The presence of weeds in a vineyard can cause several problems, including competition for resources between the weeds and the vines, which ultimately leads to a reduction in yield of up to 37% (up to 68% for cane weight). In order to resolve this issue, vineyards adopt special weed management practices, which span a wide range of control solutions. With conventional farming practices, herbicides are used under the vines, with cover crops planted between the rows.
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Weed management not only changes the structure of the plant community in the vineyard, but also affects the arthropod (insects, spiders, etc) community as well. It has been well established that increased plant diversity leads to increased arthropod diversity. So, while increasing the number of weeds present in the vineyard has negative effects on the growth of the vines, increasing weed diversity creates an ideal environment for many arthropods that are natural enemies of certain pests. It is therefore very important that vineyards are managed in a way that controls the population of weeds that compete with the vines without damaging the populations of plants that are beneficial for the arthropod community.
Studies have shown that the type of weed management practice affects the ground cover vegetation in the vineyard, which affects the diversity of arthropods in the community. Arthropods are known to consume weed plant species seeds, which is a benefit to having an increased arthropod diversity present in the community. This seed consumption has the ability to alter the weed species community, and could be used in weed management program. Though we know these dynamics occur in many different types of environmental communities, surprisingly little is known about the effects of weed management on these beneficial arthropods or crop pests.
The goals of the study summarized today were threefold: 1) to describe this relationship of weed management practices on plant and arthropod diversity; 2) to identify those arthropod species that could be weed seed predators, and 3) to determine the importance of plant species composition and density on arthropod weed seed predation.
Methods
(I’m leaving out some details of the experiment for space considerations, however, if you have questions about a particular method, just ask!).
This study was conducted in a commercial grape vineyard in Paso Robles, CA during the 2006 and 2007 vintages. The vineyard was planted in 1997 with Zinfandel grapes (clone P1) on 110R rootstock, with 2.13m between vines, and 2.44m between rows. Drip irrigation was used, and sprinklers were available for the cover crops. Irrigation and fertilization was applied uniformly throughout the vineyard, based on commercial production practices.
The experiment was a randomized complete block design, with 5 treatments and 3 blocks in 2006, and 5 treatments and 4 blocks in 2007 (all the same block with the exception of the new one). One experimental unit was four vine rows that were 170m long, with two additional adjacent buffer rows. Treatments were applied to a 1.3m section right under the vine rows.
The treatments were as follows: 1) flumioxazin; 2) simazine; 3) cultivation; 4) cover crops; and 5) untreated control. Flumioxazin and simazine were applied using a commercial sprayer in February, as a tank mix with oxyfluorfen and glyphosate. Cultivation occurred once per year in late spring when weeds covered 75% of the ground and were 30cm in height (this technique is often used in organic vineyards). In the aisles, the common cover crop Bromus carinatus was planted and mowed each year during the spring.
The cover crop was ten low growing species, with winter annual or perennial growth, and adapted to local conditions. Cover crop seeds were sown by hand. Control plots were left untreated throughout the growing season. Weeds under the vines of buffer rows were controlled using a tank mix of oxyfluorfen, glyphosate, and simazine in February. Paraquat was applied to all treatments and buffer rows (except for the cover crop treatment) after harvest in November. This treatment was applied to kill any weeds that managed to escape the other herbicide treatments.
Plant species density was recorded once per month from March to November in four randomly chosen points within each experimental unit (in 0.25m2 frames). The arthropod community density was recorded each month between February and October using pitfall traps.
Laboratory experiments were performed to evaluate the potential of two arthropods (Tanyastoma maculicolle and Calathus ruficollis) to predate on the seeds of Amaranthus retroflexus L. (common tumbleweed), Anagallis arvensis L. (red pimpernel), Brassica nigra (L.) W.D.J. Koch (black mustard), Capsella bursa-pastoris (L.) Medik. (shepherd’s purse), Eragrostis spp. (a grass), Malva parviflora L. (cheeseweed), Picris echioides L. (bristly ox-tongue), and Sonchus oleraceous L (sow thistle). Seeds and beetles were collected in the field and brought back to the lab. There were 6 replications for each beetle and weed combination. The experiment ran for 48 hours in a growth chamber at 21oC and was repeated once. The number of intact seeds were counted at the end of the experiment.
A field study was performed to evaluate if higher densities of seed predators in the non-herbicide treatments resulted in higher seed predation rates. Seed predation was measured in May and August of 2007. There were two treatments: one with a vertebrate exclusion cage, and one with no cage. The seeds used for this experiment were from black mustard and shepherd’s purse. Thirty seeds of each were collected and placed on sand-paper seed cards and secured in the soil. Each experimental unit had three caged cards and three no cage cards that were randomly placed under the vines.
Results
Plants
- Plant density was highest in the cover crop, then the control and cultivation treatments.
o Plant density in the herbicide treatments was only 26% (2006) and 2% (2007) of that of the density in the other treatments.
- Plant species richness ranged from 15 to 31 species in 2006, and 6 to 25 species in 2007.
o Plant species richness in the herbicide treatment was 60% of that of the richness in the other treatments in 2006, and 29% in 2007.
- Plant community evenness was upwards of 4 times higher in non-herbicide treatments compared to herbicide treatments for both 2006 and 2007.
- Plant diversity was highest in the cover crop and untreated control, followed by the cultivation treatment then finally, the herbicide treatment (lowest diversity).
o Plant diversity in the cover crop and untreated control was 5 to 15 times higher compared to herbicide treatments.
o Plant diversity in the cultivation treatment was 10% lower than the cover crop and untreated control treatments, and 13 times higher than the herbicide treatment.
Arthropods
- The number of arthropods collected in 2007 was twice as high in the cultivation and untreated control as in the herbicide treatment (note: 2007 was a dry year).
- Arthropod richness in the cover crop and untreated control was 10 species higher (on average) than in the herbicide treatments in 2006, and 6 species higher in 2007.
o Arthropod richness was higher in the cultivation treatment than the herbicide treatment, but only in 2007.
- Differences in arthropod diversity between treatments were small (compared to plant diversity differences).
o Arthropod diversity was higher in the cover crop and untreated control than in the herbicide treatment.
- Arthropod species evenness was not affected by treatment or year.
Relationships between Plants and Arthropod Diversities
- There was a significant relationship between plant density and diversity, and arthropod diversity.
o Activity-density of arthropods was higher in cover cropped treatments.
§ The authors speculate this means that vegetation ground cover and species composition may be important driving factors in this plant-arthropod diversity relationship.
- There were significant differences in arthropod composition between herbicide and non-herbicide treatments.
- The arthropod community composition was similar in the two herbicide treatments, the cultivation treatment showed similarities to all treatments, and the untreated control differed from the herbicide treatments.
o The arthropod community composition of the cover crop treatment differed from all other treatments.
Seed Predation Experiments
- 2 of the 8 weed species were actively consumed by the beetles.
- In the field, seed removal was the same for both the caged and non-caged treatments, indicating that invertebrates (i.e. arthropods) removed the seeds, and not larger vertebrates.
- Seed removal in the cover crop and untreated control treatments were twice as high as in the herbicide treatments.
o Seed removal in the cultivation treatment was consistently intermediate, compared with the other treatments.
Authors’ Interpretations of Results
According to the results of this study, the different treatments showed significant differences in plant richness, density, and diversity. The authors noted that drought was the likely cause of the decline in plant species richness and density in 2007.
In regards to the arthropod community, the differences between treatments were not as clear as they were in the plant community. The authors speculated one issue may have been with the pitfall trapping method, in that pitfall traps may select only more active arthropods, and those arthropod species collected may not represent the community as a whole. In some treatments, more dense vegetation could have prevented the arthropods from moving around as much and falling into the pitfall traps, even though the diversity may have been high. As a result of the possible selection bias on the part of the pitfall trap method, diversity may have been underestimated. I believe that a better solution would be to perform multiple types of collection methods, to ensure collection of a wide range of arthropods that would more accurately represent the diversity of the community.
In regards to the drought in 2007, the variation in arthropod activity-density may have been affected. Studies have shown that drought negatively affects the abundance of arthropods (in a farm settting) through changes in vegetation complexity. This study found that in the drought year of 2007, plant species diversity/richness did decrease thus may be playing a large role in the abundance of arthropods as well. The overall difference in arthropod activity between the herbicide and non-herbicide treatments were more pronounced in the drought year, since the herbicide treatment had significantly lower ground cover during that year.
The results of this study also found that arthropod activity-density and diversity were proportionally related to plant species richness (amount of vegetation cover) and diversity, therefore the authors were not able to determine which parameter was more important in enhancing arthropod diversity, though they are guessing it’s possibly the plant density parameter. Also, even though the cover crop and untreated controls had similar plant diversity indices, the actual species present were different for each treatment, thus potentially affecting the arthropod diversity in each treatment as well.
Based on the results of this study, the authors recommended two options for vineyard management changes. First, in order to minimize competition between weeds and vines, while preserving the complexity of the habitat, vineyards could plant a diverse mix of drought-tolerant cover crops between rows, and use cultivation practices under the vines (instead of conventional herbicide treatments). Second, and finally, the authors suggest that cover crops could be planted during the winter and spring, and then cut to use as mulch under the vines.
Overall, this study found that by allowing vegetative ground cover which increases plant density and diversity, arthropod density and diversity is effectively maintained, which is overall beneficial to the health of the vineyard environment. If species diversity is indicative of a healthy environment, then it is clear that alternative vineyard management practices, such as cover cropping (and to a lesser extent, cultivation), are superior to that of conventional vineyard management practices that utilize potentially harmful chemicals.
I’d love to hear what you all think about this study (methods, results, authors’ interpretations, what have you). Please feel free to leave your comments below.
Source: Sanguankeo, P.P., and León, R.G. 2011. Weed management practices determine plant and arthropod diversity and seed predation in vineyards. Weed Research 51: 404-412.
DOI: 10.1111/j.1365-3180.2011.00853.x
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