Determining the efficacy of foliar applications of EPNs to control the vine mealybug Planococcus ficus

PROJECT TITLE:    Determining the efficacy of foliar applications of EPNs to control the vine mealybug Planococcus ficus

Project leader:       Annika Pienaar (ExperiCo)

Contact:                     annika@experico.co.za

Duration:                   1 August 2022 – 31 May 2024

Problem identification and project objectives:

The vine mealybug, Planococcus ficus, is a pest of grapevines in the Western Cape, causing problems for growers due to phloem feeding, deposition of honeydew (leading to sooty mould) and the deposition of a white waxy substance onto grapes. Wax deposition is of particular relevance to table grapes, as it is a phytosanitary export concern and standards are extremely strict. The vine mealybug can also transmit the grapevine leafroll-associated virus type III (GRLaV-3) which leads to the rolling and discolouration of leaves. Control programs targeting the vine mealybug are mainly focused on chemical control methods such as the use of insecticides. Due to the mealybug’s choice of cryptic habitats, its hydrophobic white coating providing protection from liquid pesticides, as well as its potential to develop resistance to pesticides (due to its high reproductive rate), alternative control methods need to be investigated. Biological control of P. ficus is therefore of interest to grape farmers, as these can be highly species-specific and do not leave chemical residues.
Entomopathogenic nematodes (EPNs) have been assessed as a candidate for biological control of P. ficus. They are capable of actively seeking out hosts, which means that they have the potential to infect mealybugs in their cryptic environments. EPNs are however soil-adapted organisms that are susceptible to UV and desiccation on foliage. Above-ground environments generally have lower relative humidity, more extreme temperatures and higher UV exposure, which can lead to the desiccation and death of the EPNs. The main factor affecting EPN survival above ground appears to be humidity. Research to improve the survival of EPNs in above-ground environments has mainly focused on weather forecasting, applying EPNs early in the morning or late afternoon, wetting before and after application to increase humidity, or the use of adjuvants. Adjuvants have proven promising as a means of prolonging the survival of EPNs on foliage to provide them a window to infect their targeted species. Semi-field trials were previously conducted using the adjuvants Zeba® and Nu-Film-P® in formulation with the EPN species Steinernema yirgalemense to control Planococcus citri in citrus orchards (van Niekerk & Malan, 2014) and also to control P. ficus in grapevines (Platt et al., 2019). Proof of the efficacy of adjuvant-EPN formulations against wild mealybug infestations in the field however remains unknown. This study aims to build on the work of Platt et al. (2019) where Steinernema yirgalemense was applied to control P. ficus in semi-field conditions, and apply it to a wild mealybug population in field conditions.

References
Platt, T., Stokwe, N.F. & Malan, A.P., 2019. Grapevine Leaf Application of Steinernema yirgalemense to Control Planococcus ficus in Semi-field Conditions. S. Afr. J. Enol. Vitic. 40:1-9.

Van Niekerk, S. & Malan, A.P. 2014. Evaluating the efficacy of a polymer-surfactant formulation to improve control of the citrus mealybug, Planococcus citri (Risso) (Hemiptera: Pseudococcidae), using entomopathogenic nematodes under simulated natural conditions. African Plant Protection 17:1–8.