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(1)

Deforestation, new migration pathways and outbreaks

of the Red locust Nomadacris septemfasciata

in the Sofia river basin (Madagascar)

Franc Alex, Soti V., Tran A., Leclair D., Duvallet G.,

Duranton J-F.

(2)

1 cm 1 cm

1 cm

1 cm

Nomadacris septemfasciata (Serville, 1838)

Orthoptera : Acrididae, Cyrtacanthacridinae

(3)

Distribution Map

(4)

Malagasy Bioclimates

Sofia river Basin

50 000 km²

Background:

Guillaumet et al. 1997

(5)

Red locust

distribution

in Madagascar

(6)
(7)
(8)
(9)
(10)

BANANA RAFFIA

MAIZE RICE

MANIOC

(11)

Biological cycle of the Red locust

(12)

For the first time, gregarious

populations in Sofia Area in 2003

Solitarious ,

Isolated and inoffensive for the crops

Gregarious ,

Grouped and devastating

Gregarisation threshold between 5 000 & 10 000 imagos /ha

(13)

To gregarise

Increase, multiplication and concentration of locust numbers,

Red locust needs 3 conditions:

– Good breeding rate, – Low mortality,

– Concentration by efficient migrations

(14)

Outbreak Area

Gregarisation occurs in specified areas :

– Geographically delimited,

– Share seasonal ecological complementarities

conditions, for the survival of the solitary locust every year,

– Migration pathways providing connectivity between ecological complementary areas

– And some years, with particular eco-meteorological

conditions, locust can gather, multiply and exceed its

gregarisation threshold.

(15)
(16)
(17)

Problematic

• Are some ecologic transformation, as deforestation, responsible of this new phenomenon?

• How did the connectivity change between Diapause and Reproduction areas?

• Does Sofia area become an Outbreak Area for the Red locust?

For the first time in Madagascar, the Red locust

reached gregarious phase between 1999 and 2003

in the Sofia area.

(18)

Methodology

• What are the ecological parameters to

delimit the red locust migration pathways?

• How to map them?

• Diachronic study using remote sensing data

to evaluate the evolution of the connectivity

between Reproduction and Diapause areas.

(19)

Ecological parameters for locust migration

Considering that:

• Solitary Red locust migrate at twilight, from 6 pm to 11 pm

• Forest is an hostile habitat to the locust.

• Optomotor reaction for the migration: locust follows the white patches distributed within black areas.

=> Deforestation corridors may show the way

(20)

Ecological parameters for locust migration

Tree main factors turn the locust migrations:

• Orography,

• Hydrography,

• Deforestation corridors: the only one may

change over time.

(21)

Methodology

Selection of SPOT data (1&4)

• Resolution of 20 meters

• 3 or 4 spectral bands (G, R, NIR, MIR).

1986 2004

April

July

(22)

Data processing

Georeferencing:

• Reference image: 1986 registered with GPS points

• Image 2004 : registered with reference

image

(23)

Data processing

The post-classification comparison method

was applied for Land-cover change detection:

• Robust even if images does not match perfectly

• Needs 2 separate classifications (1986 et 2004)

• Change-detection accuracy depends on the

accuracy of the 2 classifications

(24)

Land-cover classification

Supervised classifications using:

• GPS point for 2004,

• Photo-interpretation for 1986 (any field data available).

4 3 2 1

4 classes of Plant Community

Non Attractive: roads, water, urban habitat Grassy: savannas, meadows, steppes, crops Mixed: secondary forest, fallow

Forest: dry forest, gallery forest

(25)
(26)
(27)

Cost-distance

Cost-distance method to model the migration pathways:

• High cost for barriers

• Low cost for favourable environment

1 Non Attractive

1000 Slopes (> 30°): using MNT (90m)

1 Grassy

5 Mixed

20 Forest

Migration Cost

Plant Community

(28)

Results, classification

Evolution of Plant communities (total surface)

(29)

Results, classification

Evolution of Plant communities (mean surface)

(30)

• Classification 1986

1986 2004

(31)

Results

Evolution of migrations pathways

(32)
(33)

Conclusion (1)

• New migrations pathways due to deforestation

• Better connectivity between Diapause and Breeding areas

• Colonisation of new breeding areas now possible (west and south)

↗ ↗

↗ ↗ the number of breeding areas colonisable =>

↗ ↗

↗ ↗ the probability to find suitable breeding conditions each year =>

↗ ↗

↗ of solitary locust numbers, and possibility to

gregarise

(34)

Conclusion (2)

Conditions to fulfil for an outbreak area:

1. Seasonal ecological complementarities areas 2. Migration pathways between these areas.

Deforestation leads to a new outbreak area for a major crop pest

in the North of Madagascar

(35)

The end….

(36)

ALTITUDE

DEFORESTATION

(37)

Regional evolution of the migration

pathways

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