The Effect Of Cover Crops On Ants And Mealybugs In Vineyards

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The Effect Of Cover Crops On Ants And Mealybugs In Vineyards

Pia Addison

Pia Addison¹,², Michael Samways² & Johan Fourie¹
1. ARC Infruitec-Nietvoorbij, Stellenbosch
2. Department of Conservation Ecology and Entomology, Stellenbosch University, Matieland
Biological control of the vine mealybug (Planococcus ficus) cannot take place without ant control. Ants feed on the honeydew excreted by mealybugs and thereby prevent small parasitic wasps and predatory beetles from attacking the mealybugs (Kriegler & Whitehead, 1962). In the Western Cape Province, there are primarily four ant species that are associated with vine mealybug: the Argentine ant (Linepithema humile), two species of pugnacious ant (Anoplolepis custodiens and A. steingroeveri) and the cocktail ant (Crematogaster peringueyi) (Addison & Samways, 2000). Ants are controlled effectively using chemical stem barriers (Addison, 2002). This method of control allows the ants to remain on the ground where they are effective predators of other insects, but prevents them from entering the vine canopy to forage on honeydew.

Fig. 1. Mean number of pugnacious ants (± standard error bars for four ground cover treatments established in a vineyard in Bonnievale during two years. Numbers with the same letter do not differ significantly (ANOVA, LSD where P 0.05).
Natural enemies have been found to use cover crops as alternative refuges and so enhance biological control in agricultural systems (Tedders, 1983; Altieri & Schmidt, 1985; Bugg & Waddington, 1994 and Hoffmann, 2000). Cover crops, by reducing dust levels in vineyards, may protect natural enemies from exoskeleton abrasion (Pettigrew, 1998). In light of this, and the growing interest amongst producers to plant cover crops, this study was conducted to determine what effect various cover crops have on local pugnacious ant infestations and mealybug parasitoid populations and so in turn affect mealybug populations in vineyards in the Western Cape Province.
A mealybug infested Chenin blanc vineyard in the Bonnievale area, 1.5ha in size, was selected. Three cover crop treatments, namely grazing vetch (Vicia dasycarpa Ten.) and triticale (Triticale v. Usgen 18), controlled chemically from bud break, as well as dwarf fescue (Festuca arundinaceae v. Cochise), slashed throughout the growing season of the vines, were compared to a control treatment in which a standard chemical and mechanical weed control program was followed. Monitoring of insect populations took place from June 2001 until March 2003. The following data were collected: Ant activity on the soil surface, nest entrance counts, ant and mealybug infestations in the vine canopy, mealybug parasitoid activity. Data were analysed using Analysis of Variance (ANOVA) and Least Significant Differences (LSD) calculated to compare treatments. Soil temperature and moisture readings were taken hourly for the duration of the trial at 10 cm and 30 cm depths. A t-test was used to analyse the data separately for each depth.

Fig. 2. Average percentage pugnacious ant and mealybug infestation in vines, as monitored on four sampling dates, in a vineyard in Bonnievale where four ground cover treatments were compared. April 2001 represents the pre-treatment sampling date. Letters that differ on each column indicate a significant difference (P 0.05), analysed for each date separately (ANOVA, LSD) (Reproduced from Addison & Samways, 2006, with kind permission of Springer Science and Business Media).
Cover crops grew well except for dwarf fescue, which was dominated by weeds throughout the year. Control plots were largely free of weeds during summer, although weeds did grow towards the end of winter. Results showed that triticale consistently caused a significant increase in ant activity on the ground during both years (Fig. 1), although this did not lead to a significantly higher ant nor mealybug infestation in the vines (Fig. 2). It is possible that the ants were utilizing the seeds of triticale plants as an additional food source. Furthermore, none of the cover crops had any effect on ant nest entrance density, compared to the control, despite the fact that cover crops influenced soil temperatures and moisture significantly. Maximum and minimum soil temperatures were significantly higher in the control plot than in the cover crop plots at two soil depths: 10 and 30 cm. The highest reduction in soil temperature was 3°C, while a maximum difference of 5,21% soil moisture was found between the control and the fescue, with fescue having the higher soil moisture. The reduction in soil temperature and the increase in soil moisture arising from the use of these cover crops, was, therefore, not enough to affect ant infestations, and the main driver regulating ant abundance in this study was the food source, which dominated over any possible inhibitions imposed by the cover crops. This insensitivity to vegetation cover was also emphasized in Australia, where ants were found to be poor indicators for monitoring grassland condition (New, 2000). None of the cover crops caused a significant increase in the number of parasitoids. In the current study, most (although not significantly more) parasitoids were in the control plots (Fig. 3). This may indicate that a more diverse planting could be of high value to natural enemies in vineyards, in that they provide a greater variety of refugia and nectar sources. Weeds may therefore not be as detrimental as earlier thought, provided that they do not harbour underground mealybug populations (Walton, 2001) or provide pathways for ants to enter the vine canopy and so hamper the effect of chemical stem barriers.

Fig. 3. Mean number of mealybug parasitoids caught in sticky yellow Bugtraps^TM from July 2001 to March 2003 in a vineyard in Bonnievale where four ground cover treatments where compared.
CONCLUSIONS AND RECOMMENDATIONS
Triticale (where it is used as a viticultural ground cover) must be treated with caution, as it has the potential to increase ant infestations in vineyards already infested with A. custodiens (the common pugnacious ant). This study has clearly indicated that these viticulturally-acceptable cover crops could not be used as a management tool in an integrated mealybug control programme, but that their use did not have any significant negative effects on ant and mealybug infestations in the vine canopy either. Samways (1983) likewise found that habitat modification would not be a suitable method for managing dominant Pheidole spp. in citrus and that trunk barriers are an ecologically more appropriate method of management. The effect that cover crops have on other pest ants in vineyards, e.g. Argentine ant, was not determined in this study, but there are indications from the literature that there is little value in testing cover crops as a management tool for these other ant species.
Acknowledgements
Many thanks to staff of ARC Infruitec-Nietvoorbij, in particular E.C. du Toit and members of the Pest Management Divison for technical assistance, L. Williams for the identification and counting of mealybug natural enemies, V.M. Walton for advice on mealybug and mealybug parasitoid sampling and F. Calitz for assistance with the statistical analyses. Mr F van der Merwe of Bonnievale is thanked for his assistance in the sowing of cover crop seeds and for providing us with a trial site. This project was funded by the Agricultural Research Council (ARC) and Winetech and their contributions are gratefully acknowledged.
References
Addison, P. & Samways, M.J. (2000) A survey of ants (Hymenoptera: Formicidae) that forage in vineyards in the Western Cape Province, South Africa. African Entomology 8 (2), 251 - 260.
Addison, P. & Samways, M.J. (2006) Surrogate habitats demonstrate the invasion potential of the African pugnacious ant. Biodiversity and Conservation 15: 411 - 428.
Addison, P. (2002) Chemical stem barriers for the control of ants (Hymenoptera: Formicidae) in vineyards. South African Journal of Enology and Viticulture 23(1), 1 - 8.
Altieri, M.A. & Schmidt, L.L. (1985) Cover crop manipulation in Northern California orchards and vineyards: Effects on arthropod communities. Biological Agriculture and Horticulture 3, 1 - 24.
Bugg, R.L. & Waddington, C. (1994) Using cover crops to manage arthropod pests of orchards: A review. Agriculture, Ecosystems and Environment 50, 11 - 28.
Hofmann, U. (2000) Cover crop management in organic viticulture. Winepress 86,12 - 17.
Kriegler, P.J. & Whitehead, V.B. (1962) Notes on the biology and control of Crematogaster peringueyi var. angustior Arnold on grape vines (Hymenoptera: Formicidae). Journal of the entomological Society of southern Africa 25, 287-290.
New, T.R. (2000) How useful are ant assemblages for monitoring habitat disturbance on grasslands in south eastern Australia? Journal of Insect Conservation 4, 153- 159.
Pettigrew, S. (1998) Cover crops in Integrated Pest Management. The Australian Grapegrower and Winemaker, February, 26 - 27.
Samways, M.J. (1983) Community structure of ants (Hymenoptera: Formicidae) in a series of habitats associated with citrus. Journal of applied Ecology 20, 833 - 847.
Tedders, W.L. (1983) Insect management in deciduous orchard ecosystems: Habitat manipulation. Environmental Management 7(1), 29 - 34.
Walton, V.M. (2001) Wingerdwitluis: Biologie en beheerstrategie. Wynboer tegnies 140, 75 - 78.
Way, M.J. (1953) The relationship between certain ant species with particular reference to biological control of the coreid, Theraptus spp. Bulletin of Entomological Research 44, 669 - 691.
ABSTRACT
Biological control of the vine mealybug (Planococcus ficus) cannot take place without ant control. Ants are controlled effectively using chemical stem barriers, but currently no non-chemical control methods are available. Due to renewed interest in planting cover crops, this study was conducted to determine what effect various cover crops have on local pugnacious ant infestations and P. ficus parasitoid populations and so in turn affect mealybug populations in vineyards in the Western Cape Province. Three cover crops, creeping vetch Vicia dasycarpa, triticale Triticale v. Usgen 18 and a permanent seed mixture (dwarf fescue, creeping red Harold, SR-4-200 and Santiago medic, were compared to a control treatment, in which a standard chemical and mechanical weed control program was followed. Ant, mealybug and natural enemy data were collected during two years. Soil temperature and moisture were also monitored. This study has clearly indicated that these cover crops could not be used as a management tool in an integrated mealybug control programme, but that their use did not have any significant negative effects on ant and mealybug infestations in the vine canopy either. Triticale should, however, be used with caution, especially in areas where the pugnacious ant is a problem as it could lead to higher ant activity.

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