Extension Bulletin WQ-61 • New • June 2009 Biological Control of Eurasian Watermilfoil: A Review of the Native Watermilfoil

Tom Alwin, Department of Fisheries and Wildlife

Kendra Cheruvelil, Department of Fisheries and Wildlife and Lyman Briggs College Tom Alwin Tom

Matt Fox

E. milfoil in Clear Lake, Mecosta Co., Mich.

Tom Alwin Tom Alwin Tom Boat prop entangled with E. milfoil.

Adult watermilfoil . Looking down into a dense E. milfoil bed.

1 Biological Control of Eurasian Watermilfoil:

urasian watermilfoil ( spica- the northern United States and southern Canada2. tum), hereafter referred to as “E. milfoil”, is The weevil’s natural host is a native variety an invasive exotic that, once of milfoil2, but once it’s exposed to E. milfoil, the E 3 introduced, can reach nuisance levels in ponds, weevil prefers it over native . This high affin- lakes, rivers and reservoirs. Dense E. milfoil growth ity for E. milfoil means that weevils will have little can alter the physical and chemical conditions of impact on other aquatic plants. On E. milfoil, weevil water bodies and drastically change fish and wild- survival and reproductive rates are high, and weevil life habitat. Dense E. milfoil beds can also impede populations can reach levels capable of controlling swimming, fishing and boating. Also, E. milfoil ac- E. milfoil. On native milfoil plants, however, weevil cumulating at the water surface or broken E. milfoil numbers remain low, and they generally do not neg- stems collecting on shore can reduce the aesthetic atively affect the native milfoil population4. Weevil quality of the lake. Significant amounts of time survival on hybrids of E. milfoil and native milfoil and money are invested in E. milfoil management is intermediate, and weevils’ impact on hybrids is because of these potential impacts. likely somewhere between their impact on native and invasive milfoils5. In small-scale laboratory Preventing E. milfoil introduction and early detec- studies, milfoil weevils have been found to be effec- tion need to be the first lines of defense. However, if tive at controlling E. milfoil. These results indicate your water body is already infested with E. milfoil, that the watermilfoil weevil may be an ideal biologi- then deciding on a control strategy may be the next cal control agent for E. milfoil. step. Several methods are available for whole-lake E. milfoil control. Each method has pros and cons Life cycle of the weevil associated with it (see Table 1 for details). This publication focuses on one of these methods: bio- The weevil life cycle — egg, larva, pupa, adult — is logical control. Biological control is defined as the closely coupled to milfoil. Adults feed on the use of biological means (such as parasites, viruses and stems of milfoil, reducing the plants’ ability to 6 or predators) to control a pest. The benefits of this photosynthesize . Females can lay an average of approach are selectivity — the control affects only two eggs per day on the growing tips of milfoil, and the target pest — and the potential for long-term a maximum of five generations can be completed 7 control. Also, biological control is generally consid- each summer . Of all stages of the weevil’s life cycle, 6 ered a natural and sustainable method of control. the larval stage has the largest impact on the plant . This bulletin provides general information on a After hatching, larvae tunnel into the stem and eat native watermilfoil weevil, lecontei, a the inner tissue. Their feeding interrupts the flow commercially available that has shown some of nutrients through the plant. Lower nutrient promise as a biocontrol agent for E. milfoil. concentrations in the roots may reduce E. milfoil’s overwinter survival8. Larval tunneling also creates holes in the stem walls, which release the gases that keep the plant upright6,9. This reduced buoyancy (“yoo-RICK-e-op-sis la-CON-tee-eye”) – can cause the plant to sink out of the water column the Milfoil Weevil and cause entire E. milfoil beds to collapse. Indi- vidual larvae then hollow out a pupal chamber in The watermilfoil weevil, hereafter referred to as “the the upper portion of the plant (2 to 3 feet from the weevil”, is an aquatic beetle native to North Amer- tip of the plant) to complete development. Pupal ica. Adult weevils reach a length of 3 millimeters chambers, as well as the larval stem mining, have (about the size of a sesame seed) and are dark- been found to increase stem breakage8, but stem colored with brown to yellow stripes on their fragments that result from weevil damage rarely backs1. They can be found from coast to coast in succeed in developing into new plants10.

2 A Review of the Native Watermilfoil Weevil

When the adult beetle emerges from the pupal and research into these areas is ongoing. The fol- chamber, it moves back to the E. milfoil canopy to lowing are possibilities. feed and reproduce. Each season, the last genera- tion of adults foregoes reproduction and instead Overwintering habitat stores fat for overwintering, which occurs in near- A lack of suitable overwintering sites has been sug- 7,11 shore soil and litter . During the spring, gested as a factor limiting weevil densities from adults emerge from the soil and return to the water one summer to the next. One study suggested that to reproduce. Because all weevil life stages feed ex- natural shoreline may be needed to maintain weevil clusively on milfoil, the weevil may be particularly populations through the winter months. However, well-suited as an agent for E. milfoil biocontrol. overwintering was not the focus of the study, and the impact of shoreline type on overwintering could E. milfoil control, not eradication not be tested because each lake was sampled only once. In fact, several lines of evidence suggest that Although the weevil can be an effective control overwintering conditions may not affect weevil den- agent for E. milfoil under some conditions, it will sities. First, 10 years of commercial weevil stock- not completely eliminate E. milfoil from a lake. ing efforts have not seen any sizeable differences In fact, some remaining E. milfoil is an important in weevil population densities between lakes hav- component necessary for long-term E. milfoil sup- ing natural shorelines versus lakes with developed pression. If E. milfoil is completely removed from shorelines (Hilovsky, EnviroScience Inc., personal the lake, weevil populations will not persist, leaving comment). Second, the weevils’ high reproduc- the lake vulnerable to reinfestation. Therefore, some tive rate should allow small spring populations to E. milfoil remaining provides resources to maintain reach high levels within a single season13. Third, a a small weevil population that can keep the E. mil- study of two Minnesota lakes with natural shoreline foil in check for years to come. As in most natural determined that weevil densities were not limited by systems, the weevil-milfoil interactions can fluctu- overwintering habitat14. ate from year to year. In years when conditions are particularly good for plant growth, the E. milfoil The amount and type of habitat necessary for suc- population may grow too fast for complete control. cessful weevil overwintering are poorly understood. There may be a slight lag before the weevil popula- Additional scientific study on overwintering survival tion builds up in response and again suppresses the success in landscaped habitat (maintained lawn, E. milfoil to a low level. beds, mulch, etc.) is necessary. Maintaining a wide buffer zone (25+ feet deep) of natural veg- Factors that may limit weevil density etation along the shore, where possible, may help weevil populations and may indirectly improve Although the weevil’s life history and host specificity E. milfoil management by taking up incoming nutri- suggest that it may be a promising biocontrol agent, ents from the surrounding landscape and reducing effective E. milfoil suppression depends on weevil nutrient availability for E. milfoil growth. grazing rates, which, in turn, are influenced by the total number of weevils present. Weevils can be Predation found in many lakes at low densities, but they are Predation by fish is another potential explanation rarely found in high enough numbers to control for naturally low weevil populations and may also 12 E. milfoil . So, if weevils are already present in a limit the effectiveness of weevil stocking efforts. Be- lake at low levels and there are abundant food re- cause larvae and pupae spend the majority of their sources (E. milfoil), why are populations not natu- time inside E. milfoil stems, they are protected from rally increasing to a level that can control E. milfoil? predation. Larvae are susceptible when they occa- Several explanations for this have been proposed,

3 Biological Control of Eurasian Watermilfoil: sionally leave the stem, and within E. milfoil beds, tions without artificial augmentation, several ap- adult weevils are exposed to predation12. proaches can be implemented to improve E. milfoil control using weevils. The following three manage- Several studies have investigated the effects of fish ment approaches can be applied to either increase predation on weevil populations. Neither black weevil densities or reduce E. milfoil growth rates: crappie nor yellow perch consumes weevils, but sunfish larger than 2 inches will eat the weevils15. 1. Conservation biocontrol modifies the existing In fact, lower sunfish densities have been associated habitat or management practices to conserve or with higher weevil densities15, and in Cenailko Lake, enhance existing biocontrol agent populations. Minnesota, during the early 1990s, an unexplained For weevils, three strategies discussed in detail decrease in sunfish density coincided with the below are reducing nutrient inputs that promote weevil population reaching densities high enough to E. milfoil growth, practicing integrated pest man- control E. milfoil16. These studies provide evidence agement and reducing predation pressure. that sunfish consume weevils and may negatively • Reducing nutrient inputs: Reducing nutrient affect weevil densities. Dense and complex E. milfoil inputs to a lake will not directly benefit weevil beds, however, may limit predator success, provide populations but may be an important step in a refuge for weevils and positively affect weevil den- reducing E. milfoil growth over time. Nutrients sities. Computer modeling found that both the sun- entering a lake from the surrounding land- 13 fish and E. milfoil density affect weevil density . No scape act as fertilizer and encourage vigorous field studies to date have addressed the interactions E. milfoil growth. Actions by the surrounding between fish predation, E. milfoil stem density and landowners to reduce nutrient input include weevil density, which may be an important factor in applying lawn fertilizer as per manufacturer the weevils’ ability to suppress E. milfoil. directions, not washing cars on the driveway or street, maintaining a buffer of “wild” vegetation Reproduction rate at the water’s edge and properly maintaining Another potential explanation for small naturally septic systems. For more information on these occurring weevil populations is difficulty finding and other steps to reduce nutrient loading to mates. At naturally low densities, weevils may have your lake, check out Extension bulletin WQ57, difficulty locating mates and reproducing, especially “Lakescaping for Wildlife and Water Quality,” in dense E. milfoil beds. This situation is an area of published through the Minnesota Department weevil research that has not been investigated but of Natural Resources and available through the may be important. Michigan State University Extension Bulletin Office. Milfoil hybrids • Practicing integrated pest management: Hybridization between E. milfoil and native milfoil Biocontrol can be part of a strategy that also species can occur. In fact, reports of lakes infested includes any combination of physical, with milfoil hybrids are increasing. These hybrid mechanical or chemical control methods. plants can possess the qualities of both species and Lake-wide herbicide application and harvesting pose a serious threat to the effectiveness of both are likely counterproductive to weevil popula- weevil control and current herbicide treatments. tions because, in the short term, they eliminate E. milfoil and leave no food for the weevil. Improving biocontrol efforts: integrated However, strategies such as targeted herbicide pest management application or benthic barriers (light-blocking material anchored to the lake bottom) can be Although natural weevil populations rarely reach incorporated with biocontrol management the numbers necessary to control E. milfoil infesta-

4 A Review of the Native Watermilfoil Weevil

practices. These applications can be beneficial be ineffective. More research on what limits weevil in high-traffic areas (e.g., boat launches) or populations is necessary to improve control efforts other areas not suitable for weevils and may using the watermilfoil weevil, and to determine lead to more rapid lake-wide E. milfoil control. where and under what circumstances weevils are • Reducing weevil predation: Although it’s often most suitable for E. milfoil control. impractical at best, reducing the densities of predator sunfish may help increase weevil Frequently asked questions populations. Approaches that could be taken about weevil stocking to decrease sunfish numbers include increas- ing the population of large fish (greater than 12 Q: How many weevils are needed per acre of inches) that prey on sunfish (e.g., largemouth E. milfoil? bass), either by reducing fishing pressure and/ A: This question is difficult to answer because each or stocking, or increasing fishing pressure on lake has a unique set of conditions. Even under sunfish. More practically, a lake with high controlled laboratory conditions, there is no sunfish density may require a different stock- consensus on the minimum number of weevils ing strategy or may not be suitable for use of necessary to control E. milfoil. Therefore, the weevils for E. milfoil control. goal of stocking has not been to attain a specific Additional strategies that may be beneficial are density but rather to establish areas of the lake limiting the production and spread of E. milfoil with permanent and ecologically significant fragments (keeping boat traffic out of E. milfoil subpopulations. The subpopulations can then beds) and improving weevil overwintering spread and increase over time to achieve lake- habitat. wide suppression of E. milfoil. Establishing significant subpopulations typically requires 2. Inoculative biocontrol is intentional release of several thousand weevils to be stocked in close an organism as a biocontrol agent with the intent proximity. that it will multiply and control the pest for an extended period17. Weevil stocking has been ap- Q: How long will it take to achieve lake-wide plied to many lakes, including 70 Michigan lakes E. milfoil control? during 1998-2006 (EnviroScience, Inc.), with A: This is another tough question to answer. Many varying degrees of success. Enhancing natural factors play a role in determining the time weevil populations is likely necessary in most needed for control, including lake size, quantity lakes to achieve weevil densities high enough to and density of the E. milfoil, and the number control E. milfoil. of weevils stocked. However, in most stocked Michigan lakes, lake-wide control (not eradica- 3. Combining the two strategies above will likely tion) has been achieved in one to four years. improve the success rates of E. milfoil biocontrol. Q: Will the weevils become a nuisance? Specifi- Conclusion cally, do they bite or swarm homes in the fall? Evidence from laboratory research, 10 years of com- A: The answer to these questions is no. Typically, mercial stocking in Michigan lakes and multiple the only sign indicating that weevils are present examples of E. milfoil declines attributed to weevils in a lake is the impact they have on E. milfoil. all suggest that weevils can be an effective biologi- Early indicators of weevil activity include brown cal control agent. However, the results of stocking or blackened stems and stems that are collaps- efforts have been variable. In most cases, some ing or “candy-caning” in the water column. decline in E. milfoil can be seen within one to four years, but in other cases, stocking efforts appear to 5 Biological Control of Eurasian Watermilfoil:

Q: What does it cost to stock the weevil? treatment, determined by lake area and volume, A: Weevils are stocked in increments of 1,000 indi- has been estimated to average $80 to $350 per viduals. The current market price is $1,200 per acre of E. milfoil (http://www.co.midland.mi.us/ thousand. Stocking is often done over the course departments/extra.php?id=9&pid=513). of several years, with the number stocked per Q: What time of the year is best for stocking? year determined on a case-by-case basis. Mak- A: Mid-May through early August is best. Stocking ing a direct comparison between weevil stocking weevils by midsummer allows several genera- and herbicide treatment is difficult. Herbicide tions to be completed before overwintering.

Literature Cited:

1. Johnson, R.L., and B. Blossey. 2002. Eurasian Water- 9. Creed, R.P., S.P. Sheldon and D.M. Cheek. 1992. The milfoil. Pages 79-90 in R. Van Driesche (ed.), Biological effect of herbivore feeding on the buoyancy of Eurasian Control of Invasive Plants in the Eastern United States. watermilfoil. Journal of Aquatic Plant Management 30: USDA Forest Service, FHTET. 75-76. 2. Creed, R.P., and S.P. Sheldon. 1994. Aquatic weevils 10. Sheldon, S.P., and R.P. Creed. 1995. Use of a native (Coleoptera: ) associated with Northern as a biological control for an introduced weed. Ecological watermilfoil (Myriophyllum sibricum) in Alberta, . Applications 5(4): 1122-1132. Entomological News 105(2): 98-102. 11. Sheldon, S.P., and L.M. O’Bryan. 1996. The effects of 3. Sheldon, S., and K.N. Jones. 2001. Restricted gene flow harvesting Eurasian watermilfoil on the aquatic weevil according to host plant in an herbivore feeding on native Euhrychiopsis lecontei. Journal of Aquatic Plant Manage- and exotic watermilfoils (Myriophyllum: ). ment 34: 76-77. International Journal of Plant Sciences 162(4): 793-799. 12. Newman, R.M. 2004. Invited review biological control of 4. Sheldon, S., and R.P. Creed, Jr. 2003. The effect of a na- Eurasian watermilfoil by aquatic : Basic insights tive biological control agent for Eurasian watermilfoil from an applied problem. Archiv Fur Hydrobiologie on six North American watermilfoils. Aquatic Botany 76: 159(2): 145-184. 259-265. 13. Ward, D.M., and R.M. Newman. 2006. Fish predation on 5. Roley, S.S., and R.M. Newman. 2006. Developmental Eurasian watermilfoil () herbi- performance of the milfoil weevil, Euhrychiopsis lecontei vores and indirect effects on macrophytes. Canadian (Coleoptera : Curculionidae), on northern watermilfoil, Journal of Fisheries and Aquatic Sciences 63(5): 1049- Eurasian watermilfoil, and hybrid (Northern x Eurasian) 1057. watermilfoil. Environmental Entomology 35(1): 121-126. 14. Newman, R.M., D.W. Ragsdale, A. Milles and C. Oien. 6. Creed, R.P., and S.P. Sheldon. 1993. The effect of feeding 2001. Overwinter habitat and the relationship of overwin- by a North-American weevil, Euhrychiopsis-lecontei, on ter to in-lake densities of the milfoil weevil, Euhrychiopsis Eurasian watermilfoil (Myriophyllum spicatum). Aquatic lecontei, a Eurasian watermilfoil biological control agent. Botany 45(2-3): 245-256. Journal of Aquatic Plant Management 39: 63-67. 7. Mazzei, K.C., R.M. Newman, A. Loos and D.W. Ragsdale. 15. Sutter, T.J., and R.M. Newman. 1997. Is predation by sun- 1999. Developmental rates of the native milfoil weevil, fish Lepomis( spp.) an important source of mortality for Euhrychiopsis lecontei, and damage to Eurasian watermil- the Eurasian watermilfoil biocontrol agent Euhrychiopsis foil at constant temperatures. Biological Control 16(2): lecontei? Journal of Freshwater Ecology 12(2): 225-234. 139-143. 16. Newman, R.M., and D.D. Biesboer. 2000. A decline of 8. Newman, R.M., K.L. Holmberg, D.D. Biesboer and B.G. Eurasian watermilfoil in Minnesota associated with the Penner. 1996. Effects of a potential biocontrol agent, milfoil weevil, Euhrychiopsis lecontei. Journal of Aquatic Euhrychiopsis lecontei, on Eurasian watermilfoil in ex- Plant Management 38: 105-111. perimental tanks. Aquatic Botany 53(3-4): 131-150. 17. Eilenberg, J., A. Hajek and C. Lomer. 2001. Suggestions for unifying the terminology in biological control. Bio- Control 46: 387-400.

6 A Review of the Native Watermilfoil Weevil

Table 1. Methods for E. milfoil control.

Additional Method Effectiveness Advantages Disadvantages Costs comments

Mechanical Not effective • Rapid results • High cost to purchase • High initial No permitting harvesting • Boat lane harvester investment required creation • Short-term relief, may • Significant ongoing need multiple applica- costs: equipment tions per year maintenance, labor, • Generally not selective transport/disposal • Not useful in shallow of cuttings areas • Cuttings need land disposal site • Resulting fragments may promote spread of milfoil

Herbicides Highly • Rapid control • Reapplication required • High ongoing MDEQ permit effective • Selective every 1-3 years costs: chemicals, required control • Certified applicator permitting, required applicator fees • Some can be • Restrictions may be spot applied associated with waters used for drinking and irrigation • Some milfoil popula- tions are showing signs of herbicide resistance • Some non-target impacts are possible • Avoiding skin contact (swimming) for 1-2 days after application may be advisable

Weevils Effective, but • Has the poten- • Not a quick fix – time • Variable cost, No permitting some lake-to- tial to provide lag of 1-4 years from depending on required in lake variability long-term, stocking to control number of weevils Michigan sustainable • May have limited stocked control usefulness in very • Generally less • Very selective, high traffic areas expensive than no non-target • May not be other methods impacts appropriate for over 3- to 5-year • Environment- all lakes period ally friendly, uses native North American insect • High degree of flexibility in application and cost

7 Tom Alwin Tom Alwin

Adult watermilfoil weevils. Looking down into a dense E. milfoil bed. Tom Alwin Tom Alwin

Collecting samples in an E. milfoil bed. Underwater view of an E. milfoil bed.

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