The vine mealybug, Planococcus ficus, is a major pest of South African grapevines. Sooty moulds grow on honeydew excreted by the mealybugs, disfiguring fruit and making them unsuitable for export or sale within the highly selective table grape industry. Mealybugs also feed on plant sap, and serve as vectors for viruses such as the grapevine leaf roll-associated virus 3 (GLRaV-3). Chemical pesticides are the main tool used in their control, though their waxy coating and sheltered habitats often make pesticide application problematic. Natural enemies (such as parasitic wasps) are used as part of integrated pest management schemes for the control of the vine mealybug, but more biological control methods are sought.
Entomopathogenic nematodes (EPNs) are insect pathogens which are currently used commercially for the control of soil-based insect life stages. EPNs exist in the soil as an infective juvenile life stage (IJs) which seek out and parasitise soil-based insect hosts. However, EPNs are ill-suited to environments outside of the soil, being prone to desiccation, which has limited research into their use on pests of foliage. One possible means of promoting EPN survival on foliage is through the use of adjuvants, two of which are Zeba®, an anti-desiccant, and Nu-Film-P®, a spreader/sticker. These could provide EPNs with a long-lasting, thin film of water, allowing them to survive longer on foliage and providing them with a medium in which to move.
The focus of this study was in the above-ground application of EPN treatments to control the vine mealybug on grapevine foliage. Steinernema yirgalemense was selected for testing going forward after assessing its temperature and humidity requirements, as well as comparing its ability to infect and kill P. ficus against three newly-described indigenous EPN species. Subsequently, Zeba and Nu-Film-P were tested in solution with S. yirgalemense, where it was found that both in combination were able to cause significantly more IJs to be deposited onto grapevine leaves than any other treatment.
S. yirgalemense IJs were then formulated in three treatments (water only, Zeba, and Zeba + Nu-Film-P) and applied to mealybugs in mesh pockets. These pockets were then hung in the test environment, to simulate foliar conditions. Tests in the growth chamber and glasshouse environments gave promising results, with the IJ formulation containing both Zeba and Nu-Film-P resulting in over 80% mealybug mortality in each environment. The protocol was then taken to Welgevallen Experimental Farm in Stellenbosch, where this treatment achieved 66% mortality in the field. Another trial was performed to investigate the effects of morning versus afternoon application on IJ survival, where it was found that IJs applied in the morning (high relative humidity, low temperature) survived longer than those applied in the afternoon (low relative humidity, high temperature).
The overall conclusion was that there is potential for the use of S. yirgalemense as a biocontrol agent of P. ficus on grapevine foliage, though further testing is required and methods need to be refined. As new EPN species are discovered, and new adjuvants formulated, these must be tested in this use case. Methods for managing environmental conditions are also key. Shade netting, which is already in use to protect grapevines from excess UV radiation, also raises relative humidity and reduces temperatures in grapevines on which it is used. Finally, these methods must be put into practice in a full field trial on a natural infestation.
This study was a research project of the Agricultural Research Council (ARC) at Infruitec-Nietvoorbij, Stellenbosch. The study was presented as an MSc thesis of the student, Thomas Platt, carried out at the Department of Conservation Ecology and Entomology of the Stellenbosch University. The study and was supervised by Prof Antoinette P. Malan and Dr Nomakholwa F. Stokwe. Winetech, the South African Table Grape Industry (SATI) and the Technology and Human resources for Industry Programme, provided funding (THRIP: TP14062571871).