One of the problems encountered by organic grape growers in the Finger Lakes region is a complex of four leafhopper species of the genus Erythroneura. These Erythroneura spp. are collectively known as “grape leafhoppers;” the species found on concord grapes, Erythroneura comes, is called “eastern grape leafhopper.”
In many sites, these leafhoppers are effectively kept in check by a minute wasp, Anagrus spp., that parasitizes their eggs. In spite of this biological control agent, leafhoppers were often a serious problem for growers of wine and juice grapes in New York before the age of synthetic pesticides, often causing premature defoliation of the vines, and impeding accumulation of sugar in the grapes.
At the experimental site, leafhopper numbers have been building over the past several years. In spite of efforts to control them with soap and other organically approved materials, populations have increased to the point that fifty leafhopper nymphs are commonly observed on a single leaf, with up to 200 nymphs on the most seriously infested leaves.
The purpose of this trial was to try an approach that has worked in California—mixing pyrethrum and rotenone with insecticidal soap, and applying them at appropriate times (early morning) that maximize their activity. This approach was used in combination with releases of green lacewing eggs as a biological control agent.
Rich Figiel applied the pyrethrum/rotenone/soap sprays starting on June 9, 1999. A total of nine applications was made between June 9 and August 27. Starting with the June 16 spray a partial row was left untreated as a check.
Weekly, starting on June 29, twenty-five leaves were collected from three areas and brought to the laboratory for enumeration of leafhoppers. The total number of live and dead leafhopper nymphs, and their stage of development, was recorded for each leaf. Samples were collected from the north and south ends of the treated portion of the vineyard, and also from the untreated edge row. From these data, the average numbers of live and dead nymphs and percentage of dead leafhoppers per leaf were calculated. No statistical analysis was attempted, because plots were not replicated.
Leafhopper populations averaged 35 nymphs per leaf during the first count in the untreated area, and 25 nymphs per leaf in both the north and south treated areas. Throughout the season, counts were consistently lower in treated areas than in untreated areas. The number of dead leafhoppers was consistently higher in treated vs. untreated areas. The proportion of dead nymphs was always below 10% in the untreated areas, and consistently above 50%, and often up to 95%, in the north and south treated areas. Timing of the counts with respect to spray applications undoubtedly influenced the percentage of dead leafhoppers found.
Lacewing eggs had been released in the vineyard for biological control. No lacewing nymphs were recovered.
These results demonstrate that tank mixes of pyrethrum/rotenone and insecticidal soap can kill leafhopper nymphs and substantially reduce populations. The nine applications were necessary, in my opinion, to break the cycle and end up with the lowest number of overwintering leafhopper adults possible. In subsequent seasons, it should not be necessary to make as many applications. However vigilance, and possibly from one to three well-timed applications, may be necessary in some years.
Addition of the pyrethrum along with the soap probably enhanced efficacy of both. Soap disrupts the wax layer surrounding the exoskeleton, leading to desiccation, and probably allowing more effective entry of the pyrethrum through the exoskeleton.
Native populations of Anagrus were also present in the vineyard. While we did not assess the proportion of eggs parasitized, I estimate that by early August at least 90% of eggs were parasitized. This spray program is compatible with biological control by native parasitic wasps.
A possible refinement of this program would be to test JMS stylet oil, a food-grade horticultural spray, at certain points during the season. This would be contingent upon official approval by the organic materials research institute for its use in organic vineyards. Other spray oils have been approved for organic orchards; the rules regarding this material are somewhat unclear.