1. The study examines the larvae of the beetle Hexodon unicolor unicolor in rice crops in Madagascar and finds it can act as a "soil engineer". 2. Experiments show the abundance of H. unicolor larvae depends on the season and type of crop residues present, with lower densities in crops with Raphanus sativus and Crotalaria grahamiana residues. 3. The mortality of H. unicolor larvae is higher in systems without residues and lower in those with legume residues like soybeans and Stylosanthes guianensis. Rice growth is better with those residues and cow manure, irrespective of larvae presence.

1. 1
Larvae
of
Scarabeoidea
(called white grubs) can be
beneficial to rice growth: case
study on Hexodon unicolor
unicolor (Dynastidae) on the
Highlands of Madagascar
Randriamanantsoa R., Raharinindrina S., Rabary B.; Blanchart
E., Quaranta B.; Naudin K.., Ratnadass A., Rafamatanantsoa E. ;
Rakotosolofo H.
1

2. 2
WHITE GRUBS IN UPLAND RICE
CROPS IN MADAGASCAR
Taxonomic and
functional diversity
White grubs= main
pests on upland crops

3. 3
Damages
Crown dilaceration
Main families
causing damages on
upland rice:
Dynastidae and
Melolonthidae
Root attack

4. 4
Rhizophagous :
Apycencya
waterlotii
White grubs
feeding diet
Saprophagous:
Euryiomia
argentea
Facultative
rhizophagous:
Heteroconus
paradoxus

5. 5
Case study: Hexodon unicolor unicolor
acting as a soil engineer
Abundance in rice crops
PITFALL TRAPS
Rice-based agrosystems with:
Brachiaria mulato, Crotalaria
grahamiana; Raphanus sativus;
no mulch
Behaviour and rice growth
Residues of different types:
Brachiaria mulato, Crotalaria
grahamiana; Raphanus sativus;
no mulch

6. 6
Mean of adults density/m2
80
RESULT
S
70
60
Brachiaria mulato
50
40
Crotalaria
grahamiana
30
Raphanus sativus
20
No residu
10
Abundance of
H. unicolor
unicolor in
different
systems
0
June
August
Dates of observation
Adult =omnivorous, present
in fields only during the
rainy season
Mean of larval density/m2
March
80
70
60
50
Brachiaria
40
Crotalaire
30
Population of larvae
20
depending on the season
10
Low density in systems with
0
Raphanus sativus et Crotalaria
grahamiana
Radis fourrager
Sans résidus
Mars
Juin
Sampling
Août

7. 7
Burrowing
activity
Burrow
opening
Residues of
Stylosanthes
guianensis
Incorporation of
residues in the
soil
Residues of Soya
bean

8. 8
Effect of residue
types on the
mortality of H.
unicolor larvae
Larval mortalty rate
100
80
60
40
20
0
Cow dung
Maize
No residu
Soya bean
Stylosanthès
guianensis
Types of residu
The mortality of H. unicolor unicolor larvae is higher in
the absence of residues and lower for legumes residues

9. 9
35
d
30
d cd
bcd bcdbcd
Indirect effects of H.
unicolor larvae on rice
growth in systems with
dead mulches
25
(cm)
abc ab a
20
a
Rice growth higher with soya
and stylosanthes residues or
with cow manure than with
maize residues or absence of
residues, irrespective of the
presence of larvae
15
10
5
0
R
RH
RM RMH
RJ
RJH
RS
RSH
RF
RFH
Treatments
RF= rice+cow dung; RFH= rice+cow dung+larva ; RJ= rice+soya
residu; RJH= rice+soya residu +larva; RM= rice+maize;
RMH= rice+maize residu+larva ; RR= rice; RRH= rice + larva;;
RS= rice+stylosanthes residu; RSH= rice+stylosanthes residu
+larva ;
Légumes residues and
cow manure much more
decomposed by larvae

10. 10
CONCLUSIONS
Not all Scarabeoidea larvae are harmful
for plants
Viability of larvae depends on the
presence/absence and th type of organic
matter (residues or manure)
Larvae could have beneficial functions for
soil and plants by incorporating and
regulating the mineralisation of soil
organic matter.

11. 11