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Influence of Landscape Diversity and Flowering Cover Crops on Biological Control of the Western Grape Leafhopper (Erythroneura elegantula Osborn) in North Coast Vineyards

  • Author(s): Wilson, Sam Houston
  • Advisor(s): Altieri, Miguel A.
  • et al.
Abstract

Modern agriculture is characterized by specialized production and the use of monoculture

cropping practices. These agroecosystems are at once concentrating habitat for crop pests and

eliminating habitat for natural enemies. Multiple studies have demonstrated that such changes

can lead to a decrease or total loss of biological control of pests. At the same time, expansion of

monoculture cropping systems across entire agricultural regions has led to the creation of

landscapes that are entirely dominated by a small number of crops and devoid of natural

habitats. In the same way, entire regions can experience a reduction or loss of biological control

to agriculture.

As such, a number of studies have compared crop fields with high and low habitat diversity and

found that diversified cropping systems tend to have enhance natural enemy populations and

increased biological control of pests. At the same time, another set of studies have demonstrated

that monoculture cropping systems can still experience high levels of biological control so long

as they are situated in a landscape with high levels of habitat diversity surrounding them. More

recently, it has been proposed that the use of on-farm habitat diversification to enhance

biological control will likely be influenced by the area and quality of natural habitat surrounding

the farm (i.e. landscape diversity).

This dissertation was designed to evaluate the influence of habitat diversity at the local and

landscape scale on biological control of the Western grape leafhopper (Erythroneura elegantula

Osborn; Hemiptera: Cicadellidae) in North Coast wine grape vineyards. The key parasitoids of E.

elegantula are Anagrus eryhthroneurae S. Trjapitzin & Chiappini and A. daanei Triapitsyn

(Hymenoptera: Mymaridae). These Anagrus parasitoids are intimately tied to the natural habitats

that surround vineyards due to the fact that in order for them to successfully overwinter they

must parasitize an alternate leafhopper host that overwinters in an egg stage (E. elegantula

overwinters in the vineyard as an adult). These alternate leafhopper hosts are known to reside in

the natural and semi-natural habitats that surround North Coast vineyards. As such, it is thought

that biological control of E. elegantula in vineyards is particularly sensitive to changes in

landscape diversity. At the same time, the use of monoculture cropping practices results in a

vineyard environment that is very inhospitable to natural enemies of E. elegantula, including

Anagrus spp. Previous studies have demonstrated that without floral nectar (or an analogous

solution) the lifespan of Anagrus parasitoids can be less than two days and it may be that the

introduction of flowering cover crops into vineyards could possibly increase biological control of

E. elegantula by enhancing Anagrus longevity in the field. In this way, increased habitat diversity

at both the field and landscape scale may support increased natural enemy populations which

would lead to increased biological control of E. elegantula.

For this dissertation, a series of studies were conducted in order to evaluate how changes in

habitat diversity at the field and landscape scale could affect natural enemy populations and

ultimately influence biological control of E. elegantula. First, overwintering habitat of Anagrus

spp. was evaluated to identify the specific host plant species that contained leafhopper eggs that

these parasitoids were attacking in natural habitats during the winter, as well as throughout the

rest of the year. Second, vineyards that were adjacent to riparian habitat were studied in order

to evaluate how distance away from a large natural habitat patch influenced the timing, density

and impact of natural enemies in the vineyard. Third, in order to isolate the influence of

landscape diversity, a multi-year study was conducted to monitor biological control of E.

elegantula in a number of vineyard monocultures that were situated in low, intermediate and

high diversity landscapes. Finally, over the course of several years the use of flowering summer

cover crops was developed in collaboration with commercial wine grape growers and vineyard

trials were subsequently conducted to evaluate the ability of these flowering cover crops to

enhance biological control of E. elegantula. In order to evaluate how changes in the landscape

influenced the effectiveness of this on-farm habitat diversification practice, these cover crop

studies were conducted at multiple vineyards that were situated in low, intermediate and high

diversity landscapes.

Results from these studies indicate that the area and composition of natural habitats surrounding

vineyards can have a significant influence on biological control of E. elegantula. Reduced pest

populations in more diverse landscapes is thought to be the result of both reduced crop vigor as

well as increased natural enemy impact during the overwintering period. Early season

populations of Anagrus wasps were found in all vineyards regardless of landscape diversity,

implying a strong dispersal capacity from overwintering sites. The Anagrus demonstrated a

strong density dependent relationship with E. elegantula and this appeared to drive their

densities in vineyards much more so than changes in landscape diversity.

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