UPDATE: POTATO LEAFHOPPER RESISTANT ALFALFA
W-L Research, Madison, WI
Potato Leafhopper (PLH) is the most economically damaging insect pest in alfalfa hay production in the Eastern half of the United States. Damage from this insect can cause significant losses in both forage yield and forage quality. Although crop scouting and timely insecticide application can effectively control the leafhopper in alfalfa, chemical control is not widely used, even under heavy PLH infestations. To address this significant problem, alfalfa breeders have developed glandular-haired varieties to provide genetic resistance to potato leafhopper.
Damage Caused by the Potato Leafhopper
The PLH is a small insect that over winters in the Gulf Coast regions. Late each Spring, weather fronts from the Gulf carry the PLH northwards and deposit this damaging insect onto alfalfa fields in the Midwest. When feeding, the PLH secretes a toxin into the alfalfa plant that causes the vascular system to “plug up”, reducing water and nutrient flow. As a result, leafhopper feeding on alfalfa stunts growth and reduces yields, and in severe cases PLH feeding can kill seedling alfalfa. Yield losses of 30-40% have been reported due to PLH feeding in the establishment year. For the entire growing season on an established stand, yield reductions of 25% are possible when the PLH feeds without insecticide controls. University of Wisconsin researchers (Hogg et al 1997) have shown that severe PLH infestations in the seeding year may have a significant, negative impact on alfalfa yields for the life of the stand These results have led to recommendations for planting PLH-resistant varieties when alfalfa is seeded with small grain nurse crops, and chemical control of the insect is not practical.
The PLH has few natural enemies in the field; the best methods to control the leafhopper are application of insecticides, use of resistant alfalfa varieties, or a combination of these two approaches. Since many alfalfa producers are reluctant or unwilling to use insecticides to control PLH,, this paper will focus on the continuing development and improvement of PLH resistant alfalfa varieties.
Glandular Hairs To the Rescue
Glandular hairs are specialized hairs with a multicellular tip, which produce sticky, resin-like exudates. The mechanisms of PLH resistance are complex and involve physical entrapment, antibiosis, nonpreference, and tolerance. The glandular hairs appear to be a critical factor for each of these resistance mechanisms. The glandular-hair trait was originally discovered in exotic Medicago germplasm, and researchers at Purdue University and the USDA worked with this original material and released glandular- haired alfalfa germplasms to industry in the mid-1980’s. Since then, several private breeding programs have introgressed the glandular-hair trait into adapted alfalfa germplasm. A number of glandular-haired alfalfas have been released over the last seven years, with varying levels of both glandular-hair expression and leafhopper resistance.
The initial PLH-resistant (1st generation) varieties displayed 35% resistance, at best. Glandular hair expression on stems and leaves was low to moderate, and “resistant” varieties often required insecticide application under moderate to heavy PLH infestations. Since then, the alfalfa breeding industry has come a long way towards developing true “no-spray” PLH resistant alfalfas. The latest (5th generation) PLH-resistant varieties display over 75% resistance to this damaging insect. Ohio State researchers found that the use of these newest PLH-resistant varieties reduced leafhopper feeding and insect numbers and increased growth rate when compared to susceptible varieties. In this research, the unsprayed resistant variety lost yield only in the first cutting of the seeding year; from then on yield of the unsprayed resistant line was identical to that of the sprayed susceptible variety. The conclusion? A farmer may only have to spray these latest PLH-resistant alfalfas once in the seeding year, realizing significant savings on insecticide costs. After the seeding year, the latest PLH-resistant varieties do not appear to require insecticide control, even under high leafhopper pressure.
Improved Agronomics Completes the Package
The first PLH-resistant releases provided significant benefits to growers who were unwilling to spray for leafhopper. These varieties maintained yield potential better than susceptible varieties under moderate PLH infestations. However, these early PLH-resistant releases were quite dormant (FD ~2), recovery after cutting was slow, disease resistance profiles were marginal, and yield potential in the absence of PLH was average at best. As discussed above, the alfalfa breeding industry has come a long way in improving levels of leafhopper resistance from the 30% level up to and surpassing 80% resistance. However, of equal or greater importance are the significant improvements made in agronomics (yield, recovery after cutting, persistence) and disease resistance in the newest (5th generation) PLH-resistant alfalfas. Intense selection for later fall dormancy (e.g. FD = 4), much faster recovery after cutting, improved persistence, and higher levels of disease resistance have made the latest releases much better all-round alfalfas. In the absence of significant PLH pressure, the latest PLH varieties display overall agronomic performance similar to commercial check varieties. Add significant PLH pressure, and these newest leafhopper resistant varieties really shine, capable of delivering true economic benefits to alfalfa producers.
The potato leafhopper is the most serious insect pest on alfalfa in the Midwest, year in and year out. Damage caused by PLH feeding includes significant reductions in both yield and quality, and potential loss of new stands under especially severe infestations. Recent developments in breeding for PLH-resistant alfalfas have proven to have significant economic benefit to alfalfa growers. The combination of much higher levels of PLH resistance and much improved agronomics in the latest PLH-resistant varieties should lead to broader adaptation and greater use of resistant alfalfas where producers are unwilling or unable to use insecticides. With these most recent releases, farmers are much better off planting resistant varieties if they are seeding with an oats nurse crop, or are not scouting or managing leafhopper populations in their fields. Higher yields, improved disease resistance profiles, faster recovery after cutting, longer stand life, and much greater resistance to the leafhopper suggests that these latest PLH-resistant alfalfa varieties deserve new consideration from alfalfa growers in the Midwestern U.S.