Host Plant Resistance

1
UNDERSTANDING THE ROLE OF INSECT PESTS ON
AFLATOXIN CONTAMINATION OF MAIZE IN WEST AFRICA
ACHIEVEMENTS AND PERSPECTIVES
M. SETAMOU1, C. BORGEMEISTER2 AND N. A.
BOSQUE-PEREZ3
1Texas AM University, TAES, 2415 E. Highway,
Weslaco, TX 78596
2University of Hannover, IPP, 30419 Herrenhaeuser
St. Hanover, Germany
3University of Idaho, Department of Plant, Soil
and Entomological Sciences, Moscow, IDA
2
Sub-Saharan Africa

• 500 million people
• 45-50 live below poverty line
• 30 children under 5 suffer from chronic
  malnutrition
• Annual population growth rate 2.7
• 1 billion people by 2025

Source World Bank
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Sub-Saharan Tropical Africa

• Poor soils
• High disease pest pressure
• Mostly low-input agriculture
• Labor intensive
• Small-scale farmers
• Complex farming systems

4
Sub-Saharan Tropical Africa

• 79 of population depend on agriculture for their
  livehood
• Lack of investment on agricultural research,
  training and extension

5
Sub-Saharan Tropical Africa

• Pest and disease control in food crops based on
• cultural practices
• biological control
• host plant resistance

6
Maize in Africa

• Most important cereal crop
• 15 million ha
• 19 million tons
• 1.2 tons/ha
• Staple for 100 million people

7
Maize processing
Maize is prepared in numerous different ways
before consumption
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Maize storage
Benin Republic
Ivory Coast

• Storage of grain in West Africa varies greatly
  between countries and ecological regions

9
Insect pests of maize

• Numerous insect pests damage maize in the fields
  and in storage
• Losses are substantial (10-100) and vary with
  agro-ecology

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Field insect pests of maize

• Lepidopterous borers are the most important field
  insect pests of maize in Africa
• They cause direct damage and increase incidence
  of ear and stalk rots

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Priority pests and their damage on maize
Damage symptoms caused to
- stems
- cobs
- by tunneling larvae
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Priority pests and their damage on maize
Main pest species are

• Busseola fusca Fuller (Lep. Noctuidae)
• Mussidia nigrivenella Ragonot (Lep. Pyralidae)
• Sesamia calamistis Hampson (Lep. Noctuidae)
• Eldana saccharina Walker (Lep. Pyralidae)
• Cryptophlebia leucotreta Meyrick (Lep.
  Tortricidae)

Mussidia
Sesamia
Eldana
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Priority pests and their damage on maize

• Two other lepidopterous pests commonly found on
  maize ears
• Mussidia nigrivenella
• Cryptophlebia leucotreta

Mussidia nigrivenella
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Insect pests of maize in storage
Main species

• Numerous insects attack maize ears pre and
  post-harvest
• Losses in storage are substantial and may reach
  25 or more

Sitophilus zeamais
Damage by Prostephanus truncatus
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Insect pests of maize in storage
Secondary species
- Carthatus quadicollis
- Carpophilus spp.
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Fungi infecting maize grains

• Numerous fungal species are also found infecting
  maize ears during pre and post-harvest
• Main species in West Africa
• - Aspergillus spp.
• - Fusariam spp.
• - Penicellium sp.

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Fungi infecting maize grains

• Severe ear rots infestations are common
• Drying and storage of grain are particularly
  difficult in humid areas, and this aggravates the
  problem

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Aspergillus flavus

• Dominant aflatoxin-producing fungus in Africa
• Prevalent on maize grain pre- and post -harvest
• Aflatoxin contamination is serious, but varies
  greatly between zones

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Fusarium moniliforme

• Prevalent on maize in Africa
• Produces the toxin fumonisin
• Like A. flavus it may invade kernels via cracks
  insect-damaged areas

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Interactions borer-fungi

• Fungal and borer infestations often occur together

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Objectives

• Determine incidence and distribution of A.
  flavus in Benin Republic
  
  
  
  
  
• Elucidate the role of damage of maize by insects
  before harvest on infection by A. flavus and
  contamination of grain with aflatoxin

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Methods

• Country-wide surveys in Benin in 1994-95
• 140 fields sampled
• Field trial to study interaction between M.
  nigrivenella and A. flavus
• Ears and kernels examined for borer damage and
  fungal presence
• Kernels assayed for A. flavus infection and
  aflatoxin contamination

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Results Infestation and distribution of Mussidia
nigrivenella in Benin

• Ear borer damage is recording in all the
  agro-ecological zones
• M. nigrivenella is the most abundant and
  damaging species

Setamou et al. 2000
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Results
A. flavus and aflatoxin contamination were
detected in all ecological zones Highest
infection rates and aflatoxin contamination
levels were detected in the southern Guinea
savanna kernels damaged by M.
nigrivenella was significantly correlated to A.
flavus infection and aflatoxin contamination
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Aspergillus flavus infection and aflatoxin
contamination in Benin
Setamou et al. 1997
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Aspergillus flavus infection as affected by grain
damage in pre-harvest maize in Benin
Setamou et al. 1998
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Aflatoxin contamination as affected by grain
damage in pre-harvest maize
Setamou et al. 1998
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Results

• Field trial revealed of grain infected by A.
  flavus and of samples contaminated with
  aflatoxin increased with increasing borer numbers

Mussidia nigrivenella damage
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Ear borer damage, A.flavus infection and
aflatoxin contamination in Benin
Setamou et al. 1998
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Results summary

• Grain damage by borers, specially M.
  nigrivenella, increased the susceptibility of
  maize to A. flavus and subsequent aflatoxin
  contamination.

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Hypothesis

• Control of insect pests in the field will not
  only result in a substantial increase of yield,
  but will also improve the grain quality through
  reduction of fungal infection and subsequent
  mycotoxin production

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Objectives

• Determine whether selection for improved plant
  type, ear aspect and borer resistance had
  resulted in improvement of resistance to
  infection by A. flavus and/or F. moniliforme
• Study relationships among grain flora fauna

Breeding populations
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Methods

• Two field trials in 1996
• 6 replications 6-row plots
• Four genotypes were compared
• TZBR-Eldana 1
• TZE Comp. 4
• Pool 16
• Gbogbe x TZSR-W-1

Selection for improved plant type
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Methods

• Artificial inoculation with fungal suspensions
• Row 2 A. flavus 5 10d past mid-silk
• Row 5 F. moniliforme 10 15d past mid-silk
• Rows 3 4 controls inoculated with water

A. flavus-infected ear
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Methods

• At harvest, 5 ears per treatment per rep taken
  at random rated for damage. Beetles and borers
  identified and counted by species.
• 25 kernels from the 5 ears were incubated and
  assessed for fungal infection by species.
• Remaining kernels ground and assayed for
  aflatoxin using Thomas method or for fumonisin
  using a Veratox kit.

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Fungal and insect infectionFirst season, Ibadan,
1996
Cardwell et al. 2000
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Fungal and insect infectionSecond season,
Ibadan, 1996
Cardwell et al. 2000
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Fungal and insect infection
Beetle numbers were significantly higher in
ears inoculated with F. moniliforme than in
other treatments on both seasons Borer numbers
were significantly higher in F. moniliforme-
inoculated ears than in other treatments during
1st season
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Effect of fungal inoculation on beetle density
Mean no. beetles/ five ears
Cardwell et al. 2000
Fungal inoculation
40
Effect of fungal inoculation on borer density
Mean no. borers/ five ears
Cardwell et al. 2000
Fungal inoculation
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Effect of fungal inoculation on borer density
The stem borer-resistant population,
TZBR-Eldana, had significantly lower borer
numbers than the other genotypes in the A.
flavus-inoculated rows Conversely, in the F.
moniliforme -inoculated rows TZBR-Eldana had the
highest number of borers of all genotypes
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Effect of fungal inoculation on borer density
In the presence of F. moniliforme, TZBR-Eldana
had increased numbers of E. saccharina and S.
calamistis and greater vulnerability to M.
nigrivenella Compared to the control, in the F.
moniliforme -inoculated rows TZBR-Eldana had a
35 increase in numbers of M.
nigrivenella
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How is this mediated?
Possible explanations Insects attracted to F.
moniliforme Fungal infection alters the host
plant resulting in greater attraction or
increased insect survival Multiple mechanisms
operating
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Follow-up studies
Higher survivorship of E. saccharina has been
observed on maize stems infected with F.
moniliforme All the potential mechanisms of
the interaction have not been explored
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Implications
These interactions had not been taken into
consideration in the past when breeding for
insect resistance Successful resistance
breeding requires understanding interactions
between different biotic constraints, as well as
between biotic and abiotic ones