weevils ewm: a 4 year field experiment fileweevils and ewm: a 4‐year field experiment susan knight...
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Weevils and EWM: A 4‐year Field Experiment
Susan Knight and John Havel
April 1, 2016
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Can Weevils Control EWM?
Little Bearskin Lake, Oneida County
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AUTUMN adult flight
SPRING adult flight
SUMMERreproduction (≥ 10°C)weak flight muscles
WINTERadult diapause
Larval stem mining
Milfoil weevil life cycle
© J. Havel & A. Wempe 2014
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Can Weevils Control EWM?Experimental design
• 2012 baseline year
• 2013 augmentation with weevils
• 2014, 2015 planned augmentation years, but cancelled Little Bearskin Lake, Oneida County
• 4 Lakes
• 4 Beds/lake
• 2 beds/lake augmented with weevils (provided by EnviroScience)
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Boot
Little Bearskin
Manson
Long
AB
CD
A BC
D
AB
CD
A
F
CD
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Weevil density will increase following augmentation
EWM biomass will decline
Native biomass will increase, but more slowly than EWM declines
% EWM biomass will decline
Native diversity will eventually increase
Hypotheses to Test
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EWM biomass is highly variable, both among lakes and among beds.Trends are similar among years.
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0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
A B C D A B C D A B C D A F C D
L Bearskin Manson Boot Long
WEE
VIL
S P
ER
STEM
Background DensityAugmented Density
In some beds, extra weevils were a majoraddition to the existing weevil population but in
many they were only a minor addition.
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Weevils/Stem
0.0 0.5 1.0 1.5 2.0 2.5 3.0
% S
tem
Dam
age
0
20
40
60
80
100
r = 0.846p < 0.001
More EWM stems are damaged with higher weevils density
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Weevils/stem
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
EWM
bio
mas
s (g/
m2 )
0
50
100
150
200
250
201320142015
rho = -0.449p = 0.002
There is less EWM biomass with increased density of weevils
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Results• Weevils are common
in EWM beds• Weevils damage
EWM • There is less EWM
biomass in beds with more weevils
• Experimentally, we could not detect a change in biomass consistent with weevil augmentation
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Conclusions • Biomass of native plants and EWM is highly variable among beds within a lake, among lakes and among years.
• Though weevils clearly harm EWM, weevils are not abundant enough to affect biomass of bed
• Our “treatment” was more of a nudge than a hammer. Lakes are messy and we needed a hammer.
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Why didn’t we see more of an effect?
• This was an experiment– It didn’t not work, it was an experiment.
• Lake and bed selection – Lakes: not random (no herbicides)– Bed: not random (not too large)– Treatments to beds: random
• Background weevil population high in many beds
• Weevils may be working– EWM may be lower than it would be otherwise
– These weevils cannot reproduce at a prolific rate
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Should we spend money on weevils?• No one offering weevils for sale in WI• ES sold eggs, not adults• $$$, but no harm• Lake groups raise their own weevils?• To be effective to combat EWM
weevils need:• High EWM density • Absence of physical and chemical
disturbance• Few weevil‐eating fish• Proximity to good shore habitat• No chemical treatment
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Lessons Learned• Lakes are messy and
complicated
• Field experiments are tough
• Find a very large hammer