The impact of weed diversity on insect population dynamics and crop yield in collards, Brassica oleraceae (Brassicaceae)

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 (cm) 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 (cm), 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.

Plant species diversity for sustainable management of crop pests and diseases in agroecosystems: a review

“…Such conflicting effects were reported by Theunissen and Schelling (1996) in the case of leek when undercropped with clover (against onion thrips and leek rust), Smith (1976) in the case of Brussels sprouts with weed cover and Schellhorn and Sork (1997) in the case of collards interplanted with weeds.…”

“…Finch and Kienegger 1997) This is limited by competition by non-productive plants for resources with the main crop (e.g. Schellhorn and Sork 1997) Deterrent diversion Repellent semiochemicals produced by a plant that is intercropped or undersown result in bottom-up effects against pests of the main crop (e.g. Kimani et al 2000) This process may also attract non-target pests to the crop (e.g.…”

Population densities of herbivorous lepidopterans in diverse cruciferous cropping habitats: Effects of mixed cropping and using a living mulch

“…Certain insect pests associated with Brassica crops (e.g., flea beetles, aphids, and cabbage root flies) are consistently found at lower densities on Brassica plants in the presence of non-host vegetation (Smith, 1976;Horn, 1981;Altieri and Gliessman, 1983;Andow et al, 1986;Hofsvang, 1991;Costello and Altieri, 1995;Langer, 1996). The responses of other Brassica pests (e.g., lepidopterans) are less predictable, and the type of non-host plant present affects their population responses (Latheef and Irwin, 1979;Theunissen and Schelling, 1996;Schellhorn and Sork, 1997).…”

“…Several studies have investigated the population dynamics of insect herbivores in vegetationally diverse Brassica plantings. In most instances, herbivore populations were lower in diversified plantings compared with monospecific stands (Pimentel, 1961;Bach and Tabashnik, 1990;Theunissen et al, 1995;Schellhorn and Sork, 1997). Root (1973) proposed two hypotheses why herbivores would be less abundant in more diverse cropping systems.…”

Management options for the conservation of rare arable plants in Europe

“…Such adaptation may not only have occurred in plants, but, due to functional relations, also in associated fauna such as pollinators, avifauna and other groups of organisms linked to the arable flora. By supporting ecological services such as insect pollination , soil conservation (Weil 1982), food and habitat provision for natural enemies of pests (Schellhorn and Sork 1997;Nentwig, Frank, and Lethmayer 1998), and the provision of aesthetic and social ecosystem services, arable plants have also benefited human wellbeing. For all these reasons, the rationale for conservation of the arable flora is more evident in regions where arable plants, although introduced, have been long-established, compared to regions where arable farming has been introduced more recently.…”