Mealybug and scale insects are currently the only proven vectors of leafroll virus. All producers do not have the financial means to remove and replace leafroll infected grapevines. New grapevines from certified virus-free plant material are consequently being established amongst infected blocks.
How to determine the presence and population of mealybug
• Physical monitoring.
• Pheromone traps.
• Degree day model.
The following control measures may be applied for mealybug:
NB: Only active ingredients are listed – consult your agricultural chemical advisor. Read and follow the prescriptions and safety periods on the label. Spray judiciously – only sections where infection occurs. Withholding period is indicated in brackets.
This is the first step in the control of mealybug. Ants and mealybug live in symbiosis with each other – ants eat the honeydew secreted by mealybugs.
The following chemicals may be used:
• Alpha-cypermethrin (28 days).
• Chlorpyrifos (28 days).
• Fipronil + Lambda Cyhalothrin (28 days).
This entails the release of natural predators that parasitise the mealybug.
Locally bred predators are available for this purpose:
• Anagyrus pseudococci.
• Cryptolaemus montrouzieri.
• Coccidoxenoides perminutus.
Chemical control at root level in the soil
• Imidacloprid (112 days).
• Clothianidin (90 days).
• Thiamethoxam (application one month after harvest).
Soil application at the time of establishment of new blocks. Some registrations are available for autumn application.
Winter and summer chemical control on sections of the plant that are above the soil
This is a good option considering the fact that the population of natural enemies during winter is low:
• Chlorpyrifos – before budburst.
• Prothiophos – before budburst.
• Profenofos – application during dormancy of grapevine.
NB: This is the last resort, seeing that in the process not only mealybug predators, but also other beneficial insects are killed:
• Acetamiprid (45 days) – table grapes only.
• Borax + Orange oil – no withholding period.
• Carbaryl (14 days).
• Chlorpyrifos (28 days).
• Dichlorvos (7 days).
• Dimethoate (28 days).
• Methidathion (8 days) – only one application.
• Mevinphos (7 days).
• Spirotetramat – not after pea berry stage.
• Sulfoxaflor (28 days).
We have the knowledge and the means to control ants and mealybug. If mealybug is absent, the leafroll virus cannot spread to healthy vines. The only distribution can then take place by the vector(s) from already infected plant material.
With thanks to Petrie de Kock and Pieter le Roux for their technical input.
– For more information, contact Lucinda Heyns at email@example.com.