Introduction
The potato leafhopper (PLH) is Wisconsin's most serious alfalfa insect pest. A review of the scientific literature confirms that PLH can impact alfalfa stands in the form of reduced dry matter yields, reduced nutrient yields, reduced crude protein concentration, plant stunting, leaf yellowing (otherwise known as "hopperburn"), reduced root carbohydrate levels, slower regrowth after cutting, and decreased plant survival. The degree of negative impact on the alfalfa stand is a function of PLH population and timing of the infestation. Infestation of PLH early in the regrowth cycle usually results in the greatest damage. The only way a grower could avoid these losses was to scout fields and apply insecticides when PLH populations reached economic threshold levels. However, alfalfa varieties with resistance to leafhopper feeding will be on the market in 1997.
Background
The ability to get PLH resistant varieties on the market was the result of many years of research. In the mid-1980's, researchers at Purdue University and Kansas State University released varieties they had developed for PLH resistance. However, these varieties did not have many of the other desirable traits like disease resistance, good plant vigor, and persistence. At this point, commercial plant breeders took these resistant varieties and incorporated the trait into top performing varieties that are adapted to Midwest conditions. This process took about ten years.
The Resistance Mechanism
The new PLH resistant varieties have tiny glandular hairs on plant surfaces that are tipped with a sticky resin. This is the resistance factor. When the leafhoppers infest the alfalfa, they either die or do little feeding. Researchers have documented lower PLH adult and nymph survival, slower PLH development, and fewer eggs laid by adults in the resistant varieties. The exact mechanism of PLH mortality is not known. It's clear that entrapment in the glandular hairs is not common. More likely, the hairs and resin pose somewhat of a physical barrier to the PLH but there appears to be even more to the resistance than this. Speculation is that the resin contains a chemical that may actually be toxic to the PLH. Greenhouse studies show that there is a great diversity between plants in terms of resistance and that it is not always correlated to the number of glandular hairs on the plant but rather the type of glandular hairs. Some glandular hairs seem to be more "effective" than others in providing resistance.
Names of PLH resistant varieties that growers will see in the near future include Arrest (Northrup King) and 5347LH (Pioneer). Croplan Genetics, DeKalb, Mycogen, Cargill, Agway, America's Alfalfa and Agripro are also scheduled to release resistant varieties. You will see trade names like HopperGuard, Leafhopper Stopper, and AlfalfaGuard. Most of these varieties will come with a "Resistant" (R) rating for PLH. As breeding efforts continue in this area, future varieties will likely move well into the "Highly Resistant" (HR) range.
Four State Research Project
A four-state research project to evaluate the performance of several PLH resistant alfalfa varieties was established in 1996. The cooperating states were Ohio, Indiana, Wisconsin, and Minnesota. Nine experimental PLH resistant varieties were compared to five standard commercial varieties in field plots subject to natural PLH infestations. Variety performance was compared without insecticide treatment and with preventative insecticide control. Some of the key seeding-year findings to date are as follows:
Economic Considerations
Economics is usually the driving force behind most crop input decisions. It will be no different with PLH resistant alfalfa varieties. Seed cost in 1997 will be close to $5.00 per pound. At a 15 lb. per acre seeding rate, planting PLH resistant alfalfa will translate to an additional investment of about $30 per acre. Although this may seem high, it actually only equates to about 0.3 to 0.5 tons per acre of additional alfalfa dry matter that would need to be harvested during the life of the stand to pay for the initial investment. Conversely, it could be argued that at least two insecticide treatments could be applied, if needed, with the additional $30 investment. It's still unclear how much, if any, potential yield loss might be incurred by planting a first generation PLH resistant variety compared to a top standard variety where low PLH pressure is present. We also don't know how much yield loss is incurred by sub-threshold levels of PLH that would not be lost if we planted a PLH resistant variety. All of these factors must be considered and as we gain added field experience with this new technology more definitive answers will surely be available.
Some Final Thoughts
Although there is still much to learn about the performance of these new varieties, there is clearly great potential to benefit alfalfa growers economically. We still need to answer some questions on a production field basis. Future generations of PLH resistant alfalfa varieties will have improved qualities compared to those that are being released in 1997. Some people have questioned whether the glandular hairs will have a negative impact on ensilability, animal acceptance, and digestion. Preliminary research trials would indicate that this is not the case. Most likely, PLH resistance will come "standard" on most improved alfalfa varieties in the future. Currently, the best advice is to seed a small acreage to a PLH resistant alfalfa and make your own assessment as to its economic utility.
For more information contact Mike Rankin|
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