Michigan Grape & Wine Industry Council 2008 Research Report Determination of action thresholds for potato leafhopper in winegrapes, and comparison of foliar insecticides for its control Rufus Isaacs 1, Steven Van Timmeren 1, and Paolo Sabbatini 2 1. Dept. of Entomology and 2. Dept. of Horticulture, Michigan State University, East Lansing, MI 48824 INTRODUCTION Grapevines grown in Michigan are at risk of infestation by a wide range of insect pests, but one of the most damaging to wine vineyards is the potato leafhopper, Empoasca fabae (PLH). This insect flies into Michigan on weather fronts during the springtime when rapid shoot elongation is underway, and infestation of vines can quickly lead to feeding damage and a hypersensitive response in susceptible cultivars. Within a few days, yellowing and cupping of leaves is seen followed by stunting of shoot growth. This rapid vine response often means that vineyard managers realize this pest has infested their vines too late, when symptoms are already visible and increasing. There is a strong desire among Michigan vineyard managers for clear guidance on thresholds that can be used to decide when too many PLH are present, and a need for information on the relative effectiveness of the various new insecticides that have promise for PLH control. This new research project is a collaboration between the MSU viticulture and entomology programs and involves a two-pronged approach to providing growers with the information they need for effective control of PLH: 1) determine how much PLH infestation vines of low, medium, and high susceptibility can withstand before there is measurable reduction in vine productivity and 2) compare currently-registered and promising insecticides to determine their efficacy and residual control of PLH. The objectives of this study are as follows: Objective 1. Determine action thresholds for PLH in grapevines with varying levels of pest susceptibility, crop load, and maturity. Hypothesis 1.1: Vine susceptibility will affect the degree of injury caused by PLH. Hypothesis 1.2: Reduced crop load will increase vine capacity to tolerate PLH infestation. Hypothesis 1.3: Tolerance to PLH will be greater in mature vines than in young vines. Objective 2. Compare foliar insecticides for their ability to control PLH. Hypothesis 2.1: PLH are more effectively controlled by neonicotinoids than by pyrethroids or botanical insecticides. Hypothesis 2.2: Residual control against PLH will be greater using neonicotinoids compared to pyrethroids or botanical insecticides. 1
Hypothesis 1.1 - Vine susceptibility will affect the degree of injury caused by PLH. In spring 2008 vines were planted in pots based on their susceptibility to PLH, including Pinot Gris (susceptible), Chardonnay (intermediate), and Vignoles (resistant). These vines were allowed to grow through the 2008 growing and were maintained with a standard program of fungicides and insecticides, as well as regular watering via a drip irrigation system and fertilizing three times during the summer. In 2009 these vines will be used in the experiments outlined in the original grant proposal. A colony of PLH (Fig. 1) has been established at the Trevor Nichols Research Complex (TNRC) in Fennville, Michigan in order to have Figure 1. Colony of potato leafhoppers on fava beans. nymphs available to utilize in experiments taking place in 2009. Leafhoppers for this colony were collected in August 2008 from alfalfa fields in Lawton and Fennville, Michigan and were placed in Plexiglas cages containing fava beans, Vicia fava. Fava bean plants are replaced weekly to ensure fresh foliage for the leafhoppers to feed on. Hypothesis 1.2: Reduced crop load will increase vine capacity to tolerate PLH infestation. A new Pinot Gris vineyard was planted at TNRC in spring 2008 in anticipation of crop load experiments for this project that will be conducted in 2009. The vines were weighed before planting and the vineyard was laid out with blocking for vine weight. Vines were maintained with a standard pesticide program as well as weekly watering, and fertilizing three times during the summer to build vine size. In late fall 2008 the vines were buried in soil to protect them from damage over the winter. Posts and trellis wires for these vines will be installed in spring 2009 with the assistance of Fenn Valley vineyards, and the shoots will be trained to the trellis in 2009. We consider this new planting a long-term investment for winegrape research at TNRC. Figure 2. New Pinot Gris vineyard planted in May 2008 at the Trevor Nichols Research Complex. 2
Hypothesis 1.3: Tolerance to PLH will be greater in mature vines than in young vines. Methods A mature Pinot Noir vineyard at Fenn Valley vineyards, in Fennville, Michigan was used to determine how crop load affects tolerance to PLH. This vineyard was chosen for high levels of PLH in previous years (based on grower experience) as well as uniformity of vine age, size and health. Vines used in the experiment were adjusted to either a high crop load (3.3 tons per acre) or a low one (1.1 tons per acre) on 13 August 2008. Within each of these crop loads, vines were either treated with foliar insecticides to prevent infestation by PLH or were left untreated to allow for PLH infestation, using a randomized complete block design. The four treatments (low crop low PLH; low crop high PLH; high crop low PLH; high crop high PLH) were applied to six-vine plots and were replicated four times. Insecticides were applied to low PLH treatments on 24 June 2008 (Venom 70SG at 3 oz/acre) and on 1 July 2008 (Sevin XLR at 2 qt/acre) using a CO 2 backpack sprayer set at 50 psi, outfitted with a single head boom and a TeeJet 8003VS nozzle, delivering spray solution equivalent to 50 gallons per acre. The effect of PLH on vines was measured by counting the total number of leaves with yellowing around the edges on the vines on 18 July 2008. Once leaf yellowing assessments were conducted all treatments received the same insecticide program through harvest to prevent potential variability due to Japanese beetle leaf feeding and grape berry moth cluster infestation. On 29 September 2008 cluster samples were taken from the vines and were analyzed for Brix, titratable acidity (g per 100 ml), and ph to determine if PLH and crop load had an effect on fruit quality. Four clusters were sampled from the east side of each of the middle four vines in each replicate, with two clusters taken from close to the truck and two taken from farther down the cordon. Botrytis on clusters was removed to eliminate the effects it could potentially have on the cluster analyses. All data were analyzed using an Analysis of Variance followed by Fisher s Least Significant Difference Test for post hoc comparison of means (Statview v 5.0.1, Cary, NC). The number of leaves showing yellowing was significantly higher on vines that did not receive insecticide treatments for PLH than those that did receive treatments (Fig. 3) (F=7.14, df=3,12, P<0.01). Crop load did not significantly affect leaf yellowing. Average Number of Yellow Leaves 8 7 6 5 4 3 2 1 0 b Low Crop, Low PLH a Low Crop, High PLH High Crop, Low PLH High Crop, High PLH Treatment Figure 3. Average (±SE) number of Pinot Noir leaves with potato leafhopper feeding-induced yellowing on six-vine plots under differing crop loads and leafhopper pressure. Values with the same letter are not significantly different at α=0.05. b a 3
Analysis of cluster samples taken just before harvest showed no significant differences among the different treatments for Brix, titratable acidity, or ph (Table 1) (F<2.93, df=3,12, P>0.077). The lack of significant differences was not unexpected due to the low amount of PLH pressure in this vineyard in 2008 (as seen by the relatively low amount of leaf yellowing). In 2009 PLH populations in the high PLH treatments of the vineyard will be augmented with leafhoppers from the colony that has been established to ensure vines sustain damage. Table 1. Average (±SE) Brix, titratable acidity, and ph of Pinot Noir clusters at harvest under different crop loads and potato leafhopper pressure. Values followed by the same letter within a column are not significantly different at α=0.05. PLH level Crop level Brix ph TA (g/l) Low Low 20.14 ± 0.3 3.92 ± 0.04 5.84 ± 0.3 Low High 19.79 ± 0.3 3.79 ± 0.02 5.64 ± 0.3 High Low 19.88 ± 0.3 3.86 ± 0.03 5.93 ± 0.3 High High 20.59 ± 0.2 3.88 ± 0.03 6.39 ± 0.6 F 3,12 1.43 2.93 0.69 P 0.28 0.077 0.57 H 2.1: PLH are more effectively killed by neonicotinoids than by pyrethroids or botanical insecticides. H 2.2: Residual control against PLH will be greater using neonicotinoids compared to pyrethroids or botanical insecticides. Potato leafhopper spray trials took place in an Aurore vineyard at TNRC in Fennville, MI. Five insecticides were tested including Actara 25WDG (3.5oz/acre), Baythroid XL (3.2 oz/acre), Clutch 50WDG (2 oz/acre), Pyganic EC (32 oz/acre), and Sevin XLR (2 qt/acre) as well as an untreated control. Treatments were applied to three-vine plots and were replicated four times. Insecticide application took place on 18 June 2008 using a CO 2 backpack sprayer set at 50 psi, outfitted with a single head boom and a TeeJet 8003VS nozzle, delivering spray solution equivalent to 50 gallons per acre. Treatments were assessed by counting the total number of potato leafhopper adults and nymphs on 10 leaves per vine for the three vines in the replicate. Leafhoppers were assessed the day before insecticides were applied and six days after application Due to low PLH populations, there were no significant differences among any of the treatments six days after treatment (F=2.0, df=5,18, P=0.13). In addition, the lack of PLH in large enough numbers made clip cage experiments difficult to carry out. Experiments in 2009 will utilize PLH from the colony that has been established and was described previously in this report. Conclusions Experiments on crop load and PLH pressure showed significantly higher amounts of leaf yellowing in the high PLH treatments, but this stress did not result in any significant differences in Brix, titratable acidity, or ph in clusters at harvest. This was possibly due to low PLH pressure this year; the experiment will continue in 2009 with PLH population augmentation from the colony if it proves necessary. Preparations were made in 2008 for experiments related to varietal susceptibility and crop load and their relationship to PLH feeding. Grape vines were planted in pots as well as in a new vineyard at TNRC in Fennville, Michigan in anticipation of these experiments. In addition, a PLH colony has 4
been established to provide a reliable source of leafhoppers for experiments to take place in 2009. Spray trials in 2008 suffered from low numbers of PLH and will be run again in 2009 using PLH from the colony that has been established. Acknowledgements We thank the staff at TNRC as well as Adam Young, Robert Young, Ryan Renkema, and Jon Wyma for technical help this study. We also thank Doug Welsch for the use of his vineyard and Tim Seppala for use of his alfalfa fields for collection of potato leafhoppers, as well as the Michigan Grape and Wine Industry Council and GREEEN for financial support of this study. 5