1991 Annual Report for LNC91-039
A Landscape Ecological Perspective in Insect and Weed Population Regulation in Low-Input and Conventional Systems
Summary
Objectives:
1) Characterize landscape structure of an established low-input dairy/crop farm and nearby
conventional farms.
2) Contrast insect population regulation in low-input and conventional cropping systems in
relation to farm landscape structure.
3) Characterize weed population regulation and distribution in relation to landscape structure.
Methods:
Studies were completed to determine the impact of agricultural landscape structure on the
biological regulation of pest insects and weeds. Overall goals were to determine the impact that
factors such as field size, shape, type of crop, field border composition, distance to alternative
habitats, etc., have on the biological control of crop pests. These studies tested the hypothesis
that landscape structure plays a pivotal role in determining the effectiveness of many species of
predators and parasites in agricultural systems by fostering or inhibiting their populations over
time.
Results:
An analysis of landscape structure conducted on two areas on Ingham County, Michigan in 1992
quantified significant differences in physical structure between an area of high structural
complexity vs. an identically sized area two miles to the north of low structural complexity. The
area of high heterogeneity had significantly more and smaller fields with smaller perimeters and
shorter distances to an edge from field center. Crop-hedgerow edges were significantly more
abundant in the high heterogeneity area while crop-crop interfaces were more abundant in the
low heterogeneity area.
Previous studies have shown that weeded field edges benefit Eriborus terebrans, currently the
most important parasite of the European corn borer of the Midwest. In this study it was found
that Eriborus wasps require both a source of adult food (sugar) and a moderated microclimate for
optimum survival. Corn fields do not provide these resources (during the first generation of the
wasp) and wasps are forced to obtain these resources in alternate habitats. These studies have
shown that nectar from flowering plants or the secretions of aphids feeding on weeds in
field-edge habitats provide a suitable source of adult food. Fencerows and woods surrounding
corn fields also provide a cooler, more humid microclimate than a corn field and wasps in these
habitats live significantly longer. These studies demonstrate that landscape structure can play an
important role in determining the effectiveness of natural enemies.
Studies of weed seed predation revealed that during the winter of 1991-92, vertebrates removed
6-12 percent of the seeds on the soil surface of crop fields in a six day period. In the spring,
insects and vertebrates removed 48.5 percent of the seeds within 5 meters of hedgerow and 35
percent at 100 meters from a hedgerow in a six week period. Carabid beetles were the most
abundant insect predator and rodents and birds were the primary insect vertebrate seed feeders.
In 1992-93, up to 40 percent of large weed seeds were removed over the winter and up to 66
percent of the small weed seeds in the spring.
Potential Contributions and Practical Applications:
Findings on individual objectives point towards the potential for natural population regulation
(biological control) of insects and weeds through an understanding of the impact of landscape
structure on these processes. This would result in a decreased need to rely solely on chemical
pest control, which would have both economic and environmental impacts.