Planococcus ficus (the vine mealybug) is currently regarded as the dominant mealybug species, with it being a major pest insect of the South African table grape and wine industries. The most common method of mealybug control in South Africa is the use of such chemical insecticides as organophosphates. An alternative to using chemicals in controlling P. ficus is to use entomopathogenic nematodes (EPNs) within an integrated pest management (IPM) scheme. The current rise of interest in the use of EPNs within an IPM scheme is a progressive and positive move that has been taken towards reducing chemical pesticide use, in the pursuit of sustainable agricultural practice.
The use of EPNs is generally more suited to controlling soil-dwelling insect stages than above ground insect pests. Although the greater part of the mealybug population is found above ground, they also occur on vine roots to a depth of 30 cm. This is encouraging for the use of EPNs, when considering the fact that the vine mealybug has a distinctive vertical seasonal movement on grapevines.
The main objectives of the current study were to establish the efficiency of controlling soil populations of P. ficus, using EPNs in vineyards. The persistence of nematodes was established in the laboratory, as well as after application in two vineyards.
In-field soil application and infectivity of Steinernema yirgalemense
Nematodes at concentrations of 80, 40, 20, and 0 nematodes (infective juveniles)/cm2 were applied to an area of 80 x 100 cm measured around each treatment vine in a vineyard in Welgevallen and Nietvoorbij (Fig. 1). The soil temperature, 15 cm below the soil surface during the 48 hours following on nematode treatment, was a mean of 24°C, with a minimum of 17°C and a maximum of 32°C.
The concentration of 80 infective juveniles/cm2 of S. yirgalamense was responsible for the greatest percentage of P. ficus mortality, being 50% at Welgevallen and 52% at Nietvoorbij, respectively (Fig. 2).
In-field soil persistence of S. yirgalemense
Testing for S. yirgalemense efficacy one, two, four, and 12 weeks post application showed high efficacy and persistence in the Welgevallen vineyard using codling moth larval mortality as the measurement concerned (Fig. 3). After 12 weeks, zero mortality for codling moth larvae was found in the Nietvoorbij vineyard (Fig. 3).
Laboratory persistence of Steinernema yirgalemense and Heterorhabditis zealandica
No significant difference was found in mortalities over the six dates for S. yirgalemense. However, codling moth larval mortalities due to H. zealandica were reduced from 100% to 5% (Fig. 4).
Conclusion of the study
In both vineyards, S. yirgalemense performed well, despite the fact that the mealybugs were left in the soil for only 48 hours, whereas in practice, the nematode would have had an indefinite time period for infection. In addition to the above findings, the mealybugs were buried 15 cm beneath the soil, which means that the nematodes had to detect, and to infect, the insects within a short period of time. These are promising results, when considering in South Africa, P. ficus spends the winter months in colonies on the lower trunk, under the bark, and underground, on the roots of the vine. Their habitat should make them more vulnerable targets to applied EPN in the field than under controlled laboratory conditions, such as were present in the current experiment.
To test the ability of S. yirgalemense to persist in the soil after application, codling moth larvae were buried in the soil. This was as a result of the difficulties experienced when working with P. ficus (because of its small size and sensitivity to handling), which might otherwise have affected the data obtained. In contrast, codling moth larvae are known for their susceptibility to S. yirgalemense. Generally speaking, as soon as nematodes are applied to the soil, they are exposed to an array of abiotic and biotic factors that cause nematode mortality. Over and above these many factors, some nematode species are known to be characteristically more persistent than are others.
The persistence of S. yirgalemense in the two vineyards that were studied in the current research differed dramatically, with steady persistence being obtained in the Welgevallen vineyard, and a drastic decrease being found in the Nietvoorbij vineyard over three months. The difference in persistence of S. yirgalemense could have been due to any of the many abiotic and biotic factors affecting the two differing vineyards. To hold one factor accountable for the low persistence in Block B would be almost impossible, as there might have been a multitude of factors at work at the time of the study. To make comparisons with, and to draw conclusions from, other persistence studies are also difficult, as different EPN species are usually studied under a variety of different conditions.
The decrease in the percentage of codling moth larvae mortality between two and six months as a result of H. zealandica infection, as was tested for in the laboratory, indicates a reduction in the number of infective juveniles persisting in the container. In contrast, S. yirgalemense steadily persisted at 12 weeks under laboratory conditions, from which its ability to persist can be seen as being superior to that of H. zealandica.
S. yirgalemense produced good results in the current soil field trials, in terms of the sensitivity of soil-dwelling stages of P. ficus as regarding their mortality immediately after application, and as regarding their field persistence. These findings place S. yirgalemense in a promising position as a potential biological control agent for the control of P. ficus.
In South Africa, mealybugs are a serious problem in vineyards and control is mainly based on use of chemical insecticides. Insect-parasitic nematodes or entomopathogenic nematodes (EPNs) can potentially be used within an integrated pest management scheme to control P. ficus, the vine mealybug, which is mainly found above ground, but which can also occur on grapevine roots. When the EPN, S. yirgalemense, was applied to the soil of two vineyards by means of buried, pierced Eppendorf tubes containing mealybugs, mortalities of up to 50% were obtained. This control was obtained despite the tubes being buried to a depth of 15 cm and using a short exposure time of only 48 hours of the mealybugs to the nematodes in the field. In response to the testing that was undertaken for the persistence of S. yirgalemense, control was zero in one vineyard, while in another it was found to be 70%, 12 weeks after application. These studies showed that EPNs, and specifically S. yirgalemense, have promising potential for use as a biocontrol agent for P. ficus soil populations. Foliar applications require further investigation.
The authors would like to thank Winetech and the National Research Foundation (NRF-THRIP TP2011060100026) for funding the project. We would also like to thank Prof. D. Nel for assistance with the statistical analysis.
This article was adapted from: Le Vieux, P.D. & Malan, A.P., 2015. Prospects for using entomopathogenic nematodes to control the vine mealybug, Planococcus ficus, in South African vineyards. South African Journal of Enology & Viticulture 36(1): 59 – 70.
– For more information, contact Antoinette Malan at email@example.com.