Banded Grape Bug Injury
Associated with Yield Loss in Concord Grapes

Tim Martinson, Greg English-Loeb, and Danielle Bernard
Department of Entomology
New York State Agricultural Experiment Station
Geneva, NY 14456

In 1994, researchers and extension agents were confronted with a new and confounding problem in the Lake Erie region. Grape set malady, or 'millerandage', was starting to affect a significant number of vineyards and at the same time, baffling the experts. Visits to a badly affected vineyard revealed that the block had a large infestation of a small insect called the banded grape bug. We wondered whether the feeding activity of this insect could be causally related to the malady.

This observation prompted us to take a closer look at what banded grape bug (BGB) was doing out in the vineyard. Two years of research has led us to the conclusion that BGB was definitely not associated with millerandage. We did find, however, that BGB infestations can have a very strong and definite impact on yield. We can now say with confidence that BGB, when present in vineyards, has the potential to be one of the most damaging pests found on grapes in the Northeast.

Biology

BGB is classified as a 'true bug' in the order Hemiptera by entomologists. What this means is that it has piercing-sucking mouthparts, and feeds by inserting its stylet (tubular mouthparts similar to those of a mosquito) into plant tissue and sucking out liquid food. Insects that feed in this way insert saliva into the plant tissue, which often contains enzymes and toxins that have a disproportionate effect on tissue development. This appears to be the case with BGB, based on the types of injury we have observed.

Figure 1. Banded grape bug nymph feeding on grape cluster.

BGB completes one generation per year on grapes, and is active in vineyards from shortly after bud break to early July. BGB spends most of the year as an egg, which is the overwintering stage. Eggs are laid in crevices on second-year wood and vine trunks. They hatch when shoots are approximately 3Ð5 inches long, and the nymphs (immature BGB) then begin feeding on shoot-tips and newly emerged leaves. As flower clusters start to develop, BGB nymphs appear to move to the clusters and feed almost exclusively there (Fig. 1). Feeding appears to be concentrated in the pedicels (stalks) of individual florets, and also on the rachis (cluster stem). Nymphs pass through five stages before becoming adults. Development takes about 3 weeks at prevailing spring temperatures, and adults start appearing after the first week in June. In contrast to the nymphal stage, adult BGB are predators. They feed almost exclusively on insect larvae, and do not feed on plants. For this reason, all injury to grapes occurs when BGB is in the nymphal stages.

Injury Evaluations

We have evaluated BGB injury through two types of studies. In 1995, we evaluated the effect of BGB injury on development of individual grape clusters at the Vineyard Laboratory in Fredonia, NY. The study design was simple  - on each of 25 Concord vines, we enclosed two shoots (or individual clusters) with mesh bags. Into one bag we introduced 5 BGB nymphs. The other bag served as an uninjured control. Insects were caged for one week, and the cage treatments were repeated (on different shoots) over time. We caged insects for four different tme periods: Pre-bloom (first week in June); Bloom (mid-June); Post-bloom (first week in July); and Pre-bloom to post-bloom (6 weeks from late May through the first week in July). In this way we were able to pinpoint the time at which clusters were most susceptible to injury.

Results of this study were dramatic (Figure 2). Effects of injury during the pre-bloom period started with a 25% reduction in the number of florets per cluster. Further effects became apparent at fruit set, where the number of berries per cluster was reduced by 57%. Berry weight at harvest was reduced by 14%. The combined result of these effects was that cluster weight was reduced by 63%. Feeding during the bloom and post-bloom periods had no effect on cluster development, while the pre-bloom to post-bloom treatment showed effects almost identical to the pre-bloom treatment. Although anecdotal evidence for this type of injury was noted in a 1914 publication, this was the first evidence from a controlled study that BGB had such a significant effect on cluster development.

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Figure 1. Effect of banded grape bugs placed in cages on Concord grape clusters during
prebloom, post-bloom, and prebloom-to-post-bloom on florets per cluster (A), berries
per cluster (B), berry weight (C) and overall cluster weight (D).

We followed up the 1995 study in 1996 with a commercial vineyard trial in which BGB was eliminated in late May from one of the treatments with an application of carbaryl. We then made harvest evaluations where we measured yield and associated com-ponents. Results of this study were equally as surprising as the results from the bag experiment (Table 1). Total counts of BGB nymphs before spraying showed similar infestation levels of about 20 per vine in both sprayed and unsprayed plots. After treatment, there were no BGB nymphs in the sprayed plots. Sprayed vines yielded 31 lb per vine (9.4 tons per acre), while yield in unsprayed vines was reduced by 26% to 23 lb per vine (6.9 tons per acre). Separation of yield into its components showed that the major cause of this yield difference was a 20% reduction in the number of clusters per vine. Cluster weight was slightly lower, due to a small (but not statistically different) reduction in the number of berries per cluster.

Table 1. Effect of banded grape bug injury on yield components of 'Concord' grapes in a commercial vineyard in 1996.

Dividing the difference in yield (8 lb per vine) by the number of nymphs (20 per vine) leads to the surprising conclusion that each BGB nymph was responsible for 0.4 lb/vine of the observed yield difference. On a per-acre basis, that amounts to 245 lb/acre, or 0.12 tons per acre yield loss associated with 1 nymph per vine. At $200 per ton, the dollar loss per nymph would be roughly $24 - an amount that would easily justify an insecticide application.

These results suggest that growers should examine their vineyards carefully to determine if and where BGB are present. Our experience is that this pest is most commonly observed in vineyards near woodlots, and that infestations are heaviest in border areas, much like grape berry moth. We also believe that BGB infestations are most common in the Lake Erie region. To date, we have not observed any significant infestations in the Finger Lakes region. We do not recommend routine insecticide treatments, as the abundance of BGB varies greatly from year to year and vineyard to vineyard. If an insecticide treatment is warranted, it must be applied between mid-May (6"-12" shoot growth) and early June (pre-bloom) to reduce injury. Further research and observation will help determine which types of sites and areas within vineyards are most likely to have significant infestations of this pest.