AMY THORSTENSON
FEBRUARY 2012 

Stevens Point, WI 
715/343-6215 
www.goldensandsrcd.org

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Introduction 

Biological control studies are currently underway in Wisconsin to improve the 
science of applied biological control of Eurasian watermilfoil (EWM). Many lake groups 
are eagerly awaiting the results of those studies and are interested in applying biological 
control in their lake. However, for many cash-strapped lake groups, purchasing their 
weevils outright would be cost-prohibitive. As we move forward in our understanding of 
the biological control of EWM, this mass rearing pilot study aims to move us forward in 
making milfoil weevils a more practical option for lake groups with more sweat equity 
than cash. The mass rearing method (Thorstenson 2011) is labor intensive and must 
be followed to the letter in order to maximize success. Phase I of this pilot study was 
the first year of evaluating the capability of volunteer groups to successfully produce 
weevils on a mass scale. 

Methods

Study area —Lake Holcombe (Chippewa/Rusk Co) is a 2,881-acre impoundment 
of the Chippewa River, with a maximum depth of 61 ft. Large parcels of the riparian 
properties belong to the State of Wisconsin or paper company holdings and remain in 
natural/wooded condition. The Minong Flowage (Douglas/Washburn Co) is a 1,587- 
acre impoundment of the Totagatic River, with a maximum depth of 21 feet and 
surrounding natural/wooded shoreline. Goose Lake (Adams Co) is an 84-acre seepage 
lake with a maximum depth of 22 ft and surrounding natural/wooded shoreline. 
Study Design — Weevil rearing methods were modeled after Hanson, et al. 
1995, with modifications based graduate work conducted by Amy Thorstenson at UW-

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Stevens Point (Thorstenson2011). Hanson, et al. reported that an outdoor stock tank 
performed just as well their indoor, controlled 20-gal aquariums, with less management 
time invested. Thorstenson’s studies found similar results, and developed a simplified 
method for outdoor, mass rearing. 
Each lake group set-up and maintained 10, 370-L “Freeland poly-tuf‟ stock tanks 
(79cm W x 132cm L x 63cm H), stationed in an outdoor area where full sun and access 
to a clean water supply was available. The sunniest location available was selected to 
keep the milfoil stems (food stems) healthy, but water temperatures were monitored to 
ensure they did not approach lethal temperatures (34 C / 93 F). Water temperatures 
were monitored with aquarium thermometers and recorded regularly. Fresh water was 
added as needed to top off the tanks. NoSeeUm (0.033 cm mesh) light duty fiberglass 
screening was used to cover the tanks and pools. While the primary use of the 
screening was to exclude predator/competitor insects and birds, it also functioned as 
light shade to reduce peak temperatures in the tanks during sunlight hours. 

EWM stems to be used for food were collected from the same lake that would be 
the recipient of the weevils reared. Stems were collected from the deepest milfoil beds 
available, farthest from shore, where naturally occurring weevils were less likely to be 
present, in order to avoid the inadvertent introduction of unaccounted for weevils. To 
minimize the introduction of predator or competitor insects, the collected food stems 
were laid thinly over a mesh screen and sprayed with a hose and nozzle at a pressure 
sufficient to clean the milfoil but not damage it. Cleaned stems were then be floated in a 
wading pool of clean water, sorted and untangled. Because weevils lay their eggs on 

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apical meristems, only stems with apical meristems were retained for use; stems that 
had gone to flower or had broken tips were be discarded. Stems were trimmed to a 
length sufficient to reach from the base of the rearing chamber to the surface of the 
chamber’s water (62 cm). Stems were then bundled together in groups of fifteen stems, 
and attached at the base to a rock with a rubber-band to weight the stems down and 
achieve vertical orientation in the rearing chamber. All chambers received an initial 
stocking of milfoil food bundles, with stockings repeated every 21 days to keep the 
weevils supplied with actively growing milfoil (Table 1). 
Table 1 
Weevil feeding schedule. 

# of EWM 
stems to feed 
per tank 
Day 0 
Day 21 
Day 42 
105 
165 
225 

The “starter batch” of weevils were purchased from EnviroScience, Inc., Ohio. 
EnviroScience Inc. provided weevil stock from northern Wisconsin, in order to ensure 
weevils with winter-hardy genetics. Each tank was stocked with 0.19 weevils/L (72 
weevils per 100-gal tank). The purchased weevils arrived as eggs and early instar 
larvae attached to bundles of milfoil stems in sealed plastic bags. The estimated 
number of weevils in each bag was written on the outside of each bag, however the 
number of weevils inside were assumed to be unevenly distributed amongst the milfoil 
stems within. Therefore, the stems were placed into a large tub of water and counted to 
derive an estimated average of weevils per stem. Stems were then selected randomly 

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to accumulate the number of weevils needed to stock each rearing chamber. Thus, the 
number of weevils initially stocked to each rearing chamber was an estimated average. 

Chambers were maintained for approximately 55 days, allowing enough time for 
producing two generations. Prior to releasing the weevils to their recipient lake, 
subsamples were extracted to estimate total production. A 10% subsample of the 
weevil-containing food stems were extracted from four of the ten tanks (selected at 
random), preserved in 80% isopropyl alcohol, and refrigerated until laboratory 
examination. The preserved subsample stems was examined by Thorstenson by 
floating stems in water in a glass pan over a light table, with 3x magnification goggles. 
Each stem was carefully examined for weevil eggs, larvae, pupae, and adults and the 
total number of weevils recorded. The assistance of a higher power (30x) Carson 
MagniscopeTM was used for identification of specimens when needed. Specimen 
vouchers were preserved in sample vials in 80% isopropyl alcohol. 

Data Analysis — For the each rearing site, average return rate and total estimated 
production was estimated based on the 10% subsamples. Total estimated release (total 
production – subsamples) was also calculated. Temperature records were analysed to 
calculate min, max, mean, and 90% confidence intervals, to evaluate whether volunteers were 
maintaining optimal water temperatures. 

Results

Goose Lake – Expected return rate was 9.6 weevils out per weevil stocked, and 
Goose Lake’s return rate was 0.6. (Table 2) 720 weevils were initially stocked to the10 

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rearing tanks, and total production was estimated at 400 weevils. Lab examinations 
observed: low occurrence of miscellaneous insects; substantial mixing of hybrid milfoil, 
M. sibiricum, and M. verticillatum stems; dead or bacteria-engulfed pupa; low 
occurrence of pupation sites; and low evidence of weevil damage on non-M. spicatum
stems. Due to an acute lack of available M. spicatum in Goose Lake, M. sibiricum and 
hybrid milfoil were also collected as an optional food choice when it became necessary. 
Water temperatures were monitored but not recorded. Tank temperatures were 
moderated by adding fresh groundwater as needed. 
Minong Flowage - Expected return rate was 9.6 weevils out per weevil stocked, 
and Minong Flowage’s return rate was 1.8. (Table 3) 720 weevils were initially stocked 
to the10 rearing tanks, and total production was estimated at 1,300 weevils. Lab 
examinations observed: low occurrence of miscellaneous insects; no non-M. spicatum
mixed in; heavy weevil damage to stems in some tanks; and fused, deformed milfoil 
leaflets and hardened, opaque stems (indicative of exposure to herbicides) in some 
tanks. Tank temperatures were moderated by adding fresh groundwater as needed. 
Water temperature ranged from 60 - 80 F, with a mean of 71 F. (Table 4) These 
temperatures were similar to temperatures expected (per Thorstenson 2011), but lower 
than the temperatures optimal for weevil production. (Figure 1) 
Lake Holcombe - Expected return rate was 9.6 weevils out per weevil stocked, 
and Lake Holcombe’s return rate was 3.1. (Table 5) 720 weevils were initially stocked 
to the10 rearing tanks, and total production was estimated at 2,090 weevils. Lab 
examinations observed: low occurrence of miscellaneous insects; no non-M. spicatum
species mixed in; poor stem health; heavy weevil damage to stems in some tanks; 

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limited available oviposition sites; and fewer eggs than expected. Tank temperatures 
were moderated by adding fresh groundwater as needed. Water temperature ranged 
from 70 - 90 F, with a mean of 82 F. (Table 6) These temperatures were higher than 
temperatures expected (per Thorstenson 2011), and similar to temperatures optimal for 
weevil production. (Figure 1) 

Discussion

Goose Lake production was substantially lower than expected, and the optional 
feeding on non-M. spicatum species was likely the key problem. Temperatures were 
closely monitored (although not recorded), and not believed to be a problem. 
Subsample observations noted few miscellaneous insects, ruling out a predation 
problem. Subsample examinations confirmed several species of milfoil were used in 
feeding, including: M. sibiricum, hybrid milfoil (northern x M. spicatum), M. verticillatum. 
M. heterophyllum is also present in Goose Lake and may also have been fed, although 
subsample examinations did not confirm this. Subsample examinations noted problems 
with pupation (bacteria-laden pupa, dead pupa, few pupal chambers observed), and 
weevil damage observed on M. spicatum but not the other species that were mixed in. 
Weevil developmental time is longer, and developmental performance is poorer, on M. 
sibiricum than on their exotic host, M. spicatum (Newman et al. 1997). Research in the 
Midwest has found that weevil performance on hybrid milfoils was intermediate between 
the native hose (M. sibiricum) and the exotic host (M. spicatum) (Roley & Newman 
2006). Weevil developmental time is significantly longer when reared on M. 
verticillatum than on M. spicatum (37 days versus 21 days) (Solarz & Newman 2001). 

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Additionally, oviposition (where they choose to lay their eggs) preference was 
significantly less for M. sibiricum and nearly absent for M. verticillatum in females that 
were reared on M spicatum (Solarz & Newman 2001). Weevil development on or 
preference for M. heterophyllum is unknown. Therefore, the optional feeding of other 
milfoils, although unpreventable due to an acute lack of M. spicatum in 2011, was likely 
the main factor in low production. 
Minong Flowage had lower than expected production, possibly due to a 
combination of factors. One factor may have been food stem quality. The Minong site 
was the shadiest of the three sites, and subsample examinations noted stems in very 
poor condition, some limp, as if they did not get enough sunlight. Additionally, some 
tubs had stems that were deformed (fused leaflets, tough, opaque stems) as if exposed 
to herbicides. Food stem collection was in an area of the Flowage that had not been 
treated with herbicides, but was within the same bay (Serenity Bay). (Appendix B) It 
would be possible that residual herbicides were insufficient to kill the milfoil there, but 
yet sufficient to cause growth deformities. These deformities may have negatively 
affected the plant’s qualities as a host plant for successful weevil development. (Note 
the dead pupa recoded in the same tub that had the deformed stems.) 
Lake Holcombe had lower than expected production, probably due to weevil 
development time being shorter than expected. The rearing site was in open prairie, 
with all-day sun, which allowed the tubs to warm more than expected. Volunteers 
managed the temperatures frequently, adding fresh, cool groundwater twice a day if 
needed to keep tanks from getting too hot during heat waves. Their temperature 
records reflect that effort, with tank temperatures hovering around a mean of 81 F, and 

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a tight 90% confidence interval of less than 1 degree. We were expecting tub 
temperatures to average around 71 F, as in Thorstenson 2011, and for the full life cycle 
to take about 21 days. Lake Holcombe’s temperatures were closer to optimal 
temperatures for weevil development (84 F, Mazzei et al. 1999). At this temperature, 
the full life cycle takes only 17 days (Mazzei et al. 1999), which means the weevils 
should have been fed four days sooner, at each feeding cycle. Subsample 
examinations found heavy feeding damage, a shortage of healthy growing buds suitable 
for egg laying, and a shortage of healthy, fat stems suitable for pupation sites, all 
evidence that the weevils were running out of food and habitat, which certainly led to 
reduced production rates. 
Although the results of this study were well below expected, the problems 
encountered can be adjusted for with modifications to the methods. In future studies, it 
is recommended to: 
select rearing sites that have a minimum of 6 hours of sunlight to maintain 
healthy food stems; 
collect food stems well away from potential herbicide residue areas; 
avoid the optional use of other milfoil species; 
and to monitor temperatures regularly and shorten feeding cycle times at very 
sunny sites where optimal temperatures are attained. 

Acknowledgments

 
This study was funded by an Aquatic Invasive Species Grant (#AEPP-304-11) 
from the Wisconsin Department of Natural Resources. This study would not have been 

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possible without the dedication of team leaders at each site: David Blumer, SEH, Inc., 
Reesa Evans, Adams County Land Conservation Department, and “Doc” Dougherty, 
Lake Holcombe Association; and their dedicated volunteer crews at Goose Lake 
Association, Swift Nature Camp, Minong Flowage Lake Association, and Lake 
Holcombe Association. 

References

Hanson, T., C. Eliopoulos, and A. Walker. 1995. Field Collection, Laboratory Rearing 
and In-lake Introductions of the Herbivorous Aquatic Weevil, Euhrychiopsis 
lecontei, in Vermont. Vermont Department of Environmental Conservation, 
Waterbury, VT. 
Mazzei, K.C., R.M. Newman, A. Loos, and D.W. Ragsdale. 1999. Developmental rates 
of the native milfoil weevil, Euhrychiopsis lecontei, and damage to Eurasian 
watermilfoil at constant temperatures. Biological Control. 16:139-143. 
Newman, R.M., M.E. Borman, and S.W. Castro. 1997. Developmental performance of 
the weevil Euhrychiopsis lecontei on native and exotic watermilfoil host-plants. J. 
of the North Amer. Benthological Soc. 16:627-634. 
Roley, S.S., and R.M. Newman. 2006. Developmental performant of the milfoil weevil, 
Euhrychiopsis lecontei (Coleoptera: Curculionidae), on northern watermilfiol, 
Eurasian watermilfoil, and hybrid (northern x Eurasian) watermilfoil. 
Entomological Soc. of Amer. 

Solarz, S.L. and R.M. Newman. 2001. Variation in hostplant preferences and 
performance by the milfoil weevil, Euhrychiopsis lecontei Dietz, exposed to native 
and exotic watermilfoils. Oecologia 126:66-75.

Thorstenson, A.L. 2011. Biological control of eurasian watermilfoil (Myriophyllum 
spicatum) using the native milfoil weevil (Euhrychiopsis lecontei). M.S. Thesis. 
University of Wisconsin-Stevens Point, Stevens Point, WI.