Vine mealybug (VMB) infestations were first detected in North Coast vineyards in August 2002.
Since that time, eradication efforts in known infested sites have increasingly given way to control measures including less disruptive insecticides, pheromone mating disruption and managing or augmenting natural enemies. An integrated approach to reduce the abundance of all types of mealybugs found in California vineyards should also include preventing human-assisted transport of these insects into new areas.
In 2003 and 2004 we investigated the survival of VMB on infested clusters that entered the winery as well as in pomace piles consisting of unfermented berry skins, seeds and stems. We wanted to learn if there was a risk of contaminating vineyards with VMB by spreading unfermented pomace, including stems, in vineyards.
We learned VMB can survive pressures generated in whole cluster pressing, a process commonly used in white wine production. VMB can also survive in unmanaged pomace piles — piles that are later spread through vineyards.
We believed mealybugs could not survive fermentation so we focused our attention on VMB survival on clusters that were pressed and not in must. The two winery-cooperators in Sonoma County used bladder presses to process whole-cluster press loads.
Pressure regimens varied by winemaker. In each of these wineries, when the press regimens were completed, unfermented skins, stems and seeds (“fresh pomace”) were transported to a pomace holding area. It is common for these areas to also contain piles of stems (rachises), generated when clusters of red grape varieties are processed by a destemmer crusher.
This gave us the opportunity to look at VMB survival in three different types of winery waste material: in pressed clusters, in fresh pomace piles and in piles of stems only.
Survival of VMB in press loads
A Bucher press with a 150 hL capacity was used to process 6 tons of Grenache clusters infested with VMB. Prior to processing, VMB were counted on 50 randomly selected clusters. On average there were 107 insects per infested cluster (these clusters contained 1-2,000 insects), and 44 percent of the clusters in the load were infested.
A randomly selected cluster was placed in each of 50 mesh bags (approximately 7”L x 7.5”W). A second set of 50 mesh bags contained clusters that had visible insects. All bags were sewn shut and dropped into the press with the 6-ton load. After an approximately four hour press regimen that gradually increased pressure to 1.8 bars, the bags were retrieved along with several pounds of pomace.
The contents of each bag were placed on a sticky card and insects that moved off the cluster onto the card were counted. Clusters were placed on fresh cards to continue counting insects over several weeks until no insects were counted over a two week period. The 1,400 pressed clusters were handled in a similar manner, but in groups of clusters.
In bags containing 100 percent infested clusters 0.04 ± 0.20 insects survived per cluster. Adjusting this number to account for the actual incidence of VMB in the load (44 percent), the survival in the bags was similar to that of the survival in the loose pomace (0.02 ± 0.06).
In summary, on average two insects per 100 cluster stems survived the press. Although two insects survived per 100 cluster stems on loose pomace, no insect survived in the 50 bags where the clusters were selected at random. This led us to believe that the bags may have affected insect mortality. To address this concern, we conducted a second press load trial.
To a 12-ton uninfested load of Chardonnay, we added 100 mesh bags of VMB infested clusters. Each bag contained either a stem only, a stem with detached berries, or a whole cluster. The press regimen reached a maximum of 1.6 bars over a three hour period. Insects survived in all bags regardless of the contents of the bags. VMB survival in bags containing either a whole cluster or a stem only was 4 percent and 23 percent respectively. Survival in bags containing a stem with detached berries was intermediate at 10 percent. We concluded that although the bags may have affected mortality, insects did survive whole cluster pressing even under challenging (bagged) conditions.
Survival of VMB in pomace and stem piles
VMB survival was evaluated in two types of winery waste piles. Two experiments were performed utilizing round piles of pomace including stems and a single windrow containing only stems.
Four truckloads containing a mixture of pomace and stems from whole cluster press loads of white grapes were dumped separately.
Each load was divided into two equal-sized piles to achieve pairs of piles with a similar mixture of stems, skins and seeds. This produced four pairs of piles, each pile approximately 4-feet tall and 15-feet across. One pair contained mostly stems and the other three pairs consisted mostly of skins and seeds. Within each pair of piles, one was covered with 4-mil thick clear plastic and the other remained uncovered.
Four mesh bags each containing infested stems and a temperature logger were placed at two different depths in the center of each pile for a total of eight bags per pile.
VMB were counted on stems collected from the same source that provided infested stems which were placed in bags. Each stem contained an average of 1,211 VMB. Every week, for four weeks, one bag from each depth per pile was removed. Stems were placed on sticky cards and moved to clean cards every week to count VMB as previously described.
Covered round piles
In the covered piles VMB population reduction ranged between 99.9 percent to 100 percent, regardless of the position of the bag in the pile, the week it was removed or pile composition.
Temperature in the mostly stems covered pile fluctuated daily between 68 F and 130 F. In mostly skins and seeds covered piles, temperatures rose in the first few days to 100 F and then slowly increased to 120 F to 130 F in the following four weeks.
Uncovered round piles
In the mostly stems uncovered pile we obtained the lowest VMB reduction, ranging from 61 percent after one week to 89 percent after four weeks. In this pile, temperatures fluctuated almost daily between approximately 55 F and 85 F.
In the uncovered mostly skins and seeds piles VMB reduction ranged from 99.9 percent to 100 percent. In these piles, temperatures fluctuated for the first two weeks and in most cases, remained above 100 F for the following two weeks.
Several truckloads of cluster stems of red grape varieties collected from the destemmer were used to create an east-west oriented windrow 4-feet tall, 80-feet long and 15-feet wide. The windrow was divided into eight 10-foot sections. A total of 32 mesh bags were each filled with five infested stems and sewn shut. In the center of each windrow section, four bags were placed at a 2-foot depth.
Starting from the west end, a 10-foot section was covered with 4-mil clear plastic and the adjacent section remained uncovered. This pattern was continued to create four covered and uncovered sections. Three covered sections had plastic across the top and sides, while the west end had plastic on three sides and the top. Each week, for four weeks, one bag was removed from each section and the contents placed on sticky cards to count surviving VMB.
The greatest population decrease occurred in the west end where plastic covered three sides and the top of the windrow. In all remaining sections, there was a reduction in the number of VMB through time but there was no significant difference between VMB survival in the sections covered on both sides and top and the uncovered sections. Temperatures in the uncovered sections closely followed ambient air temperatures.
Temperatures in the covered sections fluctuated less and remained cooler than the uncovered sections. The plastic was not effective at raising internal temperatures because air flux occurred between covered and uncovered sections.
Vine mealybugs survived whole cluster pressing in these experiments with approximately two live insects for every 100 bunch stems unloaded from the press.
Although this reduction from 107 insects per cluster is substantial, fresh pomace or cluster stems should not be spread directly into vineyard middles given the large numbers of clusters processed by wineries. Instead, this material should be covered for at least one week with thick, clear plastic that is secured at the bottom to prevent air flow and increase pile temperatures.
If possible, pomace and stems should be mixed to avoid “stemmy” piles that generate less heat when covered than piles with skins and seeds.