Nine experimental varieties having the glandular-hair trait were compared with five standard commercially available varieties in a trial seeded 20 May 1996 at the OARDC Western Branch near South Charleston, OH. Variety performance was compared without insecticide treatment and with preventative insecticide applications to control PLH. Data are presented as means of the five standard varieties and the six glandular-haired varieties to be released in 1997. Potato leafhopper populations at the site were very high in 1996. Significant differences in yield were observed in both the first and second cuttings among glandular-haired varieties ('Resistant') and commercial varieties ('Checks'), and among treated and untreated sets of plots (Table 1). In untreated plots, the average seasonal dry matter yield of the six resistant varieties was nearly twice the average yield of the five commercial check varieties. In treated plots, resistant varieties averaged 12 % less than the commercial checks. There was an economic benefit to treating all varieties in both cuttings. Insecticide treatment increased average seasonal yield of resistant varieties by 0.5 tons/acre and of commercial check varieties by 1.6 tons/acre. Differences in leafhopper yellowing and canopy height were similar to the differences in forage yield (Table 1).
Table 1. Total seasonal forage yield, canopy height, and potato leafhopper (PLH) yellowing ratings of resistant and commercial check varieties grown without (Untrt) and with (Trt) insecticide treatment for PLH at South Charleston, OH in 1996.
| Harvest/ | Yield (T/A) | Height (in) | PLH yellowing1 | |||
|---|---|---|---|---|---|---|
| Variety | Untrt | Trt | Untrt | Trt | Untrt | Trt |
| First Growth | ||||||
| Resistant | 1.01** | 1.29* | 11.6** | 17.4** | 1.1** | 0 |
| Checks | 0.39 | 1.39 | 7.3 | 18.8 | 5.6 | 0 |
| Second Growth | Resistant | 0.57** | 0.76** | 9.5** | 11.4** | 1.1** | 0 | Checks | 0.36 | 0.95 | 6.9 | 4.2 | 2.9 | 0 |
| Total | Resistant | 1.57** | 2.05** | -- | -- | -- | -- | Checks | 0.75 | 2.34 | -- | -- | -- | -- |
The large yield differences observed were due to several factors. The trial was planted May 20 due to excessive spring rainfall, and PLH populations were well over economic thresholds during early seedling development in June. PLH pressure remained high throughout the remainder of the growing season (even after first harvest), and dry weather also limited alfalfa growth. Two applications of insecticide were necessary to prevent PLH damage in the treated plots in the first crop, and one application was made to the second crop.
Potato leafhopper nymph populations were dramatically reduced on glandular-haired varieties compared with the commercial check varieties in untreated plots when PLH pressure was high (data not shown). This agrees with previous greenhouse and field studies showing reduced egg-laying, nymph survival, and retarded nymph development on individual plants having the glandular-hair trait. Adult populations did not differ greatly between resistant and commercial check varieties. The reduction in nymph populations in the resistant varieties is significant, because the greatest PLH injury to alfalfa usually occurs with the advent of high nymph populations.
PLH resistant varieties represent a significant new tool in alfalfa pest management, especially if these varieties prove to have acceptable forage yield in the absence of PLH. These varieties and future ones with even greater PLH resistance will provide significant economic and environmental benefit to alfalfa producers and society at large by reducing or eliminating the use of insecticides to control this pest. Although PLH is recognized as an important pest of alfalfa in Ohio and other midwestem states, growers often fail to regularly scout alfalfa and apply insecticides in a timely manner when PLH activity warrants treatment. These data demonstrate that the impact of PLH feeding is much less on untreated PLH resistant varieties than on untreated susceptible varieties. Therefore, these varieties with improved PLH resistance show promise in providing a significant measure of insurance in situations where timely control of PLH is not practiced, which is often the case. However, these resistant varieties will probably benefit from timely insecticide treatments during periods of intense PLH activity. Thus, it should be recognized that these glandular-haired alfalfa varieties should still be scouted periodically to prevent economic loss under conditions of intense PLH activity. Further research is needed to determine whether action thresholds currently recommended for susceptible varieties will need to be adjusted for PLH resistant varieties. Future breeding efforts may lead to further improvements in host resistance to the point where insecticide treatment will never be needed.
These results should be interpreted with care, as this is only seeding year data. Alfalfa performance should be evaluated over several years before concrete conclusions can be drawn regarding productivity and persistence. These results also need to be confirmed in larger on farm plots using commercial seed lots, as this study was conducted using small plots (5 x 20 ft) due to the limited quantity of seed available in 1996.
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