Weevils and EWM: A 4‐year Field Experiment
Susan Knight and John Havel
April 1, 2016
Can Weevils Control EWM?
Little Bearskin Lake, Oneida County
AUTUMN adult flight
SPRING adult flight
Larval stem mining
WINTER
adult diapause
SUMMER
reproduction (≥ 10°C)
weak flight muscles Milfoil weevil life cycle
© J. Havel & A. Wempe 2014
Can Weevils Control EWM?
Experimental design
• 4 Lakes
• 4 Beds/lake
• 2 beds/lake augmented with weevils (provided by EnviroScience)
Little Bearskin Lake, Oneida County
• 2012 baseline year
• 2013 augmentation with weevils
• 2014, 2015 planned augmentation years, but cancelled Boot
Long
B
A
C
D
Little Bearskin
A
B
F
C
D
Manson
D
A
B
C
D
A
C
Hypotheses to Test
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
EWM biomass is highly variable, both among lakes and among beds.
Trends are similar among years.
In some beds, extra weevils were a major
addition to the existing weevil population but in many they were only a minor addition.
3.5
WEEVILS PER STEM
3.0
Background Density
2.5
Augmented Density
2.0
1.5
1.0
0.5
0.0
A B
C D
L Bearskin
A B
C D
Manson
A B
C D
Boot
A
F
C D
Long
More EWM stems are damaged with higher weevils density
100
% Stem Damage
80
r = 0.846
p < 0.001
60
40
20
0
0.0
0.5
1.0
1.5
Weevils/Stem
2.0
2.5
3.0
There is less EWM biomass with increased density of weevils
EWM biomass (g/m2)
250
200
2013
2014
2015
150
rho = -0.449
p = 0.002
100
50
0
0.0
0.5
1.0
1.5
Weevils/stem
2.0
2.5
3.0
3.5
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
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. 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
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
Lessons Learned
• Lakes are messy and complicated
• Field experiments are tough
• Find a very large hammer