A single infected mealybug can spread leafroll virus from one vine or vineyard to another. For this reason, mealybug control should form an integral part of any grapevine leafroll virus management strategy. Winetech has developed a protocol for mealybug control in vineyards.
Monitoring can be done by regular vine inspections or by using pheromone traps. Monitor 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 of ARC-Infruitec Nietvoorbij suggests that farmers draw a map of the vineyard showing the rows and number of plots in each row. Subdivide large blocks into units of 2ha at most. Then select around 20 plots containing five vines each for inspection purposes. The plots should be evenly distributed through the vineyard. Areas where mealybug problems occurred previously should definitely be included in the inspections.
Pay special attention to new growth when inspecting the vines, as this is one of the favourite homes of these little culprits. Only female mealybugs cause damage. A vine with only one female on it, whether it is a crawler, nymph or adult, should be noted as infested. Ants are usually good indicators of whether mealybugs are present.
The total number of infested vines out of the hundred monitored gives the estimated infestation percentage for that area. For example, if five vines of the 100 have mealybugs on them, the infestation rate is 5%. The mealybug protocol for production blocks suggests that action should be taken when infestation levels are higher than 2%.
Using pheromone traps
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 using one pheromone trap to cover 1ha of vineyard. Traps in the sub-units should be more than 100m apart so that they don’t interfere with each other in larger areas.
The traps contain pheromone capsules that attract the male mealybugs. The capsules will be less effective if they are covered with glue, so farmers must ensure that the capsules are suspended above the sticky floor of the trap. The sticky trap floors should be replaced every two weeks, while pheromone capsules should be replaced every two months, according to the protocol.
Vines in the trapping area should be inspected immediately if more than 65 males are counted in the trap over 14 days. Twenty plots containing five vines per plot (as described above) should be inspected to determine the infestation percentage. Spot treatments can be applied to heavily infested vines if female infestation levels are below 2%, says Carstens, 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 depends on ambient temperature. Research indicates that a minimum temperature of 16,59ºC is required for the initiation of mealybug development. When 235 days have passed with temperatures above 16,59ºC, the mealybug is able to complete a full generational cycle. Thus an accumulation of 235 degree-days 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 to alert producers when high infestations of mealybugs are expected. More information can be obtained from www.ipwc.co.za.
Carstens warns against using the degree-day model on its own as a monitoring mechanism, because there still are some grey areas that must be researched. For example, the model still does not have a biofix – 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 to nearby vines 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.
Effective control of mealybugs is critical during the transition from virus infection to being primarily virus-free. Chemical control works, but unfortunately this severe method is unsustainable, because of increased pressure to produce wine in an environmentally friendly way. Overuse of chemicals could also lead to the development of insect resistance. In addition, the chemicals are expensive and large volumes of water must be used to dilute them.
Instead, farmers should concentrate on removing infected vines and applying biological controls as an alternative to chemicals. Control through the use of parasitic or predatory insects can maintain mealybug populations below 1%.
Farmers must also avoid the formation of dust in vineyards, as this can reduce the effectiveness of natural enemies. Unfortunately, biological controls are not effective under outbreak conditions.
Ant control is also important
Ants must also be controlled, because they feed on the honeydew excreted by mealybugs and protect the bugs against their natural enemies. Producers should therefore monitor the number of ants on vineyard stems and leaves by dividing the vineyard in the same way as 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 of the Department of Conservation Ecology and Entomology at the University of Stellenbosch. Addison adds that it is better to monitor stem infestations, as this is where mealybugs usually occur early in the season.
This type of infestation 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 are other measures to prevent ants from moving up into the vines. These methods are ideal as they do not affect the ants’ natural enemies. Thorough weeding is also important to prevent ants from using tall weeds to get up into vines. Contact the Pest Management Division of ARC-Infruitec Nietvoorbij on (021) 809 3458, Jan Booysen at Winetech on (021) 807 3324 or e-mail [email protected].