Weak link of vine mealybug

Vine mealybug, Planococcusficus, continues to spread throughout California’s table, raisin and wine grape growing areas.

It has become the primary arthropod pest both through direct damage and transmission of leafroll virus.

Managing this key pest necessitates a knowledge of its behavior to identify weak links in its life cycle. One such behavioral weakness has been the stage of distribution on the vine. Over the past three years, through leaf sampling, we have only found immature stages during the first two weeks of leaf infestation. Monitoring using pheromone traps began April 11 and continued weekly through July shows that only the immature stages are found early in the season. It is critical that sprays, particularly insect growth regulators (IGR) (like Applaud) are timed to this movement. If leaf infestation is managed, cluster infestation is managed.

The 2008 findings indicate movement to leaves later than in previous years.

• Insecticide trials

A systemic insecticide trial was initiated on June 7, 2008, to test the efficacy of three insecticides in controlling the vine mealybug. The Thompson seedless test vineyard was located in Del Rey, Calif. The vineyard is under drip irrigation and soils are sandy. There are 454 vines per acre. Each treatment consisted of one complete vine. On June 7, Admire was applied at the rate of 14 ounces acre, Platinum at 3.67 ounces per acre, and Venom at 6 ounces per acre. There was an untreated check. Treatments were evaluated by using leaf sampling and two harvest/cluster evaluations.

The average number of mealybugs (all immature) per leaf found on June 27 was 21.05 (untreated), 1.75 (Admire) 3.1 (Platinum), and 17.25 (Venom). A July 10 cluster evaluation showed Admire (1 cluster infested and 1 rating) and Platinum (0 clusters infested and 0 rating) to be significantly better than the untreated check (4.25 clusters infested and 1.88 rating) and Venom (2.5 clusters infested and1.67 rating). There was no statistical difference between the Control and Venom treatments. A final harvest cluster evaluation was done on Aug. 15. The average number of infested clusters was 5 (Untreated), 1.8 (Admire), 0.4 (Platinum), and 4.6 (Venom) Average ratings of treatments were; Untreated, 2.24; Admire, 1.14; Platinum, 1; Venom, 2.17. Both the Admire and Platinum treatments had fewer infested clusters and clusters that were less severely infested than the untreated and the Venom treatment. There was not statistical difference between the untreated check and the Venom treatment.

In 2007, my lab was trying to find the relative efficacy of Applaud (buprofezin) and Movento (spirotetramat). The treatments included Movento applied at May 11 or on June 25, at the rate of 8 ounces per acre and treatments of Applaud applied on May 11 or June 25, and both May 11 and June 25. Each Applaud application was at 12 ounces per acre. There was also an untreated check. Cluster infestation was evaluated in July and August by sampling 10 clusters from the middle two vines of four vine treatments. The July Movento readings resulted in no cluster infestation while the untreated resulted in 10 clusters infested. The Applaud treatment of May 11 averaged 2.75 clusters infested per 10, while the June 25 treatment averaged 6.25 clusters infested per 10. The treatment with Applaud made on May 11 and June 25, averaged 0.75 clusters infested per 10. The two applications of Applaud resulted in no statistical difference in infestation than either of the single Movento treatments. Both single treatments of Applaud were significantly different from the untreated check and also different from the other three treatments.

A month later, on Aug, 21, another reading was taken. The results were very similar. The untreated check had 10 out of 10 cluster severely infested. The Movento applied on June 25 had the lowest infestation with .25 clusters per 10 infested. The double treatment of Applaud and the earlier Movento treatment each averaged 1.5 clusters out of 10 infested. The single Applaud treatment on May 11 averaged 5.5 clusters infested per 10 sampled and the single Applaud treatment on June 25 resulted in 8.25 clusters infested per 10 sampled. Each of the treatments was statistically different from the untreated. The three least infested treatments (double Applaud, and each of the Movento treatments) were not statistically different from each other, but different from the two single Applaud treatments. A trial was conducted at the Kearney Ag Center in 2008 to further study Movento applied with and without surfactants in controlling vine mealybug. Two other products used in this study were Assail at 2 ounces per acre and Clutch applied at 6 ounces per acre. The Assail application was 1 ounce over the label rate for grapes while the Clutch application is registered at the rate applied. Movento was applied at 8 ounces.

The June 30 treatment was made after veraison. On Aug. 25, an additional count of leafhoppers was done to determine effect of treatments. Although Clutch and Assail had the lowest leafhopper counts the population was never greater than two leafhoppers per leaf and no statistical difference was seen between any treatments and the untreated check. The efficacy of Movento, alone or with all adjuvants, was excellent. However, as expected, the application with no adjuvant performed more poorly than with the addition of an adjuvant. Clusters were rated on a 0-3 scale, with 0 being clean and 3 being unmarketable and completely infested. The sampling on Sept. 12 showed the following average ratings: untreated check, 2.4; Movento alone, 0.3; Movento + Dyne-Amic, 0.1; Movento + Penetrator, 0.1; Movento +Break-Thru, 0.2; Clutch, 1.2; Assail, 0.1.

No berry scarring was noted with any of the adjuvants. The number of infested clusters per 10 sampled was 9.5, 2.75, 0.5, 0.75, 1.5, 5.75, and 1 for the Untreated, Movento, Movento+ Dyne-Amic, Movento + Penetrator, Movento + Break-Thru, Clutch, and Assail, respectively. Each of the Movento treatments and the unregistered 2-ounce rate of Assail was significantly better than the untreated and Clutch treatments. Although the number of infested clusters was not significantly different between the Clutch and untreated check, the severity of that infestation was significantly less in the Clutch treatments.

There are a number of very effective insecticides available for management of vine mealybug. Those focused on in these studies are considered reduced-risk with short reentry intervals and greater worker safety. When properly timed and used at correct rates, they are as effective as products such as chlorpyrifos and methomyl.

The two systemic insecticides, applied through drip irrigation systems, Admire Pro and Platinum, are equally effective while trials have shown Venom to be ineffective. Each of these products are neonicotinoid compounds and care must be taken to avoid insecticide resistance through over use.

Previous studies have shown that Admire applied in late April or May performed best. Preliminary results indicate the same timing would also effective when using Platinum. Fortunately, two other classes of insecticide have also given excellent results. These include the insect growth regulator, Applaud, and the newly registered tetramic acid compound, Movento.

Based on work done in 2007, Applaud would require two applications of 12 ounces each to be equal to a single application of 8 ounces of Movento. Additionally, Applaud applications must be timed to the immature stage of vine mealybug when moving to leaves. Studies in 2008 indicate this movement occurs in late April or early May. When Applaud is applied to more mature stages, after June, it is less effective. These two insecticides can be rotated with each other and resistance should not develop because hey posses different modes of action.

Also, if on a drip irrigation system, either Admire or Platinum, but not both, could be utilized in a rotation with both Applaud and Movento. Movento has the benefit of systemic activity. It is applied as a spray to leaves and, when taken into the phloem is transported throughout the plant, eventually reaching the xylem tissue.

Mealybugs are phloem feeders and the initial movement into the phloem appears to benefit activity. Preliminary results show that application in June provides greater residual control than one applied in May. Two other neonicitenoids sprays were evaluated in 2008. Assail was applied above the label rate, at 2 ounces per acre instead of 1 ounce per acre. At this currently unregistered rate, it provided effective quick kill of vine mealybug. Clutch, also gave quick kill, but did not give residual control.

I believe we have insecticides that can effectively manage mealybugs in grapes, but care must be taken in their use. Proper timing is critical and rotation will be necessary, just as with fungicides for powdery mildew. If resistance to these insecticides develops, grape growers will no longer be able to manage vine mealybug.

TAGS: Grapes
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