The impact of weed diversity on insect population dynamics and crop yield in collards, Brassica oleraceae (Brassicaceae) |
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Authors: | Nancy A Schellhorn Victoria L Sork |
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Institution: | (1) Department of Biology, University of Missouri-St. Louis, 8001 Natural Bridge Road, St. Louis, MO 63121-4499, USA, US |
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Abstract: | Vegetational diversity within agricultural fields is often suggested as a means to reduce insect herbivore populations and
to increase their natural enemies. In this paper we compare population densities of herbivores, predators, and parasitoids
on collards in monocultures and on collards interplanted with two different groups of weeds, one with weed species from the
same plant family as the collards (Brassicaceae) and one with weed species from unrelated plant families (non-Brassicaceae).
The collards in the Brassicaceae weed polyculture had higher densities (number of herbivores/mean leaf area (cm2) per plant) of specialist herbivores than collards in the non-Brassicaceae weed polyculture and in collard monoculture. The
“resource concentration” hypothesis is supported by the observation of higher populations of Phyllotreta spp., acting as facultative polyphages, in the Brassicaceae weed polyculture than in the non-Brassicaceae weed polyculture
where Phyllotreta spp. are facultative monophages. Population densities of natural enemies (mostly coccinellids, carabids, and staphylinids)
were higher in the polycultures than in the monoculture: carabid and staphylinid predators may be responsible for larval mortality
in the imported cabbage worm, Pieris␣rapae, and in the diamondback larvae, Plutella xylostella. In spite of differences in densities of specialist herbivores across treatments, crop yield, leaf area (cm2), the proportion of leaf area damaged, and the number of leaves undamaged did not differ. These findings suggest that plant
competition may interfere with attempts to reduce herbivore damage. We conclude that the use of weedy cultures can provide
effective means of reducing herbivores if the crop and weed species are not related and plant competition is prevented.
Received: 25 December 1995 / Accepted: 24 February 1997 |
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Keywords: | Polyculture Weeds Plant-insect interactions Phyllotreta spp Natural enemies |
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