The effects
of a vegetational corridor on the abundance and dispersal of insect
biodiversity within a northern California organic vineyard
Clara I. Nicholls
University
of California Cooperative Extension, Alameda County. 1131 Harbor Bay
Parkway, Suite 131 Alameda CA 94502,
USA
Michael Parrella
University
of California-Davis, Entomology Department, Davis, USA
Miguel A. Altieri
University
of California-Berkeley, Environmental Science Policy and Management
Department, Berkeley, CA 94520,
USA
Abstract
During
1996 and 1997, two adjacent 2.5 has organic vineyard blocks (A and
B) were monitored to assess the distributional and abundance patterns
of the Western grape leafhopper Erythroneura elegantula
Osborn (Homoptera: Cicadellidae) and its parasitoid Anagrus epos
Girault (Hymenoptera: Mymaridae), Western flower thrips Frankliniella
occidentalis (Pergande) and generalist predators. The main
difference between blocks was that block A was cut across by a corridor
composed of 65 flowering plant species which was connected to the
surrounding riparian habitat, whereas block B had no plant corridor.
In both years, leafhopper adults and nymphs and thrips tended to
be more numerous in the middle rows of block A and less abundant
in border rows close to the forest and corridor where predators
were more abundant. The complex of predators circulating through
the corridor moved to the adjacent vine rows and exerted a regulatory
impact on herbivores present in such rows. In block B all insects
were evenly distributed over the field, no obvious density gradient
was detected from the edges into the center of the field. Although
it is suspected that A. epos
depended on food resources of the corridor, it did not display a
gradient from this rich flowering area into the middle of the field.
Likewise no differences in rates of egg parasitism of leafhoppers
could be detected in vines near the corridor or in the vineyard
center. The presence of riparian habitats enhanced predator colonization
and abundance on adjacent vineyards, although this influence was
limited by the distance to which natural enemies dispersed into
the vineyard. However, the corridor amplified this influence by
enhancing timely circulation and dispersal movement of predators
into the center of the field.