varroa mite be good for broom control & why size matters for the broom seed beetle
Post on 15-Jan-2016
43 Views
Preview:
DESCRIPTION
TRANSCRIPT
Varroa mite be good for broom control& why size matters for the broom seed
beetle
Quentin Paynter, Yvonne Buckley, Alanna Main, Simon V. Fowler, Hugh Gourlay & Paul Peterson
Scotch broom Cytisus scoparius L. (Link) Fabaceae
• Minor weed in native W Europe, but highly invasive in NZ, Australia, USA etc.
• To try to understand why, scientists have conducted extensive ecological studies in native & exotic range for many years…
Waloff & Richards (1977) J. Appl. Ecol: 10-year study in UK (native range) using insecticides resulted in: o Increased growth o Reduced mortalityo 4x higher seed production
Memmott et al. (2002) Acta Oecologia:o Fewer insects & empty niches occur on NZ
broom, compared to the native range
Enemy Release?
Rees & Paynter (1997) J. Appl. Ecol
• Simulation models predict variation in 3 factors could explain weediness in NZ 1. Disturbance (e.g. heathland fire; pig rooting)
2. Longevity of broom plants
3. Ability to recolonise after death of parent plant
• Paynter et al. (1998): In France (native range), few seedlings established beneath existing broom; establishment linked to disturbance (e.g. cultivation, fire)
• Sheppard et al. (2002): In Australia, seedlings established under broom & in absence of major disturbance
• Paynter et al. (2003): Confirmed seedlings established under mature plants in Australia & NZ but rarely in France, Spain.Native & exotic plant longevity similar.
Invasiveness in exotic range is due to higher seedling establishment, especially under existing broom stands, so broom stands persist
WHY?
Waloff & Richards (1977) indicated insect herbivory reduced seed rain by ~75%.
Modelling1 indicated that this should have no effect on broom populations...
Increased seedling establishment unlikely to be explained by higher fecundity
1Rees, M., Paynter, Q. (1997) J. Appl. Ecol., 34, 1203-1221.
Pro
po
rtio
n o
f s i
tes
wit
h b
roo
m
...but assumed seed set of 19,000 seeds m-2
Was the model realistic? Pollination & seed-set in NZ
• Broom flowers must be tripped open to set seed• 7 sites surveyed throughout 2005 & 2006 flowering
seasons (N & S Islands; sea level to ~800 m) • Only honeybees & bumblebees Bombus terrestris
opened broom flowers (honeybees ~70% of flowers)
• Mean = 4175 seeds m-2 <<Rees & Paynter (1997) assumed. Range 59-14,443 seeds m-2; strongly correlated to honeybee abundance
Seed rain
Introduced bees (mainly honeybees) drive broom invasion in NZ!
y = 6664.8x - 68.156
R2 = 0.7961; P<0.001
1
10
100
1000
10000
100000
0 1 2 3
Total honeybee visit rate (flowers/min)
Se
ed
ra
in (
see
ds/
m2 )
y = 23510x + 1649.6
R2 = 0.2132; n.s.
1
10
100
1000
10000
100000
0 0.2 0.4
Total bumblebee visit rate(flowers/min)
Se
ed
ra
in (
see
ds/
m2 )
y = 6772.4x - 764.18
R2 = 0.8704; P<0.001
1
10
100
1000
10000
100000
0 1 2 3
Total (honeybee + Bumblebee visit rate)
Se
ed
ra
in (
see
ds/
m2 )
• Bumblebee contribution minor, but Bumblebee + honeybee visitation rate fits data significantly better than honeybees alone
Varroa destructor
Photo: http://www.ars.usda.gov/is/AR/archive/jul09/mites0709.htm
Implications for broom control
• The broom seed beetle Bruchidius villosus is found throughout broom’s native range
• First released in NZ in 1987, now abundant at first release sites, where is destroys an average 73% of seed & spreading elsewhere
• We modelled the impacts of varroa & biocontrol
1) honeybees present, no biocontrol (solid line)2) honeybees & biocontrol (73% seeds eaten by B. villosus: dotted line)3) no honeybees, no biocontrol (dashed line)4) no honeybees & biocontrol (dot & dash line)
Simulated broom cover at 3 under 4 scenarios
Seed-feeders & varroa can potentially drive broom extinct!
• BUT, varroa-treated commercial beehives often maintain broom invasion, even in the presence of varroa & B. villosus
• Management of commercial beehives (e.g. banning beehives from DOC land) could be used as part of an integrated broom management programme at many sites in NZ
Implications for broom control
Seed quantity versus seed quality?
Models have only considered seed quantity
• Buckley et al (2003) Ecology: on average broom seeds are c. 30% heavier in the exotic versus native range
Importance of seed size?
• Studies on other species have shown seed size influences early growth & survival of seedlings
• Preliminary work (Buckley, unpublished data) indicates seedlings grown from large broom seeds have higher survival in shaded conditions, compared to seedlings grown from small seeds
• Assuming seed size is a heritable trait, what selection pressures might influence seed size?
One B. villosus larva develops within a single broom seed.
We hypothesised that adult beetle size is constrained by seed size, i.e. big seeds should result in bigger beetles
If bigger beetles exploit plants better than small beetles, this could explain selection for small seed size
Could a seed-feeding herbivore select for smaller
seeds?
Seed size impacts on Bruchidius
y = 0.0525x + 0.835
R2 = 0.725, P<0.001
1
1.1
1.2
1.3
1.4
1.5
1.6
6 8 10 12 14
Mean seed weight (mg)
Mea
n el
ytra
are
a (m
m2)
We collected & weighed seeds & measured beetles (elytra area) at 14 localities in NZ
Mean beetle size was closely-correlated with mean seed size at each site i.e. big seeds do produce big beetles!
Bruchidius size & fecundity
We caged individual ♀ B. villosus on broom pods with 2 ♂s, food (pollen) + water & counted eggs laid & measured female elytra area after death
Noisy data (field-collected beetles), but significant correlation: biggest beetles laid, on average, ~30-40% more eggs than smallest beetles
y = 75.58x - 21.96r² = 0.283, P<0.01
0
50
100
150
200
250
0.5 1 1.5 2
No.
egg
s la
id
Elytra area mm2
Overwintering survival: preliminary data
• We compared beetle size distributions before & after winter at Palmerston North (NZ). Small beetles had low winter survival: 54% of beetles had elytra area < 1.15 mm2 before winter versus only 16% of beetles after winter
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0.5
0.8 1 1.2 1.4 1.6 More
Elytra area (mm2)
Fre
quen
cy
Newly Emerged
Post-winterPalmerston N
Preliminary conclusions & speculation: Bruchidius & broom seed size
• B. villosus should exploit big-seeded broom better than small-seeded broom because bigger beetles that develop within bigger seeds lay more eggs & survive winter better than small beetles
• High seed destruction should result in selection pressure for broom plants that produce small seeds, with potential knock-on impacts on the competitive ability/invasiveness of broom seedlings
Summary
• Invasion meltdown: broom would not be invasive in NZ without introduced bees
• Pollinator-limitation due to varroa mite can reduce seed rain below threshold levels for seed-feeding biocontrol agents to work
• B. villosus may also reduce broom invasiveness by selecting for broom plants that produce smaller seeds
Thank you!
Acknowledgements:
Tony Scott (Hanmer Bees) & Dr Barry Donovan
This study was funded by the Foundation for Research Science & Technology, contracts C09X0504 & CO9X0905
top related