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

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Total honeybee visit rate (flowers/min)

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y = 23510x + 1649.6

R2 = 0.2132; n.s.

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Total bumblebee visit rate(flowers/min)

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y = 6772.4x - 764.18

R2 = 0.8704; P<0.001

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Total (honeybee + Bumblebee visit rate)

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• 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

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Mean seed weight (mg)

Mea

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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

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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

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Elytra area (mm2)

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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