1992 Annual Report for ANC92-012
Impact of Tree Windbreaks on Distribution of Insect Pests and their Natural Enemies in Sustainable Agricultural Systems
Summary
Objectives:
1) To determine the impact of tree windbreaks on the distribution and abundance of crop pests
and their natural enemies.
2) To determine the impact on crop yield of pest populations influenced by tree windbreaks.
Methods:
Vertebrate and invertebrate (insect) natural enemies of insects, and insect pests were sampled
periodically during 1992-1994 in crop fields (corn, soybeans, grain sorghum, wheat and
cantaloupe) sheltered and unsheltered by tree windbreaks. Field margins of these fields and tree
windbreaks were also sampled. Crop fields with and without adjacent woody areas with
waterways (riparian areas) were sampled for bird species, using standard census observation
procedures. Insects were sampled by sweep nets, pitfall traps, sticky traps and tree beating
techniques, depending on the habitat.
Results:
During the winter months, few bird species were observed in crop fields. However, bird species
diversity and abundance during the winter was higher in habitats with woody borders. During the
spring and summer months, bird species diversity was higher in crop fields with woody borders,
but bird abundance was not influenced by the presence or absence of woody borders. Of the 64
bird species observed in habitats with tree windbreaks during the spring and summer, 54 species
eat insects as either their sole diet or as part of their diet.
Except for three unsheltered grain sorghum fields located close to each other (which had high
numbers of aphids in July), few insects pests were collected in grain crop fields during the
summer. The most common arthropod predators were spiders, carabid beetles and ants. Spiders
were most abundant close to windbreaks and grassy field edges Spider numbers were also higher
in windbreaks containing coniferous trees compared with sites dominated by deciduous trees.
Abundance of striped cucumber beetles, southern corn rootworms and northern corn rootworms
on sticky traps was similar in exposed and sheltered muskmelon plots. Western corn rootworm
were significantly more abundant in exposed plots. More lady beetles and ichnemonid wasps
were caught on traps in sheltered plots than exposed plots, especially during June. Few spiders,
lacewings, or braconid wasps were caught at either location. Total arthropod abundance in
pitfalls was higher in sheltered muskmelon sites than in exposed sites but did not vary in alfalfa.
Total annual pest and predator abundance was similar in sheltered and exposed pitfalls in
muskmelon and alfalfa plots.
Abundance of ants, spiders, and carabids, the most common predators, and Orthoptera, the most
common pest group, varied with crop, year, month and treatment. Total pest and rove beetle
abundance in muskmelon pitfalls and total predator and rove beetle abundance in alfalfa pitfalls
varied significantly with month and treatment more than year. Total annual arthropod, pest and
predator abundance in muskmelon and alfalfa sweep samples did not vary among sheltered and
exposed sites.
Monthly abundance of total predators in muskmelon sweeps and total pests in alfalfa sweeps
varied significantly in more than one year. Availability of pests and climatic conditions
influenced predator abundance. Total annual arthropod and predator abundance within
windbreaks varied with tree species and was significantly higher on pines. Pest abundance varied
with month and was significantly higher on pines. Abundance of spiders, ants, lacewings,
harvestman, and other predators was highly variable during the year. Availability of prey, foliage
and climatic conditions may have influenced predator abundance on trees.
Impact:
The results of this project indicate that field-edge vegetation benefits a wide variety of bird
species, including insectivores and omnivores that likely have value as natural enemies of crop
pests. Further, neotropical migrant bird species, many of which are in decline, benefit from the
woody vegetation as habitat during migration or for nesting, and the woody vegetation appears
generally important to wildlife conservation in this agricultural area. Herbaceous edges
contribute habitat for additional species. Predation pressure on insect crop pests from birds and
small mammals, based on predator foraging locations, appears to be highest within 50m of field
edges but occurs throughout fields at least 200m from edges.
Woody corridors in agricultural landscapes can help reduce soil erosion, shelter crops from wind
damage and desiccation, enhance moisture conservation, and serve as filters for field run-off,
important for ground and surface water quality. They also provide wildlife habitat important to a
variety of species in intensively farmed landscapes. The woody vegetation and associated
wildlife provide recreational and aesthetic benefits in rural areas and may enhance
the quality of life for farm families.