Análisis Estadístico

Clara I. Nicholls and Miguel A. Altieri

Division of Insect Biology, 201 Wellman, University of California, Berkeley CA 94720 e-mail: [email protected]

Introduction

One the major effects of the simplification of agricultural landscapes through mono- cultures in California, is a decrease in the abundance and activity of the natural enemies of agricultural pests, due to the disappearance of habitats providing them with critical food resources and overwintering sites (Corbett & Rosenheim, 1996). Many scientists are concerned that, with accelerating rates of habitat removal, the contribution to pest suppression by biocontrol agents using these habitats will decline further (Fry, 1995; Sotherton, 1984), thus increasing insecticide use with consequent negative effects on the sustainability of agroecosystems.

To halt or reverse this decline in natural controls, many researchers have proposed ways of increasing the vegetational diversity of agricultural landscapes as it is known that biological pest suppresion is more effective in diverse cropping systems than in mono- cultures (Andow 1991, Altieri 1994). One such biodiversification method, employed in vineyards and orchards, is to plant cover crops, a tactic designed to maintain habitats for natural enemies and thus enhance their populations. Reductions in mite (Flaherty, 1969) and grape leafhopper populations (Daane et al., 1998) have been observed with winter cover crop plantings, but such biological suppression has not been sufficient from an economic point of view (Daane & Costello, 1998).

A main constraint is that winter cover crops are mowed or plowed under at the begin- ning of the growing season, leaving the systems as virtual monocultures by early sum- mer. Natural enemies need a green cover for habitat and alternative food during the ent- ire growing season. One way to achieve this condition is to sow summer cover crops that bloom early and throughout the season, thus providing a highly consistent, abun- dant and well-dispersed alternative food source, as well as microhabitats, for a diverse community of natural enemies. Our main goal was to test whether the presence of neut- ral insects and pollen and nectar in summer cover crops provides a constant and abun- dant supply of food sources for natural enemies, thus decoupling predators and parasi- toids from a strict dependence on grape herbivores, and allowing natural enemies to build up in the system and keep pest populations at acceptable levels.

Another biodiversification option is the maintenance or planting of vegetation adjacent to crop fields (Thomas et al., 1991; Nentwing et al., 1998). Ideally, such areas provide alternative food and refuge for predators and parasitoids, thereby increasing natural e- nemy abundance and colonization of neighboring crops (Altieri, 1994; Corbett & Plant 1993; Coombes & Sotherton, 1984, Fry, 1995; Wratten, 1988). Several studies indicate that the abundance and diversity of entomophagous insects within a field is dependent

on the plant species composition of the surrounding vegetation, and also on its spatial extent and arrangement, which affects the distance to which natural enemies disperse into the crop (Lewis 1965; Pollard, 1968).

The classic study by Doutt and Nakata (1973) in California was pioneering in determi- ning the role of riparian habitats, and especially of wild blackberry patches, near viney- ards in enhancing the effectiveness of the wasp Anagrus epos in parasitizing the grape leafhopper (Erythroneura elegantula). Later, research by Kido et al. (1984) established that French prunes adjacent to vineyards could also serve as overwintering sites for A.

epos, and Murphy et al. (1996) detected higher leafhopper parasitism in grape vineyards with adjacent prune tree refuges than in vineyards lacking refuges. Corbett and Rosen- heim (1996), however, determined that the effect of prune refuges was limited to a few vine rows downwind and A. epos exhibited a gradual decline in vineyards with increa- sing distance from the refuge. This finding indicates an important limitation in the use of prune trees for biological control protection in vineyards.

In order to ovrcome this limitation we borrowed from concepts of landscape ecology and tested the effects of an established vegetational corridor to enhance movement of beneficials beyond the “normal area of influence” of adjacent habitats or refuges. Corri- dors have long been used by conservation biologists for protecting biological diversity, as they provide multiple avenues for circulation and dispersal of biodiversity through the environment (Rosenberg et al., 1997). Such study is relevant in northern California’s Mendocino County, where most vineyards are interwoven in a matrix of riparian forests, thus providing ample opportunities for the study of arthropod colonization and inter- habitat exchange of arthropods, especially those restricted to the interstices between agricultural and uncultivated land.

The corridor, which was connected to a riparian forest cutting across a monoculture vineyard, allowed for testing whether such a strip of vegetation could enhance the bio- logical control of insect pests in a vineyard. We were interested in evaluating if the cor- ridor acted as a consistent, abundant, and well-dispersed source of alternative food and habitat for a diverse community of generalist predators and parasitoids, allowing pre- dator and parasitoid populations to develop in the area of influence of the corridor well in advance of vineyard pest populations. We also thought that the corridor would serve as a biological highway for the dispersion of predators and parasitoids within the viney- ard, thus providing protection against insect pests within some area of influence.

As the vineyard was also diversified with cover crops, we could test another hypothesis: that the presence of neutral insects and pollen and nectar in summer cover crops provi- des a constant and abundant supply of food sources for natural enemies, thus decoupling predators and parasitoids from a strict dependence on grape herbivores, and allowing natural enemies to build up in the system and keep pest populations at acceptable levels. We tested this hypothesis and examined the ecological mechanisms associated with insect pest reduction when summer cover crops were planted early in the season bet- ween alternate vine rows.

Materials and methods

This study was conducted in two adjacent organic Chardonnay vineyard blocks (blocks A and B, 2.5 ha each) from April to September, in 1996 and 1997. Both vineyards blocks were surrounded on the north side by riparian forest vegetation, but block A was penetrated and dissected by a five meter wide and 300 meter-long vegetational corridor composed of 65 different species of flowering plants. The vineyard was located in Hopland, 200 km north of San Francisco, California, in a typical wine-growing region . Before and during the study, both blocks were under organic management, yearly plan- ted to winter cover crops every other row, receiving an average of 2 tons of compost per hectare and preventive applications of sulfur against Botrytis spp. and Oidium spp. Corridor

To determine if the corridor influenced the species diversity and abundance of ento- mophagous insects in the adjacent vineyard we placed ten yellow and ten blue sticky traps were placed at different points within the vineyard at increasing distances from the corridor or the bare edge (rows 1, 5, 15, 25, 45) in blocks A and B respectively) to mo- nitor diversity and abundance of the entomofauna. Yellow sticky traps were used to monitor leafhoppers, the egg parasitoid Anagrus epos, and various predator species. Blue sticky traps were mainly used to assess thrips and Orius populations. Traps were oriented perpendicular to the predominant wind direction and positioned above the vine canopy. Traps were deployed beginning in April and replaced weekly throughout the 1996 and 1997 growing seasons. All traps were returned to the laboratory and examined with a dissection microscope to count the number of phytophagous insects and associa- ted natural enemies on the traps.

In the same rows where sticky traps were placed, grape leaves were visually examined in the field and the number of E. elegantula nymphs recorded. Populations of leafhop- per nymphs were weekly estimated on 10 randomly selected leaves in each row.

Cover crop blocks

Half of each block was kept free of ground vegetation by one spring and one late sum- mer disking (the monoculture vineyard). In April, the other two halves of both blocks (the cover-cropped vineyard) were undersown every alternate row with a 30/70 mixture of sunflower and buckwheat. Buckwheat flowered from late May to July and sunflower bloomed from July to the end of the season.

From April to September of 1996 and 1997, relative seasonal abundance and diversity of phytophagous insects and associated natural enemies were monitored on the vines in both treatment plots. Ten yellow and ten blue sticky traps (10 by 17 cm [Seabright La- boratories, Emeryville, CA] coated with tanglefoot) were placed in each of 10 rows se- lected at random in each block to estimate densities of adult leafhopper, thrips, Anagrus

wasps, Orius sp. and other predators.

In the same rows where sticky traps were placed, grape leaves were visually examined in the field and the number of E. elegantula nymphs recorded. Populations of leafhop- per nymphs were estimated on 10 randomly selected leaves in each row. This sampling method was carried out in sections with and without cover crops, allowing one to de-

termine quickly and reliably the proportion of infested leaves, densities of nymphs, and rates of leafhopper egg parasitization by the Anagrus wasp (Flaherty et al., 1992, Mur- phy et al., 1996).

In order to determine whether cover crop mowing forced movement of natural enemies from cover crops to vines, three different selected cover crop rows in block B were subjected to mowing three times each year. Both years, 5 yellow and 5 blue sticky traps were placed in the three random rows with cover crops every time they were mowed, and in three random rows that were not mowed.

Results and discussion

Influence of the corridor on leafhoppers and thrips

In both years in block A, adult leafhoppers exhibited a clear density gradient, reaching lowest numbers in vine rows near the corridor and forest and increasing in numbers to- wards the center of the field, away from the adjacent vegetation. The highest concentra- tion of leafhoppers occurred after the first 20–25 rows (30-40 meters) downwind from the corridor. Such gradient was not apparent in block B, where the lack of the corridor resulted in a uniform dispersal pattern of leafhoppers (Fig. 1, similar trends were obser- ved in 1997). Nymphal populations behaved similarly, reaching highest numbers in the center rows of block A in both years. Apparently, the area of influence of the corridor extended 15–20 rows (25-30 meters), whereas the area of influence of the forest on nymphs reached 10–15 rows (20-25meters)as evident from 1997 catches. Nymphs were similarly distributed over the whole block-B field.

A similar population and distribution gradient was apparent for thrips in both years. In both years catches in block A were substantially higher in the central rows than in rows adjacent to the forest; catches were particularly low in rows near the corridor. In block B there were no differences in catches between the central and bare edge rows, although catches near the forest were lowest, especially during 1997.

Response of natural enemies

Generalist predators in the families Coccinellidae, Chrysopidae, Nabidae, and Syrphidae exhibited a density gradient in block A, clearly indicating that the abundance and spatial distribution of these insects was influenced by the presence of the forest and the corri- dor, which channeled dispersal of the insects into adjacent vines (Fig. 2, similar trends were observed in 1996). Predators were more homogeneously distributed in block B, as no differences in spatial pattern in predator catches was observed between bare edge and central rows, although their abundance tended to be higher in rows close to the fo- rest (10-15meters).

Leaf examination revealed high levels of parasitism across leafhopper generations for both 1996 and 1997 in both blocks . Eggs in center rows had slightly higher mean para- sitization rates than eggs located in rows near the forest or corridor. The proportion of eggs parasitized tended to be uniformly distributed across all rows in both blocks. It is assumed that the presence of the forest and corridor was associated with the colonizati- on of A. epos but this did not result in a net season-long prevalence in E. elegantula egg parasitism rates in rows adjacent to such habitats.

Density responses of the grape leafhopper to summer cover crops

In both years, densities of adult leafhoppers were significantly lower throughout the season (except on 6/27 and 7/18 in 1996 and early in the summer in 1997) on vines with summer cover crops than on monoculture vines (Fig. 3, t=2.612, df=10, p<0.05).

Comparing the cover-cropped vineyard with the monoculture shows that increasing plant diversity also results in a decrease in the number of leafhopper nymphs. During 1996, nymphal densities were generally lower on vines in cover-cropped sections. Dif- ferences were not statistically significant, however, from August 15 until the end of the season (t=2.31, df=13, p<0.05). In 1997 significantly lower abundance levels of nymphs on cover-cropped vines were evident from July 9 onward (t=2.50, df=6, p<0.05).

Effects of cover crops on Anagrus populations and parasitization rates

During 1996 the mean densities of Anagrus present on yellow sticky traps placed on cover-cropped and monoculture vineyard sections were similar, although towards the end of the season Anagrus attained significantly greater numbers in the monoculture. Similarly during 1997, a year in which elevated capture rates were evident, sampling revealed significantly higher numbers of Anagrus in the monoculture starting in late July (Fig. 7, t=2.41, df=9, p<0.05). Clearly, A. epos was more abundant in the vineyard monocultures associated with higher host densities. There was no consistent relationship between leafhopper abundance and the measures of parasitism done in this study. No statistical differences in parasitization rates were detected between treatments in both years.

Effects of cover crops on thrips and general predators

Densities of thrips, as revealed by blue sticky trap captures in 1996, were significantly lower (t=2.37, df=9, p<0.05) in cover-cropped vineyards than in monocultures, and re- mained lower throughout the growing season . Such differences were also apparent in 1997, a year of extreme thrips pressure.

Table 1 gives the numbers of predators from cover-cropped and monoculture systems. The predators include spiders, Nabis sp., Orius sp., Geocoris sp., Coccinellidae, and

Chrysoperla sp. Generally, the populations were low early in the season and increased as prey became more numerous during the season. The table shows that, during 1996, general predator populations on the vines tended to be higher in the cover-cropped sec- tions than in the monocultures.

Table 1. Monthly mean densities* (± SE) of various arthropod predator species on vines with and without summer cover crops (Hopland, California. 1996)

Orius Spiders Coccinelli- dae Geocoris sp. Nabis sp. Chrysoperla sp. w/ cover crop June July Aug 3 ± 0.7 5 ± 1.9 4 ± 2.0 3 ± 1.3 9 ± 3.4 12 ± 3.7 0 4 ± 1.9 1 ± 0.8 0 2 ± 1.7 4 ± 2.3 1 ± 0.3 1 ± 0.6 2 ± 1.1 3 ± 2.2 5 ± 3.1 2 ± 1.0 w/o cover crop June July Aug 2 ±1.3 3 ± 0.9 2 ± 0.8 2 ± 1.1 8 ± 2.6 9 ± 3.4 2 ± 0.7 2 ± 0.4 1 ± 0.3 0 1 ± 0.5 2 ± 0.9 0 0 1 ± 0.7 2 ± 0.7 4 ± 1.5 2 ± 0.8 *Number of individuals per 25-m D-Vac transect

D-Vac sampling of cover crops in both blocks revealed that in 1996 the most abundant predator present on the flowers of buckwheat and sunflowers was Orius, followed by several species of Coccinellidae. Among the spiders, members of the family Thomisidae were the most common.

Effects of cover crop mowing on leafhoppers and A. epos

To determine if mowing influenced leafhopper abundance in 1997, leafhopper densities were assessed on vines selected before and after mowing compared to numbers on vines where cover crops were not mowed. Before mowing, leafhopper nymphal densities on vines were similar in the selected cover-cropped rows. One week after mowing, num- bers of nymphs declined on vines where the cover crop was mowed, coinciding with an increase in Anagrus densities in mowed cover crop rows. During the second week, this decline was even more pronounced (t=2.93, df=4, p<0.05), although by then differences in Anagrus numbers between mowed and not mowed rows were not significant (Fig. 4). Conclusions

Our studies showed that cover crops harbored a large number of Orius, coccinellids, thomisid spiders and a few other predator species, which tended to be more abundant in the cover cropped vineyard blocks compared to the clean cultivated systems. Our analy- sis does reveal that greater densities of predators was correlated with lower leafhopper numbers and this relationship is most clear-cut in the case of the Orius–thrips interac- tion.

The mowing experiment suggests a direct ecological linkage, as the cutting of the cover- crop vegetation forced the movement of the Anagrus and predators harbored by the flo- wers, resulting in a decline of leafhopper numbers on the vines adjacent to the mowed cover crops in both years. We suggest the need for more research in order to better de- termine the timing of mowing in relation to the biology of the leafhopper and the phe- nology of the vine and cover crops.

This research also indicates that dispersal and subsequent within-vineyard densities of herbivores and associated natural enemies is influenced by the forest edge and the corri- dor. The presence of riparian habitats enhances predator colonization of and abundance in adjacent vineyards, although this influence is limited by the distance to which natural

enemies can disperse into the vineyard (Corbett & Plant, 1993). The corridor, however, amplifies this influence by allowing enhanced and timely circulation and dispersal mo- vement of predators into the center of the field. The great availability of pollen and nectar displayed by the various flowers of the corridor, as well as the diversity and pre- valence of neutral insects, attracted high numbers of generalist predators. In turn, this increased the impact of predators, especially in vine rows close to the corridor.

The data obtained in this study point to two main conclusions:

• Habitat diversification using summer cover crops supports season-long high popu- lations of predators, thereby favoring enhanced biological control of leafhoppers and thrips in vineyards.

• The creation of corridors across vineyards can serve as a key strategy for allowing natural enemies emerging from riparian forests to disperse over large areas of o- therwise monoculture systems. Such corridors should be composed of locally adap- ted plant species exhibiting sequential flowering periods, which attract and harbor an abundant diversity of predators and parasitoids. These corridors or strips, which may link various crop fields and riparian forest remnants, can create a network of habitat allowing many species of beneficial insects to disperse throughout whole agricultural regions, transcending farm boundaries (Baudry, 1984).

Our study suggets that it is possible to restore natural controls in agroecosystems through vegetation diversification, thus providing a robust ecological foundation for the design of pest-stable and sustainable vineyards in northern California and elsewhere in the mediterranean world.

References

Altieri, M.A. 1994. Biodiversity and Pest Management in Agroecosystems. Haworth Press, New York.

Andow, D. A. 1991. Vegetational diversity and arthropod population response. Annual Review of Ento- mology 36: 561-586.

Baudry, J. (1984). Effects of landscape structure on biological communities: the cases of hedgerows network landscapes. In: J. Brandt & P. Agger (eds). Methodology in Landscape Ecological Research and Planning. Roskilde University Center. Denmark, Vol. 1., pp. 55-65.