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A Technical Guide for Wine Producers
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Mealybugs - Villians of the vine (Part 2)
Glenneis Erasmus
Mealybug control should form an integral part of any grapevine leafroll virus management strategy. Only one infected mealybug can result in the virus being spread from one vine to another vine or to other vineyards. Winetech for this reason has developed a protocol for mealybug control in vineyards.
Monitoring can be done through physical monitoring through regular vine inspections or by using pheromone traps. Vineyards or pheromone traps should be monitored every second week from the beginning of October when mealybugs start to become a problem until the end of March when populations start to decline.
Roeleen Carstens from the ARC-Infruitec Nietvoorbij suggests that farmers draw a plan of the vineyard with the rows and number of plots in each row. Large blocks should be subdivided into units of 2 hectares at the most and around 20 plots containing five vines each that are evenly distributed through the vineyard should then be selected for inspection purposes. Areas where mealybug problems occurred in the past should definitely be included in the inspections.
The total number of infested vines out of the hundred monitored will indicate the percentage estimate infestation for that area. Say for example 5 vines out of the 100 have mealybugs on them then the infestation rate will be 5%. The mealybug protocol for production blocks suggests that action should be taken when infestation levels are higher than 2%.
The drawback of physical monitoring is that it is much more labour intensive and less time efficient than the monitoring of pheromone traps. The protocol suggests that one pheromone trap is used to cover one hectare of vineyard. Traps in the sub units should be more than a hundred meters apart so that they don’t interfere with each other if more than one is used to cover an area that is larger than one hectare.
The traps contain pheromone capsules that attract the male mealybugs. The effectiveness of the capsules will be affected if it is covered with glue, farmers must therefore ensure that capsules are suspended above the sticky bottom of the traps. The sticky bottoms should be replaced every two weeks, while pheromone capsules should be replaced every 2 months, according to the protocol.
Vines in the trapping area should immediately be inspected if more than 65 males are counted in the trap over 14 days. Twenty plots containing 5 vines per plot should be identified - as is described above for physical vine inspections to determine the infestation percentage. Carstens says that spot treatments can be applied on heavily infested vines if female infestation levels are below 2%, but full action should be taken if infestation levels are above 2%.
A degree-day model can also be used to monitor mealybugs. The model is based on the fact that mealybugs’ development is dependent on ambient temperature. Research so far indicated that a minimum temperature of 16,59 Degrees Celsius is required for the initiation of mealybug development. Each day with degrees Celsius above 16,59 is accumulated to a total of 235-degree days, which are needed for mealybugs to complete a full generational cycle. This an accumulation of 235-degree days - the amount of heat accumulated over a period - can be used to signal the start of new infestations.
Several weather stations have been set up in vine producing areas in the Western Cape and these stations aim to alert producers when high infestations of mealybugs are expected. More information about this can be obtained from www.ipwc.co.za.
Carstens warns against using the degree day model as monitoring mechanism on its own, because there still are some grey areas that must be researched to optimise the effectiveness of the model. The model for example still does not have a bio fix - in other words it does not have a definite starting point, based on biological indicators, which signals the development of male or female mealybugs for the new season, she explains.
Infected mealybugs can transmit leafroll virus through their own movement in other words by crawling short distances, but they can also be dispersed by people, birds, ants, the wind and farm implements especially during the crawler stage when they are extremely small. Strategies should therefore be implemented to prevent the movement of mealybugs from infected to uninfected areas. (See First article)
Pietersen adds that these strategies are required in the transition phase while a wine estate still has a mixture of infected vineyards and non-infected vineyards, but becomes redundant when the estate is primarily virus free. This is because it is only really important to prevent the dispersal of mealybugs that actually carry the virus, he explains.
Mealybugs can survive on live virus-infected grapevine material whether it is under or above the soil. Establishing new vines on new land is as a result obviously the safest way of prevent virus carrying mealybug spread, but very few farmers still have the luxury of this option. (See First article)
The protocol advises producers to use a fallow period of at least one year between the removal and planting of new vines. Current research by Pietersen is currently underway to determine the optimal fallow period.
Effective chemical control of mealybugs is currently one of the primary methods of addressing infestation problems, and once again is critical in the transition phase of an estate from being virus infected to primarily virus free. This severe method of control is unfortunately unsustainable due to increased pressure to produce wine in an environmentally friendly way, and as an overuse of these chemicals could also lead to the development of insect resistance to them. In addition to this the chemicals are expensive and large volumes of water must be used to dilute these products.
Farmers should preferably concentrate on the removal of virus infected vines so that biological control, which unfortunately is not effective under outbreak conditions to prevent virus spread, can be used as an alternative used as an alternative to chemical control. Biological control through the use of parasitic or predatory insects can then be used to maintain mealybug populations below 1%.
For effective biological control, farmers will have to avoid the formation of dust in vineyard as this can reduce the effectiveness of natural enemies. Ants must also be controlled, because they feed on the honeydew excreted by mealybugs and protect them against their natural enemies. Producers should therefore monitor the amount of ants on vineyard stems and leaves by dividing the vineyard in the same way that is done for mealybug monitoring.
The current protocol for ant management suggests a threshold of 25%. New research however seems to suggest lower levels - 20% for stems and 21% for leaves, according to Dr Pia Addison from the Department of Conservation Ecology and Entomology at the University of Stellenbosch. Addison adds that it is better to use stem infestations as an indication of problems as this is where mealybugs usually occur early in the season and can therefore be used as an early indicator of bunch infestation. Chemical control can be used when infestations exceed thresholds. Plastic covering the soil surface should be removed in case ants are nesting under the plastic.
Sticky trunk barriers or chemical sprays around vine stems can also be used as measures to prevent ants from moving up into the vineyards. These measures are ideal, as they do not affect natural enemies. Good weed on the "bankie" is also very important to prevent ants from using tall weeds to get up into vines.
Contact Information: Contact Information: The Pest Management Division of ARC Infruitec Nietvoorbij: (021) 809 3458 or prof Gerhard Pietersen gp@cri.co.za
Glenneis Erasmus (021) 919 8583
New softer chemistry, such as imidachloprid (Confidor, Kohinor and so forth) is now available on the market for mealybug control. These products are absorbed by the roots of the vines and are released over a longer period than the older products. The major advantage of these products, according to Pieter le Roux from Villa Crop Protection, is that they are not neurotoxic or carcinogenic. In addition to controlling mealybug, the products can also be use to control aphids without harming beneficial insects rendering it ideal for biological control programmes.
Confidor applications should however be preceded by a Tokuthion or Chlorpirifos spray application during the dormant growth phase in winter. Confidor should be applied on moist soil and should be washed into the soil through irrigation. It is therefore not advisable to use it on dryland vineyards. Leaves, weeds and other organic material should be removed before application as this material can inhibit the absorbance of the product.
Treated vines should still be monitored for mealybugs and producers might have to use a corrective spray, such as Dedevap, if populations become unacceptably high. Effective ant control also remains important to protect biological control agents from ants. Confidor is quite expensive - costing more than 80c per vine - but Le Roux foresees a drop in prices as more producers start to use it and generic imidachloprid becomes available in the market.
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