The potato leafhopper (Empoasca fabae) is the most serious insect pest affecting alfalfa production in the midwestern United States. Host plant resistance achieved through the use of glandular-haired (GH) germplasm promises to be an effective and environmentally desirable IPM strategy for controlling this pest in alfalfa. Field evaluations of GH alfalfa varieties began in 1996. Our objectives this year were to: 1) continue the evaluation of GH alfalfa varieties for resistance to potato leafhopper (PLH) and agronomic performance, 2) establish new on-farm demonstration trials comparing GH varieties and standard alfalfa varieties, 3) determine what level of PLH resistance is needed to eliminate economic losses in forage yield and quality of alfalfa, and 4) determine whether strategic use of insecticides in the seeding year can reduce or eliminate pesticide usage on resistant varieties in subsequent years. The results demonstrate that glandular-haired varieties with high levels of potato leafhopper resistance show tremendous promise as an effective and environmentally benign IPM tactic to control this most serious insect pest in alfalfa.
More insecticides are applied to control PLH than for any other pest in alfalfa. Economic thresholds for this pest are commonly exceeded at least once and often twice each year in Ohio. Varieties with resistance to PLH became available in 1997. These varieties were developed by several industry breeding programs from wild glandular-haired Medicago germplasm. Evaluations of the first commercial GH varieties began in Ohio in 1996. Our results in 1996 and 1997 demonstrated that the GH varieties performed dramatically better than standard varieties in the presence of high PLH populations; however, the GH varieties still suffered economic damage, and the initial varietal releases did not meet growers' expectations in the first year of commercial production (1997). In 1998 we continued to evaluate GH alfalfa varieties and new experimental strains in small research plots and in large on-farm demonstration plots at several locations across the state. We also established a new experiment to determine what level of resistance (% resistance plants) in alfalfa is needed to eliminate economic losses to PLH, and whether strategic use of insecticides in the seeding year will be needed to achieve this goal in subsequent years.
A trial established in 1996 at South Charleston was completed in 1998. Table 1 summarizes the net return from insecticide treatment for the GH (resistant) and standard (susceptible) varieties. These data demonstrate that at several harvests insecticide treatment was not economical for the resistant varieties but was for the standard susceptible varieties. It also shows that the greatest benefit of insecticide treatment occurred in the seeding year (1996), and that a significant carryover benefit of insecticide treatment in the seeding year was observed in the first harvest the next year (1997). Leafhopper damage to unprotected plants during the establishment year resulted in lower yields at the first harvest the next year. Thus, it is reasonable to hypothesize that strategic use of insecticides to protect resistant varieties during establishment may lead to a decrease in insecticide usage in subsequent years.
The GH varieties used in the 1996 seeded trial were rated as having 18 to 35% PLH resistant plants. Data from a trial seeded in 1997 demonstrates that resistance is being increased in newer releases (Table 2). The two highest yielding varieties in the 1997 seeding were rated as having ~50% PLH resistant plants.
As resistance is improved through breeding, dramatic reductions in use of insecticides may be possible in established stands, provided plants are protected from excessive PLH damage in the seeding year. To test this hypothesis, a trial was established in 1998 using varieties differing in resistance: 3A14B (~60% resistance), 54H69 (~50%), Cleansweep (~35%), and WL324 (0%). High PLH populations were present in the plots. Forage yield was not significantly different among varieties in insecticide treated plots, but significant yield differences were observed among varieties in untreated plots (Table 3). Dramatic differences among varieties were observed in adult and especially in nymphal leafhopper populations in untreated plots. Similar results have been observed in the large, on-farm plots across the state. But even the most resistant variety (3A14B) suffered economic yield loss during the seeding year (Table 3). Thus, resistance levels will have to be even higher to eliminate economic losses to PLH in the seeding year when populations are very high, as occurred in this trial. Next year we will be able evaluate whether insecticide treatment during the seeding year can help reduce the need to apply insecticides on the most highly resistant varieties in an established stand.
PLH resistant varieties represent a significant new tool in alfalfa pest management. Although PLH is recognized as an important pest of alfalfa in Ohio and other midwestern states, growers often fail to regularly scout alfalfa and apply insecticides in a timely manner when PLH activity warrants treatment. These data confirm previous findings demonstrating that glandular-haired varieties are superior to standard alfalfa under heavy PLH pressure. Our results demonstrate that plant breeders are still making significant improvements in PLH resistance. The PLH-resistant varieties and associated management practices in an integrated control program will provide growers with significant economic benefits, not to mention the environmental benefits of reduced pesticide use. This research will be continued and expanded, through funding obtained from the USDA CSREES IPM grants program.
Field days held at the on-farm trials at several sites around the state provided growers the opportunity to view the new PLH resistant varieties first hand. Leafhopper resistance was demonstrated and results shared with producers, extension agents, and industry professionals at field days, informal plot tours, and regional extension meetings in 1998. Data were presented in February at the National Alfalfa Symposium in Bowling Green, KY. Data from the trials was distributed through the Ohio Forage Performance Trials report, which is available at county Extension Offices, and on the OSU Extension Ohioline internet site (www.ag.ohio-state.edu/~ohioline/). The data were also published in the December issue of Ohio's Country Journal. Data were distributed to interested Extension and industry agronomists in other states via e-mail. Results of the study will continue to be presented at training meetings for extension agents and industry personnel, and at extension meetings for producers. In summary, the results are being widely publicized, and are providing growers with valuable information as they consider adoption of leafhopper resistant varieties.