arthropod communities in temperate agroforestry: theory and reality w. terrell stamps, terry l....
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Arthropod Communities In Temperate Agroforestry:
Theory and Reality
W. Terrell Stamps, Terry L. WoodsRobert L. McGraw, and Marc J. Linit
Division of Plant Sciences & UM Center for AgroforestryUniversity of Missouri, Columbia, MO, USA
Temperate versus Tropical Agroforestry
Differ in almost every respectEnvironment – soil, climate, topographyPlant speciesArthropod species – pests and beneficialsDesign and management
Do theories of biodiversity proven from the tropics translate to temperate agroforestry practices?
Insect Diversity in Agroecosystems:The Theory
Animal diversity correlates with plant diversity (Murdoch et al.
1972) The tropics as an example.
Temporal and structural diversity increases arthropod diversity
Trees harbor more a diverse community than herbaceous plants (Lawton and Schroder 1977, Strong and Levin 1979, Niemala et al. 1982).
Size does matterSpecies-area relationship and the equilibrium theory of island biogeography more area, more species (Connor and McCoy 1979,
MacArthur and Wilson 1967).
Insect Diversity in Agroecosystems:The Theory
Within-field vegetation diversity reduces pest populations
E.g. intercropped systems pests compared to monocropped systems
Many theories involving all aspects of the ecology of the system have been proposed
Insect Diversity in Agroecosystems:The Theory
Associational Resistance Theory(Tahvanainen and Root 1972)
Multispecies plant associations have a synergistic interaction that reduces insect damage compared to single species plant systems.
Enemies Hypothesis
Resource Concentration Hypothesis
Appropriate-Inappropriate Landing Hypothesis
Host Plant Quality Hypothesis
Enemies Hypothesis
Predicts that natural enemies will be more abundant in complex systems vs simple systems, and that their action will result in lower herbivore population densities
Enemies Hypothesis
Complex Systems
refugia prey variety
prey abundance nectar and pollen
Cue disruption host finding rates
Complex Systems
refugia prey variety
prey abundance nectar and pollen
Cue disruption host finding rates
Simple Systems
refugia prey variety
prey abundance nectar and pollen
No cue disruptionNo effect
Simple Systems
refugia prey variety
prey abundance nectar and pollen
No cue disruptionNo effect
Resource Concentration Hypothesis
Predicts that insect herbivores are more likely to locate and remain on host plants that occur in large, dense, pure stands
Resource ConcentrationHypothesis
Complex Systems
Less apparent emigration visual cues
Chemical cue disruption
Food sources
Complex Systems
Less apparent emigration visual cues
Chemical cue disruption
Food sources
Simple Systems
More apparent emigration visual cues
Chemical cues
Single food source
Simple Systems
More apparent emigration visual cues
Chemical cues
Single food source
Arthropod Communities in Temperate Agroforestry: The Reality
Crop Polyculture vs MonocultureConsiderable evidence of the positive effects of multiple plant systems
Forestry Polyculture vs MonocultureFewer studies - some evidence of positive effects
Agroforestry vs Traditional AgricultureStudies are lacking
Our Studies
Ecological theories predict ↑ biodiversity and improved management of insect pests in agroforestry versus conventional agriculture
We are examining the impact of agroforestry alley cropping practices on insect populations
Objectives of our Research
Is arthropod diversity greater in alley cropped alfalfa than in conventionally grown alfalfa?
Are pests less abundant and natural enemies more abundant in an agroforestry setting
Economics - Is crop quality and quantity affected and can pesticide use be reduced?
Eastern Black Walnut, Juglans nigra
Valued for both the nut crop and the wood
Adds “sustainability” to agricultural practices
May provide ecological benefits when added to traditional
agricultural practices
Alfalfa, Medicago sativa
4th most widely grown crop in U.S.
Very high nutritional quality – used for feed
Commercial hay production
Dairy and beef cattle
Horses, sheep, goats
Prone to pest problems
Chemical controls
Mechanical controls
Project #1 & Project #2 SW Missouri – Sho-Neff Plantation
Project 2
Alley cropped
Monocropped
Project 1
Alley cropped
Monocropped
Study #1 Alfalfa & brome alley croppedwith black walnut
“Pilot study” small plots (12 M x 36 M)
small monocrop area, few control plots
Alfalfa, smooth bromegrass and vegetation-free alleyways included
SamplingSweeps down the center of plots
Pitfall traps diagonal across plots
Herbivore Individuals in Alfalfa: Agroforestry vs. Conventional Plots
0
50
100
150
200
300
500
1997 1998
# I n
di v
idu
a ls
AC alfalfa (left bars) MC alfalfa (right bars)
a b a b
Predator Individuals in Alfalfa: Agroforestry vs. Conventional Plots
0
5
10
15
30
1997 1998
AC alfalfa (left bars) MC alfalfa (right bars)
PredatorsPredatorsa b a b
ParasiticHymenops
a bParasitic
Hymenops
a a
# I n
di v
idu
als
Diversity Indices
Index Alley cropped Monocropped
Shannon (H’) 1.8 + 0.4a 1.6 + 0.4bSimpson (1/D) 5.0 + 2.1a 3.8 + 1.7bEvenness (E) 0.7 + 0.2a 0.6 + 0.2b
Study #1 Conclusions
2X as many natural enemies in alley cropped alfalfa
½ as many herbivores in alley cropped alfalfa
Arthropod community more diverse and even in alley cropped alfalfa
Studies #2 & #3
Study 2: 12 M (40 ft) alleyways versus traditionally-grown alfalfa
½ acre reps – 4 alley cropped, 4 monocropped
Study 3: 12 M and 24 M (80 ft) alleyways versus traditionally-grown alfalfa
Wider alleys to improve crop yields4 reps each
SamplingMultiple sweep samples down a central transectCollect pest samples for parasitoid activityAlfalfa sampling for quality / yield
Arthropods in Alfalfa: Agroforestry vs. Conventional Plots
Parasitic Hymenoptera
a
b
Predators
a a
Herbivores
a b
MC alfalfa (right bars)
0
5
10
15
20
25
30
35
AC (left bars)
# I n
di v
idu
als
0
100
200
300
400
Alfalfa Weevil Larvae Mortality (%)
Alley Cropped Monocropped
Healthy AWA 35 + 14a 42 + 12aBathyplectes 46 + 10a 37 + 14bZoophthora 17 + 5a 11 + 11aNematode 1 + 2a 1 + 2aUnk Mortality 2 + 2a 11 + 9b Total Parasitism 63 + 12a 48 + 9b
aa
Yields over the season
0
100
200
300
400
500
Dry
Wei
gh
t ( g
m/ m
2 )
September CuttingJune CuttingMay Cutting
b ba
b
Alley cropped (AC) Monocropped (MC)
Study #2 Conclusions
Arthropod communities more diverse in AC alfalfa than in MC alfalfa (trend, not significant)
AC alfalfa has fewer herbivores and more parasitic hymenoptera than MC alfalfa
Higher rates of parasitism of AWL in AC alfalfa vs MC alfalfa
Poor yields in alleyways
Taxa and individuals in AC and MC alfalfa
Treatment Total taxa Total individ
Monocrop 15 + 3a 64 + 19a
12 M alley 22 + 4b 84 + 22b
24 M alley 19 + 4b 84 + 14b
Herbivore taxa and individuals in AC and MC alfalfa
Treatment Herb taxa Herb individ
Monocrop 7 + 2a 38 + 10a
12 M alley 8 + 2a 45 + 16a
24 M alley 7 + 2a 43 + 9a
Predator taxa and individuals in AC and MC alfalfa
Treatment Pred taxa Pred individ
Monocrop 4 + 2a 5 + 3a
12 M alley 8 + 3b 10 + 4b
24 M alley 7 + 1b 9 + 3b
Parasitic hymenoptera taxa and individuals in AC and MC alfalfa
Treatment Para taxa Para individ
Monocrop 2 + 1a 2 + 1a
12 M alley 4 + 2b 5 + 2b
24 M alley 4 + 1b 5 + 2b
Alfalfa Weevil Larvae Mortality (%)31 March 2004
Monocrop 12 M alley 24 M alley
Healthy AW 48 + 10a 44 + 14a 45 + 4aBathyplectes 38 + 14a 25 + 11a 28 + 9aZoophthora 15 + 6a 31 + 15b* 33 + 7bNematode --- --- ---
Total Para 53 + 10a 56 + 14a 55 + 4a
Higher fungal parasitism in the alleyways
Alfalfa Weevil Larvae Mortality (%)16 April 2004
Monocrop 12 M alley 24 M alley
Healthy AW 41 + 12a 19 + 11b 26 + 7bBathyplectes 29 + 16a 30 + 24a 16 + 3aZoophthora 30 + 4a 51 + 32b* 58 + 9bNematode --- --- ---
Total Para 59 + 10a 81 + 11b 74 + 8a
Higher fungal parasitism in the alleyways, fewer adults
Alfalfa Weevil Larvae Mortality (%)4 April 2005
Monocrop 12 M alley 24 M alley
Healthy AW 73 + 13a 53 + 13b 59 + 6bBathyplectes 10 + 8a 33 + 9b 27 + 20bZoophthora 17 + 5a 14 + 5a 17 + 10aNematode --- --- 2 + 5
Total Para 27 + 13a 48 + 13b 42 + 11b*
Higher bathy parasitism in the alleyways, fewer adults
Alfalfa Weevil Larvae Mortality (%)18 April 2005
Monocrop 12 M alley 24 M alley
Healthy AW 13 + 13a 3 + 5a 5 + 5aBathyplectes 14 + 12a 38 + 8b 19 + 15aZoophthora 74 + 20a 57 + 6b 74 + 13aNematode --- 2 + 5 ---
Total Para 88 + 13a 95 + 6a 93 + 9a
Fungal epizootic killed most of the larvae
a bba
cb
Yield over the Season (Study 2)
a0
100
200
300
400
500
Dry
Wei
gh
t ( g
m/ m
2 )
September CuttingJune CuttingMay Cutting
b
AC12 AC24
b
Open
Conclusions
Reality seems to follow theory, so farMore diverse arthropod community in the more diverse agroforestry practice vs monocrop
Evidence for Enemies Hypothesis• More predators and parasitic hymenoptera in AC alfalfa vs
MC alfalfa• Higher parasitism rates of alfalfa weevil in AC alfalfa vs
MC alfalfa
Impact on the economics still to be determined, but yield in the wider alleyways looks promising
Recommendations?
Don’t grow alfalfa in 40 ft alleyways
Consider growing alfalfa in the center 60 ft of 80 ft alleyways
Impact on pest management unknown, but promising
“Intangible” environmental benefits numerousIncreased plant diversity
Increased arthropod diversity
Environmentally sound, sustainable practice