Species Interactions in Crop Communities - Agroecology: The Ecology of Sustainable Food Systems - page 211

TA B L E 1 5 . 2
Impact of Bellbeans ( Vicia faba ) and Cereal Rye ( Secale cereale ) on Various Factors of the Crop
Environment

Total Biomass, g/m 2 Weed Biomass, g/m 2 Cabbage Yield, kg/100m 2

Cover crop 1985 1986 1987 1986 1987 1987

Bellbeans 138 325 403 17.4 80.7 849.0

Rye 502 696 671 0.7 9.7 327.8

Rye/bellbeans 464 692 448 0.3 3.9 718.0

None (Control) n.d . 130 305 112.3 305.1 611.0

Data from Gliessman 1989; n.d. = data not determined.
Source: Gliessman, S. R. 1989. In C. H. Chou and G. R. Waller (eds.), Phytochemical Ecology: Allelochemicals, Mycotoxins and Insect
Pheromones and Allomones . pp. 69-80. Institute of Botany: Taipei, Taiwan.

Modification of the Cropping System Environment that provide habitat for the insect, and they can provide

alternative food sources such as pollen, nectar, foliage, or
prey (Marshall et al., 2003).
In a study where weed species were planted as narrow
border strips (0.25 m wide) around 5

Weeds can protect the soil surface from erosion through
root and foliar cover, take up nutrients that might other-
wise be leached from the system, add organic matter to
the soil, and selectively inhibit the development of more
noxious species through allelopathy. Most of these bene-
fits of weeds stem from the fact that ecologically weeds
are pioneer species, invading open or disturbed habitats,
and, through their reactions on the environment, initiate
the process of succession toward more complex commu-
nities. Most crop communities, especially those composed
of predominantly annual species, are simplified, disturbed
habitats. Weeds are especially well adapted to such con-
ditions. When we gain an understanding of the ecological
basis of the reactions of weeds on the crop environment,
we can utilize their interference in ways that reduce the
need for inputs from outside the crop community.

5 m plots of
cauliflower, it was found that certain pest insects were
reduced as a result of the increase in predatory or parasitic
beneficials (Ruiz-Rosado, 1984). For example, with the
weeds Spergula arvensis (corn spurry) and Chenopodium
album (lamb's quarters) planted in pure borders around
the crop, larvae and eggs of the common imported cabbage
worm ( Pieris rapae ) and the cabbage looper ( Trichoplusia ni )
were much more heavily parasitized by beneficials such
as the tachinid fly Madremyia saundersii . The adult
tachinids are attracted to the food sources provided by the
weeds, then search out prey on which to lay their eggs in
the crop nearby (Figure 15.5).
In another study, with the weed Spergula arvensis
planted in 1.0-m strips around a Brussels sprouts field, the
numbers of aphid-controlling beneficial insects collected
among the corn spurry rose considerably when the spurry
flowered (Linn, 1984). Presumably, the flowers provided
a nectar and pollen source for the beneficials. In addition,
predatory and parasitic wasps and Syrphid flies were com-
monly found feeding on the corn spurry flowers. Larger
numbers of beneficials were also found in sweep samples
on the Brussels sprouts, but only for a distance up to 5.0
m into the crop field. Other studies have shown similar
effects. Significant reductions in aphid populations
extended throughout the field when Spergula was a more
evenly distributed member of the weed/insect/crop com-
plex (Theunissen and van Duden, 1980). Leaving weedy
borders with grasses and legumes on the margins of corn
and soybean fields in Michigan was shown to greatly
increase the presence of predatory ground-dwelling
Carabid beetles in the crops (Landis et al., 2005).

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Control of Insect Pests by Promotion
of Beneficial Insects

Agriculture is usually concerned with keeping both weeds
and insects out of the production system. This takes large
amounts of external inputs to accomplish and does not
always provide the hoped-for results. When interactions
between weeds and insects are examined from an ecologi-
cal point of view, however, the possibility of retaining
weeds in the system in order to control the unwanted
insects emerges as an option. A body of literature is accu-
mulating, which supports the hypothesis that certain
weeds should be regarded as important components of the
crop community because of the positive effects they can
have on populations of beneficial insects (Altieri and
Nicholls, 2004b). Depending on the type of beneficial
insect, weeds can modify the microenvironment in ways