Data from: Native grass ground covers provide multiple ecosystem services in Californian vineyards

1. The mechanisms responsible for the success or failure of agricultural
diversification are often unknown. Most studies of arthropod pest
management focus on enhancing the effectiveness of natural enemies, but
non-crop plants can also improve or hamper pest suppression by changing
the host quality of crop plants by reducing or adding available soil
nutrients or water. Native perennial ground covers may provide resources
and long-term habitat to resident natural enemies and be more compatible
than exotic annuals for crop management in terms of competition for soil
nutrients or water. 2. A three-year study was conducted in a California
vineyard to examine the impacts of native perennial grasses on pests,
natural enemies, crop plant condition and soil properties. Three ground
cover treatments were included: bare soil with a grower standard drip
irrigation, native grasses with the drip irrigation, or native grasses
with the drip irrigation and an additional flood irrigation to keep the
grasses green and growing during the season. 3. Numbers of leafhopper
pests (Erythroneura spp.) decreased in both native grass treatments, where
parasitism rates and spider densities were higher. 4. Nitrate levels in
vine leaf petioles were lower in grass treatments, indicating competition
with vines for soil nitrogen, which is most often considered to be
detrimental. Berry weight was higher in the irrigated treatment but did
not differ between the bare soil and non-irrigated native grass
treatments. Grape quality (brix) was similar in the bare soil and native
grass treatments, suggesting that increased soil moisture in the presence
of native grasses did not compromise grape quality. In fact, leaf water
stress was lower and available soil moisture higher not only in the
irrigated native grass treatment but, at times, in the non-irrigated
native grass treatment, in comparison to the no ground cover treatment. 5.
We conclude that native grasses contributed to a reduction in leafhopper
density by reducing host quality through competition with vines for soil
nitrogen and providing food resources and/or habitat for natural enemies.
Native grasses also improved soil water content and may be part of a water
conservation program for perennial crops in dry climate regions.