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Effects of mosquito control on the nontarget fauna in the CamargueBrigitte Poulin, Tour du Valat

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1965

: Mosquito control is initiated along 300 km of Mediterranean French coast (red area) by the EID (Inter-departmental Alliance for mosquito-control) for tourism development. Various insecticides from organochlorates

(DDT) to organophosphates (temephos, fenithrotion) are successively used. The Camargue

(green

area) is already recognized for its exceptional biodiversity and is preserved from mosquito control.

Historical context of mosquito control in southern France

2003

: The Regional Natural Park of the Camargue

(PNRC) declares itself favourable to control mosquitoes in the Camargue

with Bacillus thuringiensis israelensis (Bti) after a small-scale impact study showed little effects of Bti on chironomids, a major taxa

in wetland food webs showingsensitivity to Bti.

2003 -

2005: Mapping of all mosquito-producing habitats in the Camargue and estimation of mosquito-

control cost by the EID.

April 2005: Approbation by the PNRC of mosquito control on 2500 of the 25 000 ha of mosquito-

producing habitats in the Camargue if accompanied by impact studies on the nontarget fauna.

August 2006: Launch of Bti spraying in the Camargue.

France

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Because Bti has low toxicity, is highly selective and used in temporary wetlands with a variable hydrology over time and space, the proposed studies seek for potential indirect effects through the food web based on the comparison of treated and control sites over a relatively long period (5 years).

(1)

Reed invertebrates serving as food to passerines at 15 sites including 5 treated with Bti.

(2) Population size, food provisioning rates, diet and breeding success of house martins Delichon urbicum at 3 control and 3 treated sites.

Impact studies carried out by Tour du Valat under PNRC contract

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Reed marshes are the second habitat contributing most to mosquito production in southern France. Five breeding species of passerines are reed specialists: the reed warbler

a migratory abundant and generalist

species, the great reed warbler

a migratory species vulnerable in France, the Moustached warbler

a

resident species vulnerable in Europe, the Bearded tit, a resident species with a fragmentary distribution, and a Mediterranean sub-species of the Reed bunting,

which is rare and declining

1. Reed invertebrates

Criteria justifying the selection of this biological model:

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A former study carried out by Tour du

Valat

in 1998-1999 (Biol Conserv107:315-325) showed that reed passerine abundance is positively correlated with food availability, the latter being inversely related to the duration of ground dryness between June and December of the preceding year. Hence, it is possible to predict invertebrate availability and bird abundances from reed marsh hydrology.

0

5

10

15

20

25

30

35r = 0.73

P < 0.0001

200 300 400 500 600 700

r = 0.60P < 0.001

100

200

300

400

500

600

700

800

0 10 20 30 40 50 60 70 80

Food

ava

ilabi

lity

inde

x

Food availability index

Dry ground duration (w

eeks)

Reed passerine abundance

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Methods

500 sweep-net strokes in the reed vegetation to estimate relative food availability once during the passerine breeding season (mid May-early June) in midday when the reeds are dry and the wind low at 10 control and 5 treated sites.

Monthly measures of water levels to estimate the duration of ground dryness in marshes at all sites.

p = % of prey category i in diet samples

xij = number of items from prey category i captured at site j

yi = total number of items from prey category i captured∑

pi xij / yi wheren

i = 1

Food availability estimation (J Field Ornithol. 68: 426-442):

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Mosquito-producing habitats (EID)Bti treated areas

Pin Fourcat

St-Andiol

Armellière

Sambuc

Salin de Giraud

Port

St-Louis

Control sitesTreated sites

Study sites

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Results 2009

3942523019/05/09Mourgues6043165112/06/09Pin Fourcat

3/8-18148123002124829/05/09They Roustan*5/8-20135241502017910/06/09Bélugue*

15/8-38761102242537509/06/09Palissade*0-902875197237422/06/09Palunette (MdV)0-18541732321915902/06/09Canisson (MdV)0-14841732682244702/06/09Pisci Sud (MdV)0-16228751251918512/06/09Consécanière016122263832024619/05/09Rousty0-873843.52972622110/06/09Sollac0-6848124132533518/05/09Esquineau (TdV)0-2228752652423915/05/09

Rendez-Vous (TdV)

0-16641732512238811/06/09Baisse salée (TdV)

07735244292740018/05/09Verdier (TdV)

Number of treatments 2008/2009

Observed – predicted food index

Predicted food index

Ground dryness

(months)

Food availability

index

Number of prey

categories

Number of captures

DateSites

*Treated sites: the number of treatments refers precisely to the reed marshes sampled.

77 0

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Overall, mean food availability index is +22 at control sitesand -338 at treated sites relative to the predicted value based on hydrology

when mean food index from the reference study is calibrated at 0.

This difference would correspond to a 47% decrease in reed passerine abundance at treated sites relative to control sites.

-550

-450

-350

-250

-150

-50

50

150

Control sites Treated sites

Mea

nfo

odav

aila

bilit

yin

dex

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A nested-ANOVA on the 2007-2009 data revealed a significant impact of mosquito control (F(1,19) = 36.2; P = 0.00001), which contributes to explain 91% of the observed variance, compared to 6% for years and 3% for sites, when food availability is corrected based on marsh hydrology.

Year Site Treatment0

20

40

60

80

100

% o

f exp

lain

edva

rianc

e

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60.2061.720.0 ± 0.10.1 ± 0.0Insect pupae

100.7800.080.0 ± 0.40.1 ± 0.2Neuroptera

130.5920.300.0 ± 0.20.1 ± 0.1Orthoptera

80.6760.180.0 ± 0.60.3 ± 0.2Insect eggs*

190.0564.160.0 ± 0.20.3 ± 0.1Lepidoptera

40.0236.100.5 ± 0.20.0 ± 0.1Lepidoptera larvae*

310.1771.970.2 ± 0.71.3 ± 0.3Hymenoptera (ants)

270.7000.150.9 ± 0.70.6 ± 0.3Insect larvae*

270.4680.551.0 ± 0.50.6 ± 0.2Odonata

150.0544.230.0 ± 0.81.8 ±0.3Pseudoscorpionidae

420.4980.480.5 ± 1.61.7 ±0.6Gastropoda

540.2591.362.3 ± 3.36.5 ± 1.4Heteroptera*

250.00311.169.3 ± 2.02.0 ± 0.8Acarina

900.7900.0713.7 ± 10.316.7 ±4.2Thysanoptera

980.1032.9312.7 ± 8.7128.8 ± 3.6Coleoptera

960.00410.9011.0 ± 7.336.9 ± 3.0Araneae*

980.9940.0036.8 ± 15.537.0 ± 6.4Hymenoptera (wasps)

1000.2191.6137.1 ± 31.880.7 ± 13.2Homoptera*

1000.1042.9271.7 ± 75.9211.8 ± 31.1Diptera*

Percent occurrence

PF (1,19)Treated sitesmean ± SE

Control sitesmean ± SE

Taxa

*

Taxa of which the abundance was corrected according to the hydrology

Analyses according to taxa abundance in addition to food availability

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Criteria justifying the selection of this species as biological model:

Nest in colony outside buildings in human-inhabited areas including where mosquito nuisance aims to be reduced.

35% of diet if made of Nematocera (mosquitoes and non-biting midges) = taxon showing the highest sensitivity to Bti

Low nest predation, which facilitates the study of the relationships between food availability and breeding success.

Feed on the wing on flying insects within a 500-m radius around nests.

2. House martins Delichon urbicum

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Description of the parameters studied & methods

1271 nests counted since 2006

2. Provisioning rates: number of feeding flights during 2h in late afternoon for 10 nests/site at 3 periods (early June, mid June & early July)

9051 flights observed since 2006

3. Diet: Collect of 2 feces under 10 nests/site at 3 periods (early June, mid June & early July)

380 samples totalling 14 857 identified prey since 2006

68 nests visited 26 times on average in 2009

4. Breeding success: Visit of all nests accessible from an 8-m high ladder 2 X / week using an endoscope

1. Population size: number of occupied nests in June

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Mosquito-producing habitats (EID)

House martin colonies at treated sitesHouse martin colonies at control sites

Areas treated with Bti

Breeding success estimated

Pin Fourcat

St-Andiol

Armellière

Sambuc

Salin de Giraud

Port

St-Louis

Location of the house martin colonies studied

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05928rural3. Pin Fourcat

243132urban2. Port St-Louis

1413019urban1. Salin-de-Giraud

Treated sites

122613rural3. Armellière

07835urban + rural2. Saint-Andiol

189424urban1. Sambuc

Control sites

% mosquito biotope(400-m radius)

EnvironmentColonies

Characteristics of the studied colonies

Total number of nests (2009)

Number of nests monitored (2009)

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2006 2007 2008 20090

20

40

60

80

100

120

140Sambuc

Saint-Andiol

Armellière

Salin-de-Giraud

Port St-Louis

Pin Fourcat

1. Population size

Similar trends at all sites, suggesting that population size is primarily influenced by survival during the migration and wintering periods.

Colonial swallows shown to respond to anthropogenic environmental variations with a 6-7-yr delay.

Num

ber o

f occ

upie

dne

sts

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2. Provisioning rates

Provisioning rates also associated with energetic demands of growing chicks

Rate of chick feeding significantly lower at treated sites, suggesting lower food levels

Control sites Treated sites0

0.2

0.4

0.6

0.8

1

1.2

F(1, 94) = 3.07P = 0.083

No.

of f

eedi

ng fl

ight

s / 5

min

eggs age 1 age 2 age 3 age 40

0.2

0.4

0.6

0.8

1

1.2

1.4

F(4, 99) = 8.65P < 0.00001

Development of chicks

No. of feeding flights / 5 m

in

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Initial state: chick diet similar at Sambuc and Salin in 2006

Prop

ortio

n of

pre

y ta

ken

0

10

20

30

40

50

0-2,5 2,5-5 5-7,5 7,5-10 10-15 >150

10

20

30

40

50

60

Prey taxonomyF(8, 11) = 2.43, ns

Prey sizeF(6, 13) = 0.48, ns

3. DietFl

ying

ant

s

Col

eopt

era

Was

ps

Het

erop

tera

Dip

tera

Hom

oper

a

Nem

atoc

era

Odo

nata

Ara

neae

Lepi

dopt

era

Size categories (mm)

Salin de Giraud

Sambuc Proportion of prey taken

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P = 0.001P = 0.00004control3 vs. treated3P = 0.00003P < 0.00001control3 vs. treated2P = 0.0005P = 0.0008control3 vs. treated1P = 0.003P < 0.00001control2 vs. treated3

P = 0.0006P < 0.00001control2 vs. treated2P = 0.0004P < 0.00001control2 vs. treated1P < 0.00001P < 0.00001control1 vs. treated3P = 0.00001P = 0.00001control1 vs. treated2P < 0.00001P < 0.00001control1 vs. treated12009

P = 0.00001P < 0.00001control2 vs. treated2P = 0.00001P < 0.00001P < 0.00001P < 0.00001P < 0.00001P < 0.000012008

P = 0.002P = 0.022007

nsnscontrol1 vs. treated12006

Prey size1Prey taxa1SitesYear

Pairwise intra-annual comparisons between control/treated sites (ANOVAs)

1All P-values remain significant with Bonferroni correction

control1 vs. treated1

control1 vs. treated1control1 vs. treated2control2 vs. treated1

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Chick diet comparison between control and treated sites in 2007,

2008 & 2009:Mosquito spraying alone contributes to ≈

60% of the variance

Year Period Site Nest Treatment0

10

20

30

40

50

60

70

Prey taxa

Prey size

******

***

***

*** ******

***%

of e

xpla

ined

var

ianc

e

Nested ANOVA, prey taxa: F(9,311) = 14.28, P > 0.00001; prey size: F(6,314) = 17.20, P > 0.00001

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F(1, 319) = 22.93P < 0.000010

4

8

12

16

20Nematocera

F(1, 319) = 72.97P < 0.00001

0

0.3

0.6

0.9

1.2Odonata

F(1, 319) = 9.72P = 0.00199

0

4

8

12

16

20Formicidae

F(1, 319) = 8.65P = 0.0035

Control sites Treated sites0

0.2

0.4

0.6

0.8Araneae

Control sites Treated sites

Mea

n nu

mbe

r of p

rey

foun

d in

chi

cks

faec

es

Lower intake of Nematocera and their predators at treated sites,partially compensated by higher intake of flying ants

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0 0

1

2

3

40-2.5 mm

F(1, 319) = 18.80P = 0.00002 0

2

4

6

87.5-10 mm

F(1, 319) = 22.44P < 0.00001

0

0.2

0.4

0.6

0.8

1

1.2> 15 mm

F(1, 319) = 57.74P < 0.00001

0

0.2

0.4

0.6

0.8

110-15 mm

F(1, 319) = 11.61P = 0.00074

Control sites Treated sites Control sites Treated sites

Num

ber o

f pre

y fo

und

in c

hick

faec

es

Higher intake of small prey at treated sites and of large prey at control sites: prey selection made primarily by size in swallows, with larger prey being more

profitable energetically

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0

1

2

3

4

5

6Control sites

Treated sites

Clutch size

F(1, 64) = 8.48P = 0.005

No of chicks at fledging

F(1, 64) = 15.03P = 0.0002

No of chicksat hatching

F(1, 64) =7.28P = 0.009

4. Breeding success (2009)

Num

ber o

f egg

s or

chi

cks

Similar laying date (15 May)

Similar hatching rate (80 % at control vs. 75% at treated sites)

Higher proportion of 2nd clutches at treated sites (63 vs. 40%)

Overall breeding success

F(1, 64) = 8.91P = 0.004

Clutch size and number of chicks fledged significantly lower at treated sites: first demonstration of potential long-term effect of Bti on vertebrate populations

Overall breeding success: 2.3 chicks at treated sites vs. 3.2 at control sites

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Observed number of chicks at fledging

0

1

2

3

4

5

6

0 1 2 3 4 5 6

All sites

R = 0.517P = 0.0098

0

1

2

3

4

5

6

0 1 2 3 4 5 6

Control sites

R = 0.683P = 0.015

Pre

dict

ed n

umbe

r of c

hick

s at

fled

ging

Breeding success is positively correlated with the intake of Nematocera, Odonata and Araneae at the nest level (GRM).

Link between diet and breeding success

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Conclusion

No study had ever succeeded in demonstrating a significant indirect effect of Bti on the nontarget fauna, and on birds in particular. Why?

Excellent experimental conditions with intensive and efficient mosquito control (46 and 41 aerial spraying cumulating 5093 and 5282 ha in 2008 and 2009)

and with

control areas having as much probabilities of producing Nematocera as treated ones.

For the house martin study: effect potentially accentuated in inhabited areas where the low arthropod diversity reduces alternative prey and translates into little individual variability increasing the statistical power of tests.

Bti impact studies largely limited to direct effects in the laboratory, with field studies on indirect effects at several trophic levels still being scarce.