive aquatic plant from Europe that has become a nuisance in the United States since the 1950s (Batra, 1977). This problem plant interferes with fishing, navigation and water sports and clogs engine intakes, while fostering mosquito breeding and causing a decline in property values on lakeshore real estate (Batra, 1977). After introduction, M spicatum often becomes the dominant plant forming a thick, dense wall of a uniform height (Madsen, et al. 1991), often producing a surface canopy, eliminating native plants and annoying swimmers and boaters (Kangasniemi, 1983). Milfoil beds usually support a lower abundance and diversity of invertebrates compared to native plant communities (Keast, 1984).

The aquatic weevil (Euhrychiopsis lecontei) has been identified as a possible biological control agent for milfoil (Sheldon and Creed 1995, Creed and Sheldon 1995, EPA 1997). Prior to the introduction ofM spicatum, this native weevil typically carried

1 SUNY Oneonta biology MA candidate emolled in Bio. 681. Biological Field Station, Cooperstown, NY. 135 out its life cycle on native North American Milfoil () (Johnson, 2000). Weevils are milfoil specific, with first instar larvae feeding on meristematic tissue (Sheldon and O'Bryan, 1996). Later instar weevil larvae mine the milfoil, collapsing the stem. Interest in this weevil as a biological control has risen from the lack of a successful control agent for milfoil (Creed and Sheldon, 1995). Milfoil may contain deterrent compounds that make it unpalatable to many typical herbivores (Newman, 1991 ).

Using a native weevil for the control of an exotic plant poses a potential problem as the two organisms are "out of phase" with each other (Creed and Sheldon, 1995). Weevils may be most abundant at a time when the target plant is least susceptible. Creed and Sheldon (1995) indicate this warrants the need for artificial augmentation to produce a peak in weevil populations when the plant is most susceptible to the . Additionally, Creed and Sheldon (1995) attributed declines in milfoil dry biomass to weevil densities at >1-4 weevils per stem. This augmentation strategy has been tested in several lakes across the country by collegiate researchers, the U.S. Army Corps of Engineers and aquatic environmental consultants, often at a cost of one dollar or more per weevil for 8,000-20,000 weevils per lake (Johnson, 2000). Given the significant cost, surprisingly little attention has been given to their fate once stocked. This study focuses on the impact of fish on the survival of the weevils after initial stocking.

Sutter and Newman (1997) suggest that sunfish predation may limit weevil populations, particularly in areas with low weevil densities. In exclusion experiments, yellow perch (Percaflavescens) were not a significant source ofmortality for E. lecontei. Similarly, Newbrough (1993, in Newman and Sutter, 1997) found that bluegill preyed on weevils in the laboratory but he did not observe predation on the weevils in the field. Sutter and Newman (1997) predict that sunfish predation could be most significant with low densities ofE. lecontei and high densities of sunfish, with as much as 30% of the E. lecontei population per m2 removed daily. Johnson (2000) has also witnessed differences in weevil numbers in ponds that contain pumpkinseeds compared to non-fish ponds. Ward (2000) has noted similar differences in two Minnesota Lakes, citing low sunfish density in Lake Cenaiko resulting in large weevil populations, while Cedar Lake, a Minnesota lake with high sunfish populations, has a lower weevil density despite augmentations.

The goal of this study was to determine the effects of sunfish predation on adult aquatic milfoil weevils. Concurrent aquarium tests simulating artificial augmentation experiments were compared to determine if sunfish prey on weevils and ifthe weevil population was depressed to significantly reduce the augmentation. A dietary analysis of sunfish in a known weevil pond was also performed to examine weevil predation by sunfish under normal field conditions. 136

METHODS

Four (4) standard 30 gal. aquaria were filled with Otsego Lake water for this experiment. Three were experimental, with milfoil, weevil and sunfish added. The other was used as a control with milfoil and weevils but no fish. The top of each aquarium was fitted with a ~ in. glass plate lid placed on weather stripping for a secure fit. An air stone was placed in each aquarium.

Milfoil for this experiment was obtained form Hodges Pond, Oneonta, New York using a plant grapple. Milfoil tips were washed with tap water and cut to 170 mm. No attempt was made to remove any attached fauna from the tips. Milfoil with excessive epiphytic algal growth was not included. Ten milfoil tips were bunched together and placed in a clay pot packed with sand. Five pots were placed in the bottom of each of the three aquaria, for a total of 50 tips per aquarium.

Weevils were handpicked from milfoil tips at the Cornell Research Ponds Unit #2, Dryden, New York. They were temporarily held in a cloth covered glass jar while in transport to the experimental tanks. Twenty-five adult weevils were placed in each of the aquaria and were given 24-hr. to evenly distribute in the aquarium before fish were introduced.

Bluegill (avg. TL = 77mm, 6l-83mm, n=12) and pumpkinseed (avg. TL=94mm, 7l-l02mm, n=6) were collected by shore seine form Rat Cove, Otsego Lake, NY. Fish were measured on a standard fish board in millimeters. Immediately after capture 10 sunfish were introduced into each experimental tank containing the milfoil and weevils.

Weevils were counted 48-hr after the sunfish were added by carefully observing each cluster of stems in the aquarium without removing the plants. Counts at 72-hr were preformed by removal of stems from each test aquaria. Stems were removed from the sand and placed in a white enamel tray. Individual stems were searched for weevils.

Pumpkinseed for diet analysis were shore-seined form Cornell Research Ponds #2 and #4, both known to contain weevils. Fish were placed in a plastic bag, put on ice and transported to a freezer. Fish were kept frozen until the stomach analysis was conducted. Stomachs were dissected and the contents were analyzed under a dissecting scope.

A Mann-Whitney U-test (P<.05) was performed on the three aquaria results at 0, 48 and 72-hr. intervals to demonstrate whether the results were statistically different. A paired t-test (95% CI) was then used to demonstrate the statistical difference at 48-hr. and 72-hr.

RESULTS

Results of triplicate aquaria trials containing 50 stems of milfoil, 25 weevils and ten sunfish are listed below in Table 1. There was no decline of weevils in the control tank (i.e. without sunfish). Forty-eight hours after weevils were stocked, sunfish had 137

depleted weevils an average of 77% (25 to 5.6) among the three aquaria. Seventy-two hours after stocking weevils, numbers further declined to an average of 85% (25 to 3.6) of the original number.

Trial # Sunfish # Weevils 0-hr 48-hr (% decline) 72-hr (% decline) Control 0 25 25 (0) 25 (0) 1 10 25 7 (72) 1 (96) 2 10 25 5 (80) 5 (80) 3 10 25 5 (80) 5 (80) Average Decline N/A N/A 5.6 (77) 3.6 (85)

Table 1. Weevils initially planted and those remaining after 48 and 72 hours in the control and experimental tanks, as well as the average decline in the experimental tanks.

Figure 1 presents weevil numbers at time 0 and after 48 and 72 hours for each of the experimental aquaria. Weevil numbers in each aquarium were not statistically different for the three aquaria (Mann-Whitney U test, P< 0.05). Weevil numbers in 48-hr and 72-hr observations were not statistically different (paired t-test, 95% CI, P= 0.423). Weevil numbers were statistically different from both O-hr (a) to 48-hr (b) (paired t-test, 95% CI, P= 0.001) and O-hr to 72-hr (b) (Paired t-test, 95% CI, P= 0.004).

25 ­ 25 weevils in each +/-1SE a aquarium en 'S; (1) ~ '+­ 0 15 ­ '­ (1) .0 10 sunfish added E to each aquarium ::J Z b b 5 ­ I I I , a ~ n Number of Hours after Introduction

Figure 1. Mean # +/- 1SE of E. lecontei adults present at time 0 and after 48 and 72 hours in three aquaria, each containing Eurasian Watermilfoil (Myriophyllum spicatum) stems. 138

The results ofthe stomach analyses from the fifteen pumpkinseeds collected from the Cornell experimental ponds are listed below (Table 2). No weevils were found in stomachs from pumpkinseed from the Cornell Research Ponds Unit, despite the presence of weevils in the ponds from which test fish were removed (Johnson, 2000).

Fish # Length (mm) Weight (g) Contents 1 90 11.9 Physidae 2 77 8.4 Physidae, Centrarchidae, Amphipoda 3 64 4.7 Hymenoptera 4 70 6.1 No contents 5 65 4.7 Odonata, Ephemeroptera 6 72 6.6 Physidae, Amphipoda 7 80 10.2 Physidae 8 62 4.3 Physidae, Odonata 9 61 4.0 Physidae, Ephemeroptera 10 60 4.9 Physidae, Diptera, Ephemeroptera 11 60 3.5 Physidae 12 50 2.0 Daphnia sp. 13 56 2.6 Physidae, Daphnia sp. 14 46 1.6 Odonata 15 51 2.3 Physidae, Arachnida, Odonata

Table 2. Stomach contents ofpumpkinseed sunfish (Lepomis gibbosus) collected from the Cornell experimental ponds.

DISCUSSION

These results indicate that under experimental conditions, sunfish predation on weevils is extremely pronounced. Sutter and Newman (1997) have estimated that E. leconei populations are significantly impacted (up to 30% removal per day) by sunfish predation, especially at low weevil densities. They indicate that with moderate to high weevil densities 2-5% are removed per day, which does not cause a significant decline in their populations. Weevil densities in this experiment are moderate at .5 weevils per milfoil tip and declined an average of28.4% per day over the 72-h experiment, consistent with the decline of30% removal per day observed by Sutter and Newman (1997).

During initial observations, weevils seemed particularly vulnerable to predation when swimming from tip to tip. The sunfish generally did not prey on weevils attached to tips. In an augmentation situation, this may be problematic if stocked adult weevils are 139 moving from tip to tip trying to find new milfoil because they may be particularly vulnerable at this time.

The lack of additional food items in the experiment may magnify the effects of predation by sunfish on weevils. However, no attempt was made to remove naturally occurring food from the milfoil tips. Milfoil tips were rinsed and planted without further treatment. Hatched midge adults on aquaria lids, numerous midge retreats and snails on aquarium sidewalls were observed during weevil counts, indicating the presence of some natural food. Additional aquarium tests should include natural prey items as well as weevils to demonstrate if sunfish select for the weevils. Sunfish exclusion experiments may also demonstrate if weevil populations are kept low in habitats with sunfish.

Stomach analysis results would be more conclusive with a larger sample size. In fifteen dissected pumpkinseed stomachs from a known weevil pond, no weevils were found. Similarly, (Creed and Sheldon 1992, Creed et al., 1993) found no weevils in field-collected perch stomachs. Newbrough (1993) documented bluegill predation on E. lecontei adults in the lab, but did not observe significant sunfish predation in the field. In a larger sunfish-weevil diet study (n=158), Sutter and Newman (1997) found the frequency ofoccurrence of weevils in pumpkinseed and bluegill stomachs ranged from 10.3% in September to 28.6% in August in Lake Auburn, MN. Conversely, in a similar size experiment (n=171 sunfish stomachs) Cedar Lake, MN, Sutter and Newman (1997) found adult weevils to be extremely rare in sunfish stomachs (0-1.9% frequency of occurrence). Sutter and Newman (1997) also suggest that frequency of occurrence of weevils in sunfish exhibit a seasonal effect. Taking into account the large stomach analysis sample size of Sutter and Newman (1997) and the resulting low frequency of occurrence ofweevils in stomachs, it is plausible that a larger sample size of sunfish from the Cornell Ponds was needed to demonstrate the low level ofpredation by the pumpkinseeds on the natural weevil population. Ideally, these fish should be collected over time to evaluate seasonal differences.

These data indicate that sunfish predation on weevils significantly reduces their population under conditions simulating a milfoil-dominated lake augmented with adult weevils. Stocking density of weevils was .5 weevils per tip in this experiment, lower than the>1-4 weevils per tip Creed and Sheldon (1997) required to control milfoil in a Vermont lake. In lakes supporting milfoil with high densities of sunfish and low to moderate alternative food, stocking adult weevils may not be cost effective in meeting the goal of augmenting a weevil population sufficient to control watermilfoil. 140

REFERENCES

Batra, S.W. 1977. Bionomics of the aquatic moth, Acentropus niveus (Oliver). A potential biological control agent for Eurasian watermilfoil and hydrilla. New York Entomological Society, LXXXV(3): 143-152.

Creed, R.P. and S.P. Sheldon. 1995. Weevils and watermilfoil: Did a North American herbivore cause the decline of an exotic plant? Ecological Applications, 5(4) 1113­ 1121.

Johnson, R.L. 2000. Personal communication. Director. Cornell Research Ponds Unit. Cornell University, Dryden, NY 13053.

EPA. 1997. Use of aquatic weevils to control a nuisance weed in Lake Bomoseen, VT. Watershed Protection: Clean Lakes Case Study, (3).

Kangasniemi, B.J. 1983. Observations on herbivorous that feed on Myriophyllum spicatum in British Columbia in Lake Restoration, Protection, and Management Proceedings ofthe Second Annual Conference North American Lake Management Society. U.S. Environmental Protection Agency, Washington, D.C.

Newbrough, K.L. 1993. The effect og bluegill (Lepomis machrochirrus) on the density and survival of an aquatic weevil. M.S. thesis, University of Vermont.

Newman, R.M. 1991. Herbivory and ditritivory on freshwater macorphytes by invertebrates: a review. American Benthological Society, 10(2): 89-114.

Sheldon, S.P. and R.P. Creed. 1995. Use ofanative insect as a biocontrol for an introduced weed. Ecological Applications, 5: 1122-1132.

Sutter, TJ. and R.M. Newman. 1997. Is predation by sunfish (Lepomis spp.) an important source of mortality for the Eurasian water milfoil biocontrol agent Euhrychiopsis lecontei? Journal ofFreshwater Ecology, 12(2): 225-234.

Ward, D. Graduate study: Fish predations as a factor limiting milfoil weevil populations. http://www.fur. urnn.edu/research/milfoil/milfoilbc/fishpredation.html