Margarodes prieskaensis is one of five indigenous Margarodes species found in vineyards in SA.
By Elleunorah Allsopp (ARC Infruitec-Nietvoorbij).
Margarodes are soil-dwelling scale insects that occur along the Lower Orange River in the Northern Cape and in certain areas in the Northwest, Limpopo, and Mpumalanga provinces.
Thus far, the camel thorn tree (Vachellia erioloba) is the only indigenous host plant identified along the Lower Orange River, but the search for indigenous hosts in the other regions continues. The larvae or nymphs, protected by waxy cysts feed on grapevine roots (Fig. 1) until fully developed, thus weakening the root system, and shortening the productive lifespan of vines. Flightless females and winged males emerge from the cysts during winter and move to the soil surface to mate (Fig. 2). Since adult margarodes do not have functional mouthparts, they cannot feed and die shortly after mating and laying eggs.
The bright yellow M. prieskaensis females are highly visible when they appear at the soil surface to mate and present an easy target for control (Fig. 3). Currently, there are no chemicals registered for margarodes control. The South African Table Grape Industry (SATI) and Raisins SA co-funded research to determine the efficacy of local entomopathogenic fungi (EPF) and entomopathogenic nematodes (EPN) against margarodes females in laboratory and field trials. In parallel, registration trials with selected contact and systemic insecticides for margarodes control were funded by Villa Crop Protection. The overall aim is to develop an integrated management strategy for margarodes.
The efficacy of contact insecticides, previously identified as promising candidates in laboratory trials by Dr André de Klerk, was evaluated against margarodes females in three field trials in the Northern Cape and Limpopo. The efficacy of systemic insecticides targeting the root-feeding larvae was investigated in field and pot trials. Local EPF isolates were screened for efficacy in the laboratory. The efficacy of the best performing local EPF and a local EPN was evaluated in two field trials in the Northern Cape and Limpopo.
Outcomes
Two contact insecticides, applied as soil drenches, were effective against female M. prieskaensis, and data for the product with the best efficacy (up to 93% females killed) and best environmental profile is being prepared for submission to the Registrar (Act 36, 1947) for registration.
The fact that margarodes cysts can persist and remain viable in soil for many years posed problems for field trials with systemic insecticides, since females appearing at the soil surface after the application of the systemic insecticides could have emerged from cysts that were dormant in the soil and not from larvae exposed to the insecticide while feeding on the roots. For this reason, pot trials were attempted but the margarodes females collected in the field did not lay sufficient eggs to inoculate the pots. However, field observations showed that systemic insecticides applied as soil drenches for control of grapevine mealybug can help to reduce the margarodes population by targeting the larvae feeding on the roots.
A local Metarhizium isolate was identified as the best EPF candidate in laboratory trials and was able to infect and kill margarodes females in field trials (Fig. 4). Entomopathogenic nematodes were unable to infect and kill margarodes females in field trials. Combined EPF and EPN treatments were unsuccessful and indicated antagonism between the organisms. Further trials with EPNs and combined EPF and EPN treatments are not recommended.
Conclusions and recommendations
A contact insecticide is being submitted for registration to control margarodes females at the soil surface. This will be the first and only chemical registered against margarodes.
A local Metarhizium isolate showed potential as a biocontrol agent in field trials. However, more extensive field trials with this and other local EPF isolates are needed. Specific attention needs to be given to proper formulation of the EPF spores to reduce the impact of extreme temperatures, low humidity, and solar radiation, and to improve EPF performance under field conditions.
Male pre-pupae of M. prieskaensis move close to the soil surface to pupate. While they are not as easily visible as the females on the soil surface, field trials to investigate the feasibility of also targeting male pre-pupae with contact insecticides and EPF, are strongly recommended. Development of an IPM strategy for margarodes depends on successful registration of the contact insecticide and on the future commercial availability of the effective local EPF isolate.
For further information contact Dr Elleunorah Allsopp at allsoppe@arc.agric.za or 021 809 3007.
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